Eukaryotic cells may have evolved from primitive prokaryotes about 2 billion years ago. One hypothesis suggests that some prokaryotic cells lost their cell walls, permitting the cell’s plasma membrane to expand and fold. These folds, ultimately, may have given rise to separate compartments within the cell - the forerunners of the nucleus and other organelles now found in eukaryotic cells. Another key hypothesis is known as endosymbiosis. Molecular studies of the bacteria-like DNA and ribosomes in mitochondria and chloroplasts indicate that mitochondrion and chloroplast ancestors were once free-living bacteria. Scientists propose that these free-living bacteria were engulfed and maintained by other prokaryotic cells for their ability to produce ATP efficiently and to provide a steady supply of glucose. Over generations, eukaryotic cells situated with mitochondria - the ancestors of animals - or with both mitochondria and chloroplasts - the ancestors of plants - evolved.
The first observations of cells were made in 1665 by English scientist Robert Hooke, who used a crude microscope of his own invention to examine a variety of objects, including a thin piece of cork. Noting the rows of tiny boxes that made up the dead wood’s tissue, Hooke coined the term cell because the boxes reminded him of the small cells occupied by monks in a monastery. While Hooke was the first to observe and describe cells, he did not comprehend their significance. At about the same time, the Dutch maker of microscopes Antoni van Leeuwenhoek pioneered the invention of one of the best microscopes of the time. Using his invention, Leeuwenhoek was the first to observe, draw, and describe a variety of living organisms, including bacteria gliding in saliva, one-celled organisms cavorting in pond water, and sperm swimming in semen. Two centuries passed, however, before scientists grasped the true importance of cells.
Many advances have been made in microscope technology. This article from the 1994 Collier’s Year Book begins with the microscope most young students are familiar with and tracks the breakthroughs in the development of new types of microscopes - including those that use ultrasonic imaging and those that ‘feel’ an object’s surface.
Modern ideas about cells appeared in the 1800s, when improved light microscopes enabled scientists to observe more details of cells. Working together, German botanist Matthias Jakob Schleiden and German zoologist Theodor Schwann recognized the fundamental similarities between plant and animal cells. In 1839 they proposed the revolutionary idea that all living things are made up of cells. Their theory gave rise to modern biology: a whole new way of seeing and investigating the natural world.
By the late 1800s, as light microscopes improved still further, scientists were able to observe chromosomes within the cell. Their research was aided by new techniques for staining parts of the cell, which made possible the first detailed observations of cell division, including observations of the differences between mitosis and meiosis in the 1880s. In the first few decades of the 20th century, many scientists focused on the behavior of chromosomes during cell division. At that time, it was generally held that mitochondria transmitted the hereditary information. By 1920, however, scientists determined that chromosomes carry genes and that genes transmit hereditary information from generation to generation.
During this period, scientists began to understand some of the chemical processes in cells. In the 1920s, the ultracentrifuge was developed. The ultracentrifuge is an instrument that spins cells or other substances in test tubes at high speeds, which causes the heavier parts of the substance to fall to the bottom of the test tube. This instrument enabled scientists to separate the relatively abundant and heavy mitochondria from the rest of the cell and study their chemical reactions. By the late 1940s, scientists were able to explain the role of mitochondria in the cell. Using refined techniques with the ultracentrifuge, scientists subsequently isolated the smaller organelles and gained an understanding of their functions.
The deoxyribonucleic acid (DNA) molecule is the genetic blueprint for each cell and ultimately the blueprint that determines every characteristic of a living organism. In 1953 American biochemist James Watson, left, and British biophysicist Francis Crick, right, described the structure of the DNA molecule as a double helix, somewhat like a spiral staircase with many individual steps. Their work was aided by X-ray diffraction pictures of the DNA molecule taken by British biophysicist Maurice Wilkins and British physical chemist Rosalind Franklin. In 1962 Crick, Watson, and Wilkins received the Nobel Prize for their pioneering work on the structure of the DNA molecule.
While some scientists were studying the functions of cells, others were examining details of their structure. They were aided by a crucial technological development in the 1940s, the invention of the electron microscope, which uses high-energy electrons instead of light waves to view specimens. New generations of electron microscopes have provided resolution, or the differentiation of separate objects, thousands of times more powerful than that available in light microscopes. This powerful resolution revealed organelles such as the endoplasmic reticulum, lysosomes, the Golgi apparatus, and the cytoskeleton. The scientific fields of cell structure and function continue to complement each other as scientists explore the enormous complexity of cells.
The discovery of the structure of DNA in 1953 by American biochemist James D. Watson and British biophysicist Francis Crick ushered in the era of molecular biology. Today, investigation inside the world of cells - of genes and proteins at the molecular level - constitutes one of the largest and fastest moving areas in all of science. One particularly active field in recent years has been the investigation of cell signaling, the process by which molecular messages find their way into the cell via a series of complex protein pathways in the cell.
Another busy area in cell biology concerns programmed cell death, or apoptosis. Millions of times per second in the human body, cells commit suicide as an essential part of the normal cycle of cellular replacement. This also seems to be a check against disease: When mutations build up within a cell, the cell will usually self-destruct. If this fails to occur, the cell may divide and give rise to mutated daughter cells, which continue to divide and spread, gradually forming a growth called a tumor. This unregulated growth by rogue cells can be benign, or harmless, or cancerous, which may threaten healthy tissue. The study of apoptosis is one avenue that scientists explore in an effort to understand how cells become cancerous.
Scientists are also discovering exciting aspects of the physical forces within cells. Cells employ a form of architecture called tensegrity, which enables them to withstand battering by a variety of mechanical stresses, such as the pressure of blood flowing around cells or the movement of organelles within the cell. Tensegrity stabilizes cells by evenly distributing mechanical stresses to the cytoskeleton and other cell components. Tensegrity also may explain how a change in the cytoskeleton, where certain enzymes are anchored, initiates biochemical reactions within the cell, and can even influence the action of genes. The mechanical rules of tensegrity may also account for the assembly of molecules into the first cells. Such new insights - made some 300 years after the tiny universe of cells was first glimpsed - show that cells continue to yield fascinating new worlds of discovery.
The Nervous System signifies of those elements within the animal organism that are concerned with the reception of stimuli, the transmission of nerve impulses, or the activation of muscle mechanisms.
The reception of stimuli is the function of special sensory cells. The conducting elements of the nervous system are cells called neurons; these may be capable of only slow and generalized activity, or they may be highly efficient and rapidly conducting units. The specific response of the neuron—the nerve impulse - and the capacities of the cell to be stimulated make this cell a receiving and transmitting unit capable of transferring information from one part of the body to another.
Each nerve cell consists of a central portion containing the nucleus, known as the cell body, and one or more structures referred to as axons and dendrites. The dendrites are rather short extensions of the cell body and are involved in the reception of stimuli. The axon, by contrast, is usually a single elongated extension, it is especially important in the transmission of nerve impulses from the region of the cell body to other cells.
Although all many-celled animals have some kind of nervous system, the complexity of its organization varies considerably among different animal types. In simple animals such as jellyfish, the nerve cells form a network capable of mediating only a relatively stereotyped response. In more complex animals, such as shellfish, insects, and spiders, the nervous system is more complicated. The cell bodies of neurons are organized in clusters called ganglia. These clusters are interconnected by the neuronal processes to form a ganglionated chain. Such chains are found in all vertebrates, in which they represent a special part of the nervous system, related especially to the regulation of the activities of the heart, the glands, and the involuntary Vertebrate animals have a bony spine and skull in which the central part of the nervous system is housed; The peripheral part extends throughout the remainder of the body. That part of the nervous system located in the skull is referred to as the brain that found in the spine is called the spinal cord. The brain and the spinal cord are continuous through an opening in the base of the skull; Both are also in contact with other parts of the body through the nerves. The distinction made between the central nervous system and the peripheral nervous system is based on the different locations of the two intimately related parts of a single system. Some of the processes of the cell bodies conduct sense impressions and others conduct muscle responses, called reflexes, such as those caused by pain.
In the skin are cells of several types called receptors; each is especially sensitive to particular stimuli. Free nerve endings are sensitive to pain and are directly activated. The neurons so activated send impulses into the central nervous system and have junctions with other cells that have axons extending back into the periphery. Impulses are carried from processes of these cells to motor endings within the muscles. These neuromuscular endings excite the muscles, resulting in muscular contraction and appropriate movement. The pathway taken by the nerve impulse in mediating this simple response is in the form of a two-neuron arc that begins and ends in the periphery. Many of the actions of the nervous system can be explained on the basis of such reflex arcs, which are chains of interconnected nerve cells, stimulated at one end and capable of bringing about movement or glandular secretion at the other.
The cranial nerves connect to the brain by passing through openings in the skull, or cranium. Nerves associated with the spinal cord pass through openings in the vertebral column and are called spinal nerves. Both cranial and spinal nerves consist of large numbers of processes that convey impulses to the central nervous system and also carry messages outward; the former processes are called afferent, and the latter are called efferent. Afferent impulses are referred to as sensory; efferent impulses are referred to as either somatic or visceral motor, according to what part of the body they reach. Most nerves are mixed nerves made up of both sensory and motor elements.
The cranial and spinal nerves are paired; The number in humans are 12 and 31, respectively. Cranial nerves are distributed to the head and neck regions of the body, with one conspicuous exception: the tenth cranial nerve, called the vagus. In addition to supplying structures in the neck, the vagus is distributed to structures located in the chest and abdomen. Vision, auditory and vestibular sensation, and taste is mediated by the second, eighth, and seventh cranial nerves, respectively. Cranial nerves also mediate motor functions of the head, the eyes, the face, the tongue, and the larynx, as well as the muscles that function in chewing and swallowing. Spinal nerves, after they exit from the vertebrae, are distributed in a band-like fashion to regions of the trunk and to the limbs. They interconnect extensively, thereby forming the brachial plexus, which runs to the upper extremities, and the lumbar plexus, which passes to the lower limbs.
Among the motor’s fibers may be found groups that carry impulses to viscera. These fibers are designated by the special name of autonomic nervous system. That system consists of two divisions, more or less antagonistic in function, that emerge from the central nervous system at different points of origin. One division, the sympathetic, arises from the middle portion of the spinal cord, joins the sympathetic ganglionated chain, courses through the spinal nerves, and is widely distributed throughout the body. The other division, the parasympathetic, arises both above and below the sympathetic, that is, from the brain and from the lower part of the spinal cord. These two divisions control the functions of the respiratory, circulatory, digestive, and urogenital systems.
Consideration of disorders of the nervous system is the province of neurology; Psychiatry deals with behavioral disturbances of a functional nature. The division between these two medical specialties cannot be sharply defined, because neurological disorders often manifest both organic and mental symptoms.
Diseases of the nervous system include genetic malformations, poisonings, metabolic defects, vascular disorders, inflammations, degeneration, and tumors, and they involve either nerve cells or their supporting elements. Vascular disorders, such as cerebral hemorrhage or other forms of a stroke, are among the most common causes of paralysis and other neurologic complications. Some diseases exhibit peculiar geographic and age distribution. In temperate zones, multiple sclerosis is a common degenerative disease of the nervous system, but it is rare in the Tropics.
The nervous system is subject to infection by a great variety of bacteria, parasites, and viruses. For example, meningitis, or infection of the meninges investing the brain and spinal cord, can be caused by many different agents. On the other hand, one specific virus causes rabies. Some viruses causing neurological ills effect only certain parts of the nervous system. For example, the virus causing poliomyelitis commonly affects the spinal cord, as Viruses manufacturing encephalitis attack the brain.
Inflammations of the nervous system are named according to the part affected. Myelitis is an inflammation of the spinal cord; Neuritis is an inflammation of a nerve. It may be caused not only by infection but also by poisoning, alcoholism, or injury. Tumors originating in the nervous system usually are composed of meningeal tissue or neuroglia (supporting tissue) cells, depending on the specific part of the nervous system affected, but other types of a tumor may metastasize to or invade the nervous system. In certain disorders of the nervous system, such as neuralgia, migraine, and epilepsy, no evidence may exist of organic damage. Another disorder, cerebral palsy, is associated with birth defects.
Pain, is an unpleasant sensory or emotional experience caused by real or potential injury or damage to the body or described in terms of such damage. Scientists believe that pain evolved in the animal kingdom as a valuable three-part warning system. First, it warns of injury. Second, pain protects against further injury by causing a reflexive withdrawal from the source of injury. Finally, pain leads to a period of reduced activity, enabling injuries to heal more efficiently.
Pain is difficult to measure in humans because it has an emotional, or psychological component as well as a physical component. Some people express extreme discomfort from relatively small injuries, while others show little or no pain even after suffering severe injury. Sometimes pain is present even though no injury is apparent at all, or pain lingers long after an injury appears to have healed.
The signals that warn the body of tissue damage are transmitted through the nervous system. In this system, the basic unit is the nerve cell or neuron. A nerve cell is composed of three parts: a central cell body, a single major branching fiber called an axon, and a series of smaller branching fibers known as dendrites. Each nerve cell meets other nerve cells at certain points on the axons and dendrites, forming a dense network of interconnected nerve fibers that transmit sensory information about touch, pressure, or warmth, as well as pain.
Sensory information is transmitted from the different parts of the body to the brain via the spinal cord, which is a complex set of nerves that extend from the brain down along the back, protected by the bones of the spine. About as wide as a finger, the spinal cord is like a cable packed with many bundles of wires. The bundles are nerve pathways for transmitting information. But the spinal cord is more than just a message transmitter, it is also an extension of the brain. It contains neurons that process incoming sensory information, and generates messages to be sent back down to cells in other parts of the body.
In the nervous system, a message-carrying impulse travels from one end of a nerve cell to the other by means of an electrical impulse. When it reaches the terminal end of a nerve cell, the impulse trigger’s tiny sacs called presynaptic vessicles to release their contents, chemical messengers called neurotransmitters. The neurotransmitters float across the synapse, or gap between adjacent nerve cells. When they reach the neighboring nerve cell, the neurotransmitters fit into specialized receptor sites much as a key fits into a lock, causing that nerve cell to ‘fire,’ or generate an electric message-carrying impulse. As the message continues through the nervous system, the presynaptic cell absorbs the excess neurotransmitters, and repackages them in presynaptic versicles in a process called neurotransmitter reuptake.
Information being transmitted between and within the brain and spinal cord travels through the nervous system using both chemical and electrical mechanisms. A message-carrying impulse travels from one end of a nerve cell to another by means of an electric signal. When the electric signal reaches the terminal end of a nerve cell, a gap called a synapse prevents the electric signal from crossing to the next cell. The electric signal triggers the cell to release chemicals called neurotransmitters, which float across the synapse to the neighboring nerve cell. These neurotransmitters fit into specialized receptors found on the adjacent nerve cell, much as a key fits into a lock, generating an electric impulse in the neighboring cell. This new impulse travels to the end of the long cell, in turn triggering the release of neurotransmitters to carry the message across the next synapse. Not all neurotransmitters initiate a message in a neighboring nerve cell. Some specialize in preventing neighboring cells from generating an electrical signal, while others function as helpers, facilitating the message's journey to the brain.
While most of the sensory nerves in the skin and other body tissues have special structures covering their nerve endings, those nerves that signal injury have free nerve endings. These simple nerve endings specialize in detecting noxious stimuli - a catchall term for injury-causing stimuli such as intense heat, extreme pressure, or sharp pricks or cuts. The nerve endings that detect pain are called nociceptors, and the process of transmitting pain signals when harmful stimulation occurs is called nociception. Several million nociceptors are interlaced through the tissues and organs of the body.
When a person experiences an injury, such as a stubbed toe, specialized cells called nociceptors sense potential tissue damage (1) and send an electric signal, called an impulse, to the spinal cord via a sensory nerve (2). A specialized region of the spinal cord known as the dorsal horn (3) processes the pain signal, immediately sending another impulse back down the leg via a motor nerve (4). This causes the muscles in the leg to contract and pull the toe away from the source of injury (6). At the same time, the dorsal horn sends another impulse up the spinal cord to the brain. During this trip, the impulse travels between nerve cells. When the impulse reaches a nerve ending (7), the nerve released chemical messengers, called neurotransmitters, which carry the message to the adjacent nerve. When the impulse reaches the brain (8), it is analyzed and processed as an unpleasant physical and emotional sensation.
An injury triggers pain signals in two types of nociceptors, one with large, insulated axons known as A-delta fibers and one with small, uninsulated axons known as C fibers. The large A-delta fibers conduct signals quickly, and the smaller C fibers transmit information slowly. The difference in the functions of these two fibers becomes obvious to a person who stubs a toe. At first the injured person is aware of a sharp, flashing pain at the point of injury. Generated by the A-delta fibers, this short-lived pain intrudes upon the thoughts and perceptions occurring in the brain. Just as this first pain subsides, a second pain begins that is vague, throbbing, and persistent. This sensation is derived from the C fibers.
Pain information from the A-delta and C fibers travels through the spinal cord to the brain. When it receives the pain message, the spinal cord generates impulses that travel back down to muscles, which lead to a reflexive contraction that pulls the body away from the source of injury. Other reflexes may affect skin temperature, blood flow, sweating, and other changes.
While this reflex action is underway, the pain message continues up the spinal cord to relay centers in the brain. The sensory information is routed to many other parts of the brain, including the cortex, where thinking processes occur
The Adrenal Gland is the vital endocrine gland that secretes hormones into the bloodstream, situated, in humans, on top of the upper end of each kidney. The two parts of the gland - the inner portion, or medulla, and the outer portion, or the cortex - are like separate organs: They are composed of different types of tissue and perform different functions. The adrenal medulla, composed of chromaffin cells secretes the hormone epinephrine, also called adrenaline, in response to stimulation of the sympathetic nervous system at times of stress. The medulla also secretes the hormone norepinephrine, which plays a role in maintaining normal blood circulation. The hormones of the medulla are called catecholamines. Unlike the adrenal cortex, the medulla can be removed without endangering the life of an individual.
The adrenal outer layer, or cortex, secretes about 30 steroid hormones, but only a few are secreted in significant amounts. Aldosterone, one of the most important hormones, regulates the balance of salt and water in the body. Cortisone and hydrocortisone are necessary to regulate fat, carbohydrate, and protein metabolism. Adrenal sex steroids have a minor influence on the reproductive system. Modified steroids, now produced synthetically, are superior to naturally secreted steroids for treatment of Addison's disease and other disorders.
Adrenocorticotropic Hormone is also known as corticotropin, hormones secreted by the anterior part of the pituitary gland. The specific function of ACTH is to stimulate the growth and secretions of the cortex (outer layers) of the adrenal gland. One of these secretions is cortisone, a hormone involved in carbohydrate and protein metabolisms. ACTH is used medically for its anti-inflammatory action to alleviate symptoms of allergies and arthritis. ACTH is a complex protein molecule containing 39 amino acids. Its molecular weight is approximately 5000. The biological activity of the ACTH of various animal species is similar to that of humans, but the sequence of amino acids has been found to vary somewhat among species. ACTH production is controlled in part by the hypothalamus and in part by the existing levels of adrenal gland hormones. ACTH levels increased in response to stress, disease, and decreased blood pressure.
The Pituitary Gland is the master endocrine gland in vertebrate animals. The hormones secreted by the pituitary stimulate and control the functioning of almost all the other endocrine glands in the body. Pituitary hormones also promote growth and control the water balance of the body.
The pituitary is a small bean-shaped, reddish-gray organ located in the saddle-shaped depression (sella turcica) in the floor of the skull (the sphenoid bone) and attached to the base of the brain by a stalk; it is located near the hypothalamus. The pituitary has two lobes - the anterior lobe, or adenohypophysis, and the posterior lobe, or neurohypophysis - which differ in structure and function. The anterior lobe is derived embryologically from the roof of the pharynx and is composed of groups of epithelial cells separated by blood channels; the posterior lobe is derived from the base of the brain and is composed of nervous connective tissue and nerve-like secreting cells. The area between the anterior and posterior lobes of the pituitary is called the intermediate lobe; it has the same embryological origin as the anterior lobe.
Concentrated chemical substances, or hormones, which control 10 to 12 functions in the body, have been obtained as extracts from the anterior pituitary glands of cattle, sheep, and swine. Eight hormones have been isolated, purified, and identified; All of them are peptides, that is, they are composed of amino acids. A growth hormone (GH), or the somatotropic hormone (STH), is essential for normal skeletal growth and is neutralized during adolescence by the gonadal sex hormones. Thyroid-stimulating hormones (TSH) control the normal functioning of the thyroid gland, and the adrenocorticotropic hormone (ACTH) controls the activity of the cortex of the adrenal glands and takes part in the stress reaction. Prolactin, also called lactogenic, luteotropic, or mammotropic hormone, initiates milk secretion in the mammary gland after the mammary tissues have been prepared during pregnancy by the secretion of other pituitary and sex hormones. The two gonadotropic hormones are follicle-stimulating hormones (FSH) and a luteinizing hormone (LH). Follicle-stimulating hormones stimulates the formation of the Graafian follicle in the female ovary and the development of spermatozoa in the male. The luteinizing hormone stimulates the formation of ovarian hormones after ovulation and initiates lactation in the female, in the male, it stimulates the tissues of the testes to elaborate testosterone. In 1975 scientists identified the pituitary peptide endorphin, which acts in experimental animals as a natural pain reliever in times of stress. Endorphin and ACTH are made as parts of a single large protein, which subsequently splits. This may be the body's mechanism for coordinating the physiological activities of two stress-induced hormones. The same large prohormone that contains ACTH and endorphin also contains short peptides called melanocyte-stimulating hormones. These substances are analogous to the hormone that regulates pigmentation in fish and amphibians, but in humans they have no known function.
Research has shown that the hormonal activity of the anterior lobe is controlled by chemical messengers sent from the hypothalamus through tiny blood vessels to the anterior lobe. In the 1950s, the British neurologist Geoffrey Harris discovered that cutting the blood supply from the hypothalamus to the pituitary impaired the function of the pituitary. In 1964, chemical agents called releasing factors were found in the hypothalamus; These substances, it was learned, affect the secretion of growth hormones, a thyroid-stimulating hormone called thyrotropin, and the gonadotropic hormones involving the testes and ovaries. In 1969 the American endocrinologist Roger Guillemin and colleagues isolated and characterized thyrotropin-releasing factors, which stimulates the secretion of thyroid-stimulating hormones from the pituitary. In the next few years his group and that of the American physiologist Andrew Victor Schally isolated the luteinizing hormone-releasing factor, which stimulates secretion of both LH and FSH, and somatostatin, which inhibits release of growth hormones. For this work, which proved that the brain and the endocrine system are linked, they shared the Nobel Prize in physiology or medicine in 1977. Human somatostatin was one of the first substances to be grown in bacteria by recombinant DNA.
The presence of the releasing factors in the hypothalamus helped to explain the action of the female sex hormones, estrogen and progesterone, and their synthetic versions contained in oral contraceptives, or birth-control pills. During a woman's normal monthly cycle, several hormonal changes are needed for the ovary to produce an egg cell for possible fertilization. When the estrogen level in the body declines, the follicle-releasing factor (FRF) flows to the pituitary and stimulates the secretion of the follicle-stimulating hormone. Through a similar feedback principle, the declining level of progesterone causes a release of luteal-releasing factors (LRF), which stimulates secretion of the luteinizing hormone. The ripening follicle in the ovary then produces estrogen, and the high level of that hormone influences the hypothalamus to shut down temporarily the production of FSH. Increased progesterone feedback to the hypothalamus shuts down LH production by the pituitary. The daily doses of synthetic estrogen and progesterone in oral contraceptives, or injections of the actual hormones, inhibit the normal reproductive activity of the ovaries by mimicking the effect of these hormones on the hypothalamus.
In lower vertebrates this part of the pituitary secretes melanocyte-stimulating hormones, which brings about skin-color changes. In humans, it is present only for a short time early in life and during pregnancy, and is not known to have any function.
Two hormones are secreted by the posterior lobe. One of these is the antidiuretic hormone (ADH), vasopressin. Vasopressin stimulates the kidney tubules to absorb water from the filtered plasma that passes through the kidneys and thus controls the amount of urine secreted by the kidneys. The other posterior pituitary hormone is oxytocin, which causes the contraction of the smooth muscles in the uterus, intestines, and blood arterioles. Oxytocin stimulates the contractions of the uterine muscles during the final stage of pregnancy to stimulate the expulsion of the fetus, and it also stimulates the ejection, or let-down, of milk from the mammary gland following pregnancy. Synthesized in 1953, oxytocin was the first pituitary hormone to be produced artificially. Vasopressin was synthesized in 1956.
Pituitary functioning may be disturbed by such conditions as tumors, blood poisoning, blood clots, and certain infectious diseases. Conditions resulting from a decrease in anterior-lobe secretion include dwarfism, acromicria, Simmonds's disease, and Fröhlich's syndrome. The dwarfism occurs when anterior pituitary deficiencies occur during childhood; acromicria, in which the bones of the extremities are small and delicate, results when the deficiency occurs after puberty. Simmonds's disease, which is caused by extensive damage to the anterior pituitary, is characterized by premature aging, loss of hair and teeth, anemia, and emaciation; it can be fatal. Fröhlich's syndrome, also called adiposogenital dystrophy, is caused by both anterior pituitary deficiency and a lesion of the posterior lobe or hypothalamus. The result is obesity, dwarfism, and retarded sexual development. Glands under the influence of anterior pituitary hormones are also affected by anterior pituitary deficiency.
Over secretion of one of the anterior pituitary hormones, somatotropin, results in a progressive chronic disease called acromegaly, which is characterized by enlargement of some parts of the body. Posterior-lobe deficiency results in diabetes insipidus.
Tissue
Tissue, - group of associated, similarly structured cells that perform specialized functions for the survival of the organism. Animal tissues, to which this article is limited, take their first form when the blastula cells, arising from the fertilized ovum, differentiate into three germ layers: the ectoderm, mesoderm, and endoderm. Through further cell differentiation, or histogenesis, groups of cells grow into more specialized units to form organs made up, usually, of several tissues of similarly performing cells. Animal tissues are classified into four main groups.
These tissues include the skin and the inner surfaces of the body, such as those of the lungs, stomach, intestines, and blood vessels. Because its primary function is to protect the body from injury and infection, epitheliums are made up of tightly packed cells with little intercellular substance between them.
About 12 kinds of epithelial tissue occur. One kind is stratified squamous tissue found in the skin and the linings of the esophagus and vagina. It is made up of thin layers of flat, scalelike cells that form rapidly above the blood capillaries and is pushed toward the tissue surface, where they die and are shed. Another is a simple columnar epithelium, which lines the digestive system from the stomach to the anus; Simple columnar epithelium cells stand upright and not only control the absorption of nutrients but also secrete mucus through individual goblet cells. Glands are formed by the inward growth of epithelium-for examples, the sweat glands of the skin and the gastric glands of the stomach. Outward growth results in hair, nails, and other structures.
These tissues, which support and hold parts of the body together, comprises the fibrous and elastic connective tissues, the adipose (fatty) tissues, and cartilage and bone. In contrast to an epithelium, the cells of these tissues are widely separated from one another, with a large amount of intercellular substance between them. The cells of fibrous tissue, found throughout the body, connect to one another by an irregular network of strands, forming a soft, cushiony layer that also supports blood vessels, nerves, and other organs. Adipose tissue has a similar function, except that its fibroblasts also contain store fat. Elastic tissue, found in ligaments, the trachea, and the arterial walls, stretches and contracts again with each pulse beat. In the human embryo, the fibroblast cells that originally secreted collagen for the formation of fibrous tissue later change to secrete a different form of protein called chondrion, for the formation of cartilage, some cartilage later becomes calcified by the action of osteoblast to form bones. Blood and lymph are also often considered connective tissues.
Tissues, which contract and relax, comprise the striated, smooth, and cardiac muscles. Striated muscles, also called skeletal or voluntary muscles, include those that are activated by the somatic, or voluntary, nervous system. They are joined together without cell walls and have several nuclei. The smooth, or involuntary muscles, which are activated by the autonomic nervous system, are found in the internal organs and consist of simple sheets of cells. Cardiac muscles, which have characteristics of both striated and smooth muscles, are joined together in a vast network. These highly complex groups of cells, called ganglia, transfer information from one part of the body to another. Each neuron, or nerve cell, consists of a cell body with branching dendrites and one long fiber, or axons. The dendrites connect one neuron to another; The axon transmits impulses to an organ or collects impulses from a sensory organ.
Crossing a Synapse
In the nervous system, a message-carrying impulse travels from one end of a nerve cell to the other by means of an electrical impulse. When it reaches the terminal end of a nerve cell, the impulse trigger’s tiny sacs called presynaptic vessicles to release their contents, chemical messengers called neurotransmitters. The neurotransmitters float across the synapse, or gap between adjacent nerve cells. When they reach the neighboring nerve cell, the neurotransmitters fit into specialized receptor sites much as a key fits into a lock, causing that nerve cell to fire or generate an electric message-carrying impulse. As the message continues through the nervous system, the presynaptic cell absorbs the excess neurotransmitters, and repackages them in presynaptic versicles in a process called neurotransmitter reuptake.
Reflex
Reflex, in physiology, is the involuntary response to a stimulus by the animal organism. In its simplest form, it consisted of the stimulation of an afferent nerve through a sense organ, or receptor, followed by transmission of the stimulus, usually through a nerve center, to an efferent motor nerve, resulting in action of a muscle or gland, called the effector. In most reflex action, however, the stimulus passes through one or more intermediate nerve cells, which modify and direct its action, sometimes to the extent of involving the muscular activity of the entire organism. For example, a painful stimulus applied to the hand causes a reflex withdrawal of the hand, which involves contraction of the flexor group of muscles and reflexation of the opposing extensor group; if the stimulus is strong, the coordinating nerve cells pass it to the arm muscles and also to the muscles of the trunk and legs, the result being a jump that removes not only the arm, but the entire person from the vicinity of the painful stimulus.
The system of coordinating nerve cells is such that several different kinds of stimuli may produce the same result. For example, the stimulus produced by the sight of food and that caused by the smell of food travel different afferent pathways, but both have a common final path that stimulates the salivary glands to secretion. The final common path may also be activated through associated nerve tracts by a stimulus that ordinarily is not directly connected with the response. This type of reflex was named conditioned reflex by its discoverer, the Russian physiologist Ivan Pavlov, about 1904. Pavlov found that sounding a bell every time a dog was about to be given food eventually caused a reflex flow of saliva, which later persisted even when no food was produced. Elaborations of this habituative type of reflex are regarded by some physiologists and psychologists as an important basis for many behaviors, both voluntary and involuntary.
The normal pathways of many reflexes are generally known, and the presence, absence, or exaggerations of the normal physical responses to certain stimuli are symptoms used by neurologists to determine the condition of the neural pathways involved. A familiar reflex commonly tested by physicians is the patellar reflex, in which an involuntary jerk of the knee is evoked by lightly striking the tendon of the patella, or kneecap, indicating the efficiency of certain nerve tracts in the spinal cord.
Like all other cells, neurons contain charged ions: Potassium and sodium (positively charged) and chlorine (negatively charged). Neurons differ from other cells in that they are able to produce a nerve impulse. A neuron is polarized - that is, it has an overall negative charge inside the cell membrane because of the high concentration of chlorine ions and low concentration of potassium and sodium ions. The concentration of these same ions is exactly reversed outside the cell. This charge differential represents stored electrical energy, sometimes referred to as membrane potential or resting potential. The negative charge inside the cell is maintained by two features. The first is the selective permeability of the cell membrane, which is more permeable to potassium than sodium. The second feature is sodium pumps within the cell membrane that actively pump sodium out of the cell. When depolarization occurs, this charge differential across the membrane is reversed, and a nerve impulse is produced.
Depolarization is a rapid change in the permeability of the cell membrane. When sensory input or any other kind of stimulating current is received by the neuron, the membrane permeability is changed, allowing a sudden influx of sodium ions into the cell. The high concentration of sodium, or action potential, changes the overall charges within the cell from negative to positive. The local changes in ion concentration triggers similar reactions along the membrane, propagating the nerve impulse. After a brief period called the refractory period, during which the ionic concentration returned to resting potential, the neuron can repeat this process.
Nerve impulses travel at different speeds, depending on the cellular composition of a neuron. Where speed of impulse is important, as in the nervous system, axons are insulated with a membranous substance called myelin. The insulation provided by myelin maintains the ionic charge over long distances. Nerve impulses are propagated at specific points along the myelin sheath; These points are called the nodes of Ranvier. Examples of myelinated axons are those in sensory nerve fibers and nerves connected to skeletal muscles. In non-myelinated cells, the nerve impulse is propagated more diffusely.
The nervous system has two divisions: The somatic, which allow voluntary control over skeletal muscle, and the autonomic, which is involuntary and controls cardiac and smooth muscle and glands. The autonomic nervous system has two divisions: The sympathetic and the parasympathetic. Many, but not all, of the muscles and glands that distribute nerve impulses to the larger interior organs possess a double nerve supply; in such cases the two divisions may exert opposing effects. Thus, the sympathetic system increases heartbeat, and the parasympathetic system decreases heartbeat. The two nervous systems are not always antagonistic, however. For example, both nerve supplies to the salivary glands excite the cells of secretion. Furthermore, a single division of the autonomic nervous system may both excite and inhibit a single effector, as in the sympathetic supply to the blood vessels of skeletal muscle. Finally, the sweat glands, the muscles that cause involuntary erection or bristling of the hair, the smooth muscle of the spleen, and the blood vessels of the skin and skeletal muscle are actuated only by the sympathetic division.
Voluntary movement of head, limbs, and body is caused by nerve impulses arising in the motor area of the cortex of the brain and carried by cranial nerves or by nerves that emerge from the spinal cord to connect with skeletal muscles. The reaction involves both excitation of nerve cells stimulating the muscles involved and inhibition of the cells that stimulate opposing muscles. A nerve impulse is an electrical change within a nerve cell or fiber; Measured in millivolts, it lasts a few milliseconds and can be recorded by electrodes.
The human brain has three major structural components: The large dome-shaped cerebrum, the smaller somewhat spherical cerebellum, and the brainstem. Prominent in the brainstem is the medulla oblongata (the egg-shaped enlargement at the center) and the thalamus (between the medulla and the cerebrum). The cerebrum is responsible for intelligence and reasoning. The cerebellum helps to maintain balance and posture. The medulla is involved in maintaining involuntary functions such as respiration, and the thalamus act as a relay center for electrical impulses traveling to and from the cerebral cortex. Lack of blood flow to any part of the brain results in a stroke, permanent damage that interferes with the functions of the affected part of the brain.
Movement may occur also in direct response to an outside stimulus, thus, a tap on the knee causes a jerk, and a light shone into the eye makes the pupil contract. These involuntary responses are called reflexes. Various nerve terminals called receptors constantly send impulses into the central nervous system. These are of three classes: exteroceptors, which are sensitive to pain, temperature, touch, and pressure; interoceptors, which react to changes in the internal environment; and proprioceptors, which respond to variations in movement, position, and tension. These impulses terminate in special areas of the brain, as do of those special receptors concerned with sight, hearing, smell, and taste.
Whereas most major nerves emerge from the spinal cord, the 12 pairs of cranial nerves project directly from the brain. All but 1 pair relay motor or sensory information (or both); the tenth, or vagus nerve, affects visceral functions such as heart rate, vasoconstriction, and contraction of the smooth muscle found in the walls of the trachea, stomach, and intestine.
Muscular contractions do not always cause actual movement. A small fraction of the total number of fibers in most muscles is usually contracting. This serves to maintain the posture of a limb and enables the limb to resist passive elongation or stretch. This slight continuous contraction is called muscle tone.
In 1946 Axelrod joined the laboratory of American pharmacologist Bernard Brodie at Goldwater Memorial Hospital in New York. The pair conducted research on pain-relieving drugs called analgesics. They identified a pain-relieving chemical known as acetaminophen. This drug was later developed and marketed by the drug company Johnson & Johnson under the brand-name Tylenol.
In 1949 Axelrod took a position at the National Heart Institute, a branch of the National Institutes of Health (NIH). Their Axelrod studied how the body processes certain drugs that cause behavioral changes, including amphetamines, ephedrine, and mescaline. He identified a group of enzymes that help these drugs break down in the body. These enzymes, called cytochrome-P450 monoxygenases, have been studied extensively by other scientists, particularly in cancer research.
Realizing that career advancement in the sciences requires a doctoral degree, in 1954 Axelrod took a leave of absence from his job at the National Heart Institute to attend The George Washington University. He earned his doctorate in pharmacology in 1955. That same year he was named chief of pharmacology at the National Institute of Mental Health (NIMH) another branch of NIH.
At NIMH, Joseph Axelrod began research on neurotransmitters. A nerve cell releases a neurotransmitter to spur a neighboring cell into action. In the 1950s most scientists believed that a neurotransmitter became inactive once it stimulated a neighboring cell. But Axelrod’s research found that the neurotransmitter returns to the first nerve cell, in a process known as reuptake, where it is broken down by enzymes or repackaged for reuse. This research led to the creation of a number of drugs that prevent the reuptake process, enabling a neurotransmitter to remain active for a longer period of time.
Axelrod’s research revolutionized the understanding of many mental-health disorders, including depression, anxiety, and schizophrenia. Prior to his research, psychiatry focused on the relationship of life experiences to mental health problems. But Axelrod's research proved that mental-health disorders were often the result of complicated brain chemistry. His research spurred the development of new drugs that advanced the treatment of mental-health conditions. Among these are selective serotonin reuptake inhibitors, including the antidepressants fluoxetine, sold under the brand name Prozac, sertraline(Zoloft) and paroxetine (Paxil).
The study of the biochemistry of memory is another exciting scientific enterprise, but one that can only be touched upon here. Scientists estimate that an adult human brain contains about 100 billion neurons. Each of these is connected to hundreds or thousands of other neurons, forming trillions of neural connections. Neurons communicate by chemical messengers called neurotransmitters. An electrical signal travels along the neuron, triggering the release of neurotransmitters at the synapse, the small gap between neurons. The neurotransmitters travel across the synapse and act on the next neuron by binding with protein molecules called receptors. Most scientists believe that memories are somehow stored among the brain's trillions of synapses, rather than in the neurons themselves.
Scientists who study the biochemistry of learning and memory often focus on the marine snail Aplysia because its simple nervous system allows them to study the effects of various stimuli on specific synapses. A change in the snail's behavior due to learning can be correlated with a change at the level of the synapse. One exciting scientific frontier is discovering the changes in neurotransmitters that occur at the level of the synapse.
Other researchers have implicated glucose, a sugar and insulin(a hormone secreted by the pancreas) as important to learning and memory. Humans and other animals given these substances show an improved capacity to learn and remember. Typically, when animals or humans ingest glucose, the pancreas responds by increasing insulin production, so it is difficult to determine which substance contributes to improved performance. Some studies in humans that have systematically varied the amount of glucose and insulin in the blood have shown that insulin may be the more important of the two substances for learning.
Scientists also have examined the influence of genes on learning and memory. In one study, scientists bred strains of mice with extra copies of a gene that helps build a protein called N-methyl-D-aspartate, or NMDA. This protein acts as a receptor for certain neurotransmitters. The genetically altered mice outperformed normal mice on a variety of tests of learning and memory. In addition, other studies have found that chemically blocking NMDA receptor impairs learning in laboratory rats. Future discoveries from genetic and biochemical studies may lead to treatments for memory deficits from Alzheimer's disease and other conditions that affect memory.
Alzheimer's Disease, progressive brain disorders that causes a gradual and irreversible decline in memory, language skills, perception of time and space, and, eventually, the ability to care for oneself. First described by German psychiatrist Alois Alzheimer in 1906, Alzheimer's disease was initially thought to be a rare condition affecting only young people, and was referred to as prehensile dementia. Today late-onset Alzheimer's disease is recognized as the most common cause of the loss of mental function in those aged 65 and over. Alzheimer's in people in their 30s, 40s, and 50s, called early-onset Alzheimer's disease, occurs less frequently, accountings for less than 10 percent of the estimated 4 million Alzheimer's cases in the United States.
Although Alzheimer's disease is not a normal part of the aging process, the risk of developing the disease increases as people grow older. About 10 percent of the United States population over the age of 65 is affected by Alzheimer's disease, and nearly 50 percent of those over age 85 may have the disease.
Alzheimer's disease takes a devastating toll, not only on the patients, but also on those who love and care for them. Some patients experience immense fear and frustration as they struggle with once commonplace tasks and slowly lose their independence. Family, friends, and especially those who provide daily care suffer immeasurable pain and stress as they witness Alzheimer's disease slowly take their loved one from them.
The onset of Alzheimer's disease is usually very gradual. In the early stages, Alzheimer's patients have relatively mild problems learning new information and remembering where they have left common objects, such as keys or a wallet. In time, they begin to have trouble recollecting recent events and finding the right words to express themselves. As the disease progresses, patients may have difficulty remembering what day or month it is, or finding their way around familiar surroundings. They may develop a tendency to wander off and then be unable to find their way back. Patients often become irritable or withdrawn as they struggle with fear and frustration when once commonplace tasks become unfamiliar and intimidating. Behavioral changes may become more pronounced as patients become paranoid or delusional and unable to engage in normal conversation.
Eventually Alzheimer's patients become completely incapacitated and unable to take care of their most basic life functions, such as eating and using the bathroom. Alzheimer's patients may live many years with the disease, usually dying from other disorders that may develop, such as pneumonia. Typically the time from initial diagnosis until death is seven to ten years, but this is quite variable and can range from three to twenty years, depending on the age of the onset, other medical conditions present, and the care patients receive.
The brains of patients with Alzheimer's have distinctive formations - abnormally shaped proteins called tangles and plaques - that are recognized as the hallmark of the disease. Not all brain regions show these characteristic formations. The areas most prominently affected are those related to memory.
Tangles are long, slender tendrils found inside nerve cells, or neurons. Scientists have learned that when a protein-called tau becomes altered, it may cause the characteristic tangles in the brain of the Alzheimer’s patient. In healthy brains provides structural support for neurons, but in Alzheimer's patients this structural support collapses.
Plaques, or clumps of fibers, form outside the neurons in the adjacent brain tissue. Scientists found that a type of protein, called amyloid precursor protein, forms toxic plaques when it is cut in two places. Researchers have isolated the enzyme beta-secretes, which is believed to make one of the cuts in the amyloid precursor protein. Researchers also identified another enzyme, called gamma secretes, that makes the second cut in the amyloid precursor protein. These two enzymes snip the amyloid precursor protein into fragments that then accumulate to form plaques that are toxic to neurons.
Scientists have found that tangles and plaques cause neurons in the brains of Alzheimer's patients to shrink and eventually die, first in the memory and language centers and finally throughout the brain. This widespread neuron degeneration leaves gaps in the brain's messaging network that may interfere with communication between cells, causing some of the symptoms of Alzheimer’s disease.
Alzheimer's patients have lower levels of neurotransmitters, chemicals that carry complex messages back and forth between the nerve cells. For instance, Alzheimer's disease seems to decrease the level of the neurotransmitter acetylcholine, which is known to influence memory. A deficiency in other neurotransmitters, including somatostatin and corticotropin-releasing factor, and, particularly in younger patients, serotonin and norepinephrine, also interferes with normal communication between brain cells.
The causes of Alzheimer's disease remain a mystery, but researchers have found that particular groups of people have risk factors that make them more likely to develop the disease than the general population. For example, people with a family history of Alzheimer's are more likely to develop Alzheimer's disease.
Some of the most promising Alzheimer's research is being conducted in the field of genetics to learn the role a family history of the disease has in its development. Scientists have learned that people who are carriers of a specific version of the apolipoprotein E gene (apoE genes), found on chromosome 19, are several times more likely to develop Alzheimer's than carriers of other versions of the apoE gene. The most common version of this gene in the general population is apoE3. Nearly half of all late-onset Alzheimer’s patients have the fewer in common apoE4 versions, however, and research has shown that this gene plays a role in Alzheimer's disease. Scientists have also found evidence that variations in one or more genes located on chromosomes 1, 10, and 14 may increase a person’s risk for Alzheimer's disease. Scientists have identified the gene variations on chromosomes 1 and 14 and learned that these genes produce mutations in proteins called presenilins. These mutated proteins apparently trigger the activity of the enzyme gamma secretase, which splices the amyloid precursor protein.
Researchers have made similar strides in the investigation of early-onset Alzheimer's disease. A series of genetic mutations in patients with early-onset Alzheimer's has been linked to the production of amyloid precursor protein, the protein in plaques that may be implicated in the destruction of neurons. One mutation is particularly interesting to geneticists because it occurs on a gene involved in the genetic disorder Down syndrome. People with Down syndrome usually develop plaques and tangles in their brains as they get older, and researchers believe that learning more about the similarities between Down syndrome and Alzheimer's may further our understanding of the genetic elements of the disease.
Some studies suggest that one or more factors other than heredity may determine whether people develop the disease. One study published in February 2001 compared residents of Ibadan, Nigeria, who eat a mostly low-fat vegetarian diet, with African Americans living in Indianapolis, Indiana, whose diet included a variety of high-fat foods. The Nigerians were less likely to develop Alzheimer’s disease compared to their U.S. counterparts. Some researchers suspect that health imposes on high blood pressure, atherosclerosis (arteries clogged by fatty deposits), high cholesterol levels, or other cardiovascular problems may play a role in the development of the disease.
Other studies have suggested that environmental agents may be a possible cause of Alzheimer's disease; for example, one study suggested that high levels of aluminum in the brain may be a risk factor. Several scientists initiated research projects to further investigate this connection, but no conclusive evidence has been found linking aluminum with Alzheimer's disease. Similarly, investigations into other potential environmental causes, such as zinc exposure, viral agents, and food-borne poisons, while initially promising, have generally turned up inconclusive results.
Some studies indicate that brain trauma can trigger a degenerative process that results in Alzheimer's disease. In one study, an analysis of the medical records scribed upon veterans of World War II (1939-1945) linked serious head injury in early adulthood with Alzheimer's disease in later life. The study also looked at other factors that could possibly influence the development of the disease among the veterans, such as the presence of the apoE gene, but no other factors were identified.
Alzheimer’s disease is only positively diagnosed by examining brain tissue under a microscope to see the hallmark plaques and tangles, and this is only possible after a patient dies. As a result, physicians rely on a series of other techniques to diagnose probable Alzheimer's disease in living patients. Diagnosis begins by ruling out other problems that cause memory loss, such as stroke, depression, alcoholism, and the use of certain prescription drugs. The patient undergoes a thorough examination, including specialized brain scans, to eliminate other disorders. The patient may be given a detailed evaluation called a neuropsychological examination, which is designed to evaluate a patient’s ability to perform specific mental tasks. This helps the physician determine whether the patient is showing the characteristic symptoms of Alzheimer's disease - progressively worsening memory problems, language difficulties, and trouble with spatial direction and time. The physician also asks about the patient's family medical history to learn about any past serious illnesses, which may give a hint about the patient's current symptoms.
Evidence shows that there is inflammation in the brains of Alzheimer's patients, which may be associated with the production of amyloid precursor protein. Studies are underway to find drugs that prevent this inflammation, to possibly slow or even halt the progress of the disease. Other promising approaches center on mechanisms that manipulate amyloid precursor protein production or accumulation. Drugs are in development that may block the activity of the enzymes that cut the amyloid precursor protein, halting amyloid production. Other studies in mice suggest those vaccinating animals with amyloid precursor protein can produce a reaction that clears amyloid precursor protein from the brain. Physicians have started vaccination studies in humans to determine if the same potentially beneficial effects can be obtained. There is still much to be learned, but as scientists better understand the genetic components of Alzheimer’s, the roles of the amyloid precursor protein and the tau protein in the disease, and the mechanisms of nerve cell degeneration, the possibility that a treatment will be developed is more likely.
The responsibility for caring for Alzheimer's patients generally falls on their spouses and children. Care givers must constantly be on guard for the possibility of Alzheimer's patients wandering away or becoming agitated or confused in a manner that jeopardizes the patient or others. Coping with a loved one's decline and inability to recognize familiar face causes enormous pain.
The increased burden faced by families is intense, and the life of the Alzheimer's care giver is often called a 36-hour day. Not surprisingly, care givers often develop health and psychological problems of their own as a result of this stress. The Alzheimer's Association, a national organization with local chapters throughout the United States, was formed in 1980 in large measure to provide support for Alzheimer's care givers. Today, national and local chapters are a valuable source for information, referral, and advice.
Not to long ago, most approaches to the philosophy of science were ‘cognitive’. This includes ‘logical positivism’, as nearly all of those who wrote about the nature of science would have been in agreement that science ought to be ‘value-free’. This had been a particular emphasis on the part of the first positivist, as it would be upon twentieth-century successors. Science, so it was said, deals with ‘facts’, and facts and values and irreducibly distinct. Facts are objective, they are what we seek in our knowledge of the world. Values are subjective: They bear the mark of human interest, they are the radically individual products of feeling and desire. Fact and value cannot, therefore, be inferred from fact, fact ought not be influenced by value. There were philosophers, notably some in the Kantian tradition, who viewed the relation of the human individual to the universalist aspiration of difference rather differently. But the legacy of three centuries of largely empiricist reflection of the ‘new’ sciences ushered in by Galilee Galileo (1564-1642), the Italian scientist whose distinction belongs to the history of physics and astronomy, rather than natural philosophy.
The philosophical importance of Galileo’s science rests largely upon the following closely related achievements: (1) His stunning successful arguments against Aristotelean science, (2) his proofs that mathematics is applicable to the real world. (3) his conceptually powerful use of experiments, both actual and employed regulatively, (4) his treatment of causality, replacing appeal to hypothesized natural ends with a quest for efficient causes, and (5) his unwavering confidence in the new style of theorizing that would come to be known as mechanical explanation.
A century later, the maxim that scientific knowledge is ‘value-laded’ seems almost as entrenched as its opposite was earlier. It is supposed that between fact and value has been breached, and philosophers of science seem quite at home with the thought that science and value may be closely intertwined after all. What has happened to bring about such an apparently radial change? What are its implications for the objectivity of science, the prized characteristic that, from Plato’s time onwards, has been assumed to set off real knowledge (epistēmē) from mere opinion (doxa)? To answer these questions adequately, one would first have to know something of the reasons behind the decline of logical positivism, as, well as of the diversity of the philosophies of science that have succeeded it.
More general, the interdisciplinary field of cognitive science is burgeoning on several fronts. Contemporary philosophical reelection about the mind - which has been quite intensive - has been influenced by this empirical inquiry, to the extent that the boundary lines between them are blurred in places.
Nonetheless, the philosophy of mind at its core remains a branch of metaphysics, traditionally conceived. Philosophers continue to debate foundational issues in terms not radically different from those in vogue in previous eras. Many issues in the metaphysics of science hinge on the notion of ‘causation’. This notion is as important in science as it is in everyday thinking, and much scientific theorizing is concerned specifically to identify the ‘causes’ of various phenomena. However, there is little philosophical agreement on what it is to say that one event is the cause of some other.
Modern discussion of causation starts with the Scottish philosopher, historian, and essayist David Hume (1711-76),who argued that causation is simply a matter for which he denies that we have innate ideas, that the causal relation is observably anything other than ‘constant conjunction’ wherefore, that there are observable necessary connections anywhere, and that there is either an empirical or demonstrative proof for the assumptions that the future will resemble the past, and that every event has a cause. That is to say, that there is an irresolvable dispute between advocates of free-will and determinism, that extreme scepticism is coherent and that we can find the experiential source of our ideas of self-substance or God.
According to Hume (1978), on event causes another if only if events of the type to which the first event belongs regularly occur in conjunctive events of the type to which the second event belongs. The formulation, however, leaves a number of questions open. Firstly, there is a problem of distinguishing genuine ‘causal law’ from ‘accidental regularities’. Not all regularities are sufficient lawlike to underpin causal relationships. Being a screw in my desk could well be constantly conjoined with being made of copper, without its being true that these screws are made of copper because they are in my desk. Secondly, the idea of constant conjunction does not give a ‘direction’ to causation. Causes need to be distinguished from effects. But knowing that A-type events are constantly conjoined with B-type events does not tell us which of ‘A’ and ‘B’ is the cause and which the effect, since constant conjunction is itself a symmetric relation. Thirdly, there is a problem about ‘probabilistic causation’. When we say that causes and effects are constantly conjoined, do we mean that the effects are always found with the causes, or is it enough that the causes make the effect probable?
Many philosophers of science during the past century have preferred to talk about ‘explanation’ than causation. According to the covering-law model of explanation, something is explained if it can be deduced from premises which include one or more laws. As applied to the explanation of particular events this implies that one particular event can be explained it if is linked by a law to some other particular event. However, while they are often treated as separate theories, the covering-law account of explanation is at bottom little more than a variant of Hume’s constant conjunction account of causation. This affinity shows up in the fact at the covering-law account faces essentially the same difficulties as Hume: (1) In appealing to deduction from ‘laws’, it needs to explain the difference between genuine laws and accidentally true regularities: (2) It omits by effects, as swell as effects by causes, after all, it is as easy to deduce the height of flag-pole from the length of its shadow and the law of optics: (3) Are the laws invoked in explanation required to be exceptionalness and deterministic, or is it acceptable, say, to appeal to the merely probabilistic fact that smoking makes cancer more likely, in explaining why some particular person develops cancer?
Nevertheless, one of the centrally obtainable achievements for which the philosophy of science is to provide explicit and systematic accounts of the theories and explanatory strategies exploited in the science. Another common goal is to construct philosophically illuminating analyses or explanations of central theoretical concepts invoked in one or another science. In the philosophy of biology, for example, there is a rich literature aimed at understanding teleological explanations, and there has been a great deal of work on the structure of evolutionary theory and on such crucial concepts as fitness and biological function. By introducing ‘teleological considerations’, this account views beliefs as states with biological purpose and analyses their truth conditions specifically as those conditions that they are biologically supposed to covary with.
A teleological theory of representation needs to be supplemental with a philosophical account of biological representation generally a selectionism account of biological purpose, according to which item ‘F’ has purpose ‘G’ if and only if it is now present as a result of past selection by some process which favoured item with ‘G’. So, a given belief type will have the purpose of covarying with ‘P’, say. If and only if some mechanism has selected it because it has covaried with ‘P’ the past.
Along the same lines, teleological theory holds that ‘r’ represents ‘x’ if it is r’s function to indicate (i.e., covary with) ‘x’, teleological theories differ depending on the theory of functions they import. Perhaps the most important distinction is that between historical theories of functions and a-historical theories. Historical theories individuate functional states (hence, contents) in a way that is sensitive to the historical development of the state, i.e., to factors such as the way the state was ‘learned’, or the way it evolved. An historical theory might hold that the function of ‘r’ is to indicate ‘x’ only if the capacity to token ‘r’ was developed (selected, learned) because it indicates ‘x’. thus, a state physically indistinguishable from ‘r’ (physical states being a-historical) but lacking r’s historical origins would not represent ‘x’ according to historical theories.
The American philosopher of mind (1935-) Fodor, is known for a resolute ‘realism’ about the nature of mental functioning, taking the analogy between thought and computation seriously. Fodor believes that mental representations should be conceived as individual states with their own identities and structures, like formulae transformed by processes of computation or thought. His views are frequently contrasted with those of ‘holist s’ such as the American philosopher Herbert Donald Davidson (1917-2003), or ‘instrumentalists about mental ascription, such as the British philosopher of logic and language, Eardley Anthony Michael Dummett (1925-). In recent years he has become a vocal critic of some of the aspirations of cognitive science.
Nonetheless, a suggestion extrapolating the solution of teleology is continually queried by points as owing to ‘causation’ and ‘content’, and ultimately a fundamental appreciation is to be considered, is that: We suppose that there’s a causal path from A’s to ‘A’s’ and a causal path from B’s to ‘A’s’, and our problem is to find some difference between B-caused ‘A’s’ and A-caused ‘A’s’ in virtue of which the former but not the latter misrepresented. Perhaps, the two paths differ in their counter-factual properties. In particular, though A’s and B’s both cause ‘A’s’ as a matter of fact, perhaps can assume that only A’s would cause ‘A’s’ in - as one can say - ,‘optimal circumstances’. We could then hold that a symbol expresses its ‘optimal property’, viz., the property that would causally control its tokening in optimal circumstances. Correspondingly, when the tokening of a symbol is causally controlled by properties other than its optimal property, the tokens that eventuate are ipso facto wild.
Suppose at the present time, that this story about ‘optimal circumstances’ is proposed as part of a naturalized semantics for mental representations. In which case it is, of course, essential that it be possible to say that the optimal circumstances for tokening a mental representation are in terms that are not themselves either semantical nor intentional. (It would not do, for example, to identify the optimal circumstances for tokening a symbol as those in which the tokens are true, that would be to assume precisely the sort of semantical notions that the theory is supposed to naturalize.) Befittingly, the suggestion - to put it in a nutshell - is that appeals to ‘optimality’ should be buttressed by appeals to ‘teleology’: Optimal circumstances are the ones in which the mechanisms that mediate symbol tokening are functioning ‘as they are supposed to’. In the case of mental representations, these would be paradigmatically circumstances where the mechanisms of belief fixation are functioning as they are supposed to.
So, then: The teleology o the cognitive mechanisms determines the optimal condition for belief fixation, and the optimal condition for belief fixation determines the content of beliefs. So the story goes.
To put this objection in slightly other words: The teleology story perhaps strikes one as plausible in that it understands one normative notion - truth - in terms of another normative notion - optimality. But this appearance e of fit is spurious there is no guarantee that the kind of optimality that teleology reconstructs has much to do with the kind of optimality that the explication of ‘truth’ requires. When mechanisms of repression are working ‘optimally’ - when they’re working ‘as they’re supposed to’ - what they deliver are likely to be ‘falsehoods’.
Or again: There’s no obvious reason why coitions that are optimal for the tokening of one sort of mental symbol need be optimal for the tokening of other sorts. Perhaps the optimal conditions for fixing beliefs about very large objects, are different from the optimal conditions for fixing beliefs about very small ones, are different from the optimal conditions for fixing beliefs sights. But this raises the possibility that if we’re to say which conditions are optimal for the fixation of a belief, we’ll have to know what the content of the belief is - what it’s a belief about. Our explication of content would then require a notion of optimality, whose explication in turn requires a notion of content, and the resulting pile would clearly be unstable.
Teleological theories hold that ‘r’ represents ‘x’ if it is r’s function to indicate (i.e., covary with) ‘x’. Teleological theories differ, depending on the theory of functions they import. Perhaps the most important distinction is that between historical theories of functions: Historically, theories individuate functional states (hence, contents) in a way that is sensitive to the historical development of the state, i.e., to factors such as the way the state was ‘learned’, or the way it evolved. An historical theory might hold that the function of ‘r’ is to indicates ’x’ only if the capacity to token ‘r’ was developed (selected, learned) because it indicates ‘x’. Thus, a state physically indistinguishable from ‘r’ (physical states being a-historical), but lacking r’s historical origins would not represent ‘x’ according to historical theories.
Just as functional role theories hold that r’s representing ‘x’ is grounded in the functional role ‘r’ has in the representing system, i.e., on the relations imposed by specified cognitive processes between ‘r’ and other representations in the system’s repertoire. Functional role theories take their cue from such common-sense ideas as that people cannot believe that cats are furry if they do not know that cats are animals or that fur is like hair.
That being said, that nowhere is the new period of collaboration between philosophy and other disciplines more evident than in the new subject of cognitive science. Cognitive science from its very beginning has been ‘interdisciplinary’ in character, and is in effect the joint property of psychology, linguistics, philosophy, computer science and anthropology. There is, therefore, a great variety of different research projects within cognitive science, but the central area of cognitive science, its hard-coded ideology rests on the assumption that the mind is best viewed as analogous to a digital computer. The basic idea behind cognitive science is that recent developments in computer science and artificial intelligence have enormous importance for our conception of human beings. The basic inspiration for cognitive science went something like this: Human beings do information processing. Computers are designed precisely do information processing. Therefore, one way to study human cognition - perhaps the best way to study it - is to study. It as a matter of computational information processing. Some cognitive scientists think that the computer is just a metaphor for the human mind: Others think that the mind is literally a computer program. But it is fair to say, that without the computational model there would not have been a cognitive science as we now understand it.
In, Essay Concerning Human Understanding is the first modern systematic presentation of an empiricist epistemology, and as such had important implications for the natural sciences and for philosophy of science generally. Like his predecessor, Descartes, the English philosopher (1632-104) John Locke began his account of knowledge from the conscious mind aware of ideas. Unlike Descartes, however, he was concerned not to build a system based on certainty, but to identify the mind’s scope and limits. The premise upon which Locke built his account, including his account of the natural sciences, is that the ideas which furnish the mind are all derived from experience. He thus, totally rejected any kind of innate knowledge. In this he consciously opposing Descartes, who had argued that it is possible to come to knowledge of fundamental truths about the natural world through reason alone. Descartes (1596-1650) had argued, that we can come to know the essential nature of both ‘mind’ and ‘matter’ by pure reason. John Locke accepted Descartes’s criterion of clear and distinct ideas as the basis for knowledge, but denied any source for them other than experience. It was information that came in via the five senses (ideas of sensation) and ideas engendered from pure inner experiences (ideas of reflection) came the building blocks of the understanding.
Locke combined his commitment to ‘the new way of ideas’ with a te native espousal of the ‘corpuscular philosophy’ of the Irish scientist (1627-92) Robert Boyle. This, in essence, was an acceptance of a revised, more sophisticated account of matter and its properties that had been advocated by the ancient atomists and recently supported by Galileo (1564-1642) and Pierre Gassendi (1592-1655). Boyle argued from theory and experiment that there were powerful reasons to justify some kind of corpuscular account of matter and its properties. He called the latter qualities, which he distinguished as primary and secondary - the distinction between primary and secondary qualities may be reached by two rather different routes: Either from the nature or essence of matter or from the nature and essence of experience, though practising these have tended to run together. The former considerations make the distinction seem like an a priori, or necessary, truth about the nature of matter, while the latter make it appears to be an empirical hypothesis -. Locke, too, accepted this account, arguing that the ideas we have of the primary qualities of bodies resemble those qualities as they are in the subject, whereas the ideas of the secondary qualities, such as colour, taste, and smell, do not resemble their causes in the object.
There is no strong connection between acceptance of the primary-secondary quality distinction and Locke’s empiricism and Descartes had also argued strongly for universal acceptance by natural philosophers, and Locke embraced it within his more comprehensive empirical philosophy. But Locke’ empiricism did have major implications for the natural sciences, as he well realized. His account begins with an analysis of experience. all ideas, he argues, are either simple or complex. Simple ideas are those like the red of a particular rose or the roundness of a snowball. Complex ideas, our ideas of the rose or the snowball, are combinations of simple ideas. We may create new complex ideas in our imagination - a dragon, for example. But simple ideas can never be created by us: We just have them or not, and characteristically they are caused, for example, the impact on our senses of rays of light or vibrations of sound in the air coming from a particular physical object. Since we cannot create simple ideas, and they are determined by our experience. Our knowledge is in a very strict uncompromising way limited. Besides, our experiences are always of the particular, never of the general. It is this particular simple idea or that particular complex idea that we apprehend. We never in that sense apprehend a universal truth about the natural world, but only particular instances. It follows from this that all claims to generality about that world - for example, all claims to identity what were then beginning to be called the laws of nature - must to that extent go beyond our experience and thus be less than certain.
The Scottish philosopher, historian, and essayist, (1711-76) David Hume, whose famous discussion appears in both his major philosophical works, the ‘Treatise’ (1739) and the ‘Enquiry’(1777). The distinction is couched in terms of the concept of causality, so that where we are accustomed to talk of laws, Hume contends, involves three ideas:
1. That there should be a regular concomitance between events of the type of the cause and those of the type of the effect.
2. That the cause event should be contiguous with the effect event.
3. That the cause event should necessitate the effect event.
The tenets (1) and (2) occasion no differently for Hume, since he believes that there are patterns of sensory impressions un-problematically related to the idea of regularity concomitance and of contiguity. But the third requirement is deeply problematic, in that the idea of necessarily that figures in it seems to have no sensory impression correlated with it. However, carefully and attentively we scrutinize a causal process, we do not seem to observe anything that might be the observed correlates of the idea of necessity. We do not observe any kind of activity, power, or necessitation. All we ever observe is one event following another, which is logically independent of it. Nor is this necessity logical, since, as, Hume observes, one can jointly assert the existence of the cause and a denial of the existence of the effect, as specified in the causal statement or the law of nature, without contradiction. What, then, are we to make of the seemingly central notion of necessity that is deeply embedded in the very idea of causation, or lawfulness? To this query, Hume gives an ingenious and telling story. There is an impression corresponding to the idea of causal necessity, but it is a psychological phenomenon: Our exception that an even similar to those we have already observed to be correlated with the cause-type of events will come to be in this cas e too. Where does that impression come from? It is created as a kind of mental habit by the repeated experience of regular concomitance between events of the type of the effect and the occurring of event s of the type of the cause. And then, the impression that corresponds to the idea of regular concomitance - the law of nature then asserts nothing but the existence of the regular concomitance.
At this point in our narrative, the question at once arises as to whether this factor of life in nature, thus interpreted, corresponds to anything that we observe in nature. All philosophy is an endeavour to obtain a self-consistent understanding of things observed. Thus, its development is guided in two ways, one is demand for coherent self-consistency, and the other is the elucidation of things observed. With our direct observations how are we to conduct such comparison? Should we turn to science? No. There is no way in which the scientific endeavour can detect the aliveness of things: Its methodology rules out the possibility of such a finding. On this point, the English mathematician and philosopher (1861-1947) Alfred Whitehead, comments: That science can find no individual enjoyment in nature, as science can find no creativity in nature, it finds mere rules of succession. These negations are true of natural science. They are inherent in its methodology. The reason for this blindness of physical science lies in the fact that such science only deals with half the evidence provided by human experience. It divides the seamless coat - or, to change the metaphor into a happier form, it examines the coat, which is superficial, and neglects the body which is fundamental.
Whitehead claims that the methodology of science makes it blind to a fundamental aspect of reality, namely, the primacy of experience, it neglected half of the evidence. Working within Descartes’ dualistic framework of matter and mind as separate and incommensurate, science limits itself to the study of objectivised phenomena, neglecting the subject and the mental events that are his or her experience.
Both the adoption of the Cartesian paradigm and the neglect of mental events are reason enough to suspect ‘blindness’, but there is no need to rely on suspicions. This blindness is clearly evident. Scientific discoveries, impressive as they are, are fundamentally superficial. Science can express regularities observed in nature, but it cannot explain the reasons for their occurrence. Consider, for example, Newton’s law of gravity. It shows that such apparently disparate phenomena as the falling of an apple and the revolution of the earth around the sun are aspects of the same regularity - gravity. According to this law the gravitational attraction between two objects deceases in proportion to the square of the distance between them. Why is that so? Newton could not provide an answer. Simpler still, why does space have three dimensions? Why is time one-dimensional? Whitehead notes, ‘None of these laws of nature gives the slightest evidence of necessity. They are [merely] the modes of procedure which within the scale of observation do in fact prevail’.
This analysis reveals that the capacity of science to fathom the depths of reality is limited. For example, if reality is, in fact, made up of discrete units, and these units have the fundamental character in being ’throbs of experience’, then science may be in a position to discover the discreteness: But it has no access to the subjective side of nature, since, as the Austrian physicist(1887-1961) Erin Schrödinger points out, we ‘exclude the subject of cognizance from the domain of nature that we endeavour to understand’. It follows that in order to find ‘the elucidation of things observed’ in relation to the experiential or aliveness aspect, we cannot rely on science, we need to look elsewhere.
If, instead of relying on science, we rely on our immediate observation of nature and of ourselves, we find, first, that this [i.e., Descartes’] stark division between mentality and nature has no ground in our fundamental observation. We find ourselves living within nature. Secondly, in that we should conceive mental operations as among the factors which make up the constitution of nature, and thirdly, that we should reject the notion of idle wheels in the process of nature. Every factor which makes a difference, and that difference can only be expressed in terms of the individual character of that factor.
Whitehead proceeds to analyse our experiences in general, and our observations of nature in particular, and ends up with ‘mutual immanence’ as a central theme. This mutual immanence is obvious in the case of an experience, that, I am a part of the universe, and, since I experience the universe, the experienced universe is part of me. Whitehead gives an example’ ‘I am in the room, and the room is an item in my present experience. But my present experience is what I am now’. A generalization of this relationship to the case of any actual occasions yields the conclusion that ‘the world is included within the occasion in one sense, and the occasion is included in the world in another sense’. The idea that each actual occasion appropriates its universe follows naturally from such considerations.
The description of an actual entity as being a distinct unit is, therefore, only one part of the story. The other, complementary part is this: The very nature of each and every actual entity is one of interdependence with all the other actual entities in the universe. Each and every actual entity is a process of prehending or appropriating all the other actual entities and creating one new entity out of them all, namely, itself.
There are two general strategies for distinguishing laws from accidentally true generalizations. The first stands by Hume’s idea that causal connections are mere constant conjunctions, and then seeks to explain why some constant conjunctions are better than others. That is, this first strategy accepts the principle that causation involves nothing more than certain events always happening together with certain others, and then seeks to explain why some such patterns - the ‘laws’ - matter more than others - the ‘accidents’ -. The second strategy, by contrast, rejects the Humean presupposition that causation involves nothing more than happen-stantial co-occurrence, and instead postulates a relationship ‘necessitation’, a kind of ‘cement, which links events that are connected by law, but not those events (like being a screw in my desk ad being made of copper) that are only accidentally conjoined.
There are a number of versions of the first Human strategy. The most successful, originally proposed by the Cambridge mathematician and philosopher F.P. Ramsey (1903-30), and later revived by the American philosopher David Lewis (1941-2002), who holds that laws are those true generalizations that can be fitted into an ideal system of knowledge. The thought is, that, the laws are those patterns that are somewhat explicated in terms of basic science, either as fundamental principles themselves, or as consequences of those principles, while accidents, although true, have no such explanation. Thus, ‘All water at standard pressure boils at 1000 C’ is a consequence of the laws governing molecular bonding: But the fact that ‘All the screws in my desk are copper’ is not part of the deductive structure of any satisfactory science. Ramsey neatly encapsulated this idea by saying that laws are ‘consequences of those proposition which we should take as axioms if we knew everything and organized it as simply as possible in a deductive system’.
Advocates of the alternative non-Humean strategy object that the difference between laws and accidents is not a ‘linguistic’ matter of deductive systematization, but rather a ‘metaphysical’ contrast between the kind of links they report. They argue that there is a link in nature between being at 1000 C and boiling, but not between being ‘in my desk’ and being ‘made of copper’, and that this is nothing to do with how the description of this link may fit into theories. According to D.M. Armstrong (1983), the most prominent defender of this view, the real difference between laws and accidentals, is simply that laws report relationships of natural ‘necessitation’, while accidents only report that two types of events happen to occur together.
Armstrong’s view may seem intuitively plausible, but it is arguable that the notion of necessitation simply restates the problem, than solving it. Armstrong says that necessitation involves something more than constant conjunction: If two events e related by necessitation, then it follows that they are constantly conjoined, but two events can be constantly conjoined without being related by necessitation, as when the constant conjunction is just a matter of accident. So necessitation is a stronger relationships than constant conjunction. However, Armstrong and other defenders of this view say ver y little about what this extra strength amounts to, except that it distinguishes laws from accidents. Armstrong’s critics argue that a satisfactory account of laws ought to cast more light than this on the nature of laws.
Hume said that the earlier of two causally related events is always the cause, and the later effect. However, there are a number of objections to using the earlier-later ‘arow of time’ to analyse the directional ‘arrow of causation’. For a start, it seems in principle, possible that some causes and effects could be simultaneous. That more, in the idea that time is directed from ‘earlier’ to ‘later’ itself stands in need of philosophical explanation - and one of the most popular explanations is that the idea of ‘movement’ from earlier to later depends on the fact that cause-effect pairs always have a time, and explain ‘earlier’ as the direction in which causes lie, and ‘later’ as the direction of effects, that we will clearly need to find some account of the direction of causation which does not itself assume the direction of time.
A number of such accounts have been proposed. David Lewis (1979) has argued that the asymmetry of causation derives from an ‘asymmetry of over-determination’. The over-determination of present events by past events - consider a person who dies after simultaneously being shot and struck by lightning - is a very rare occurrence, by contrast, the multiple ‘over-determination’ of present events by future events is absolutely normal. This is because the future, unlike the past, will always contain multiple traces of any present event. To use Lewis’s example, when the president presses the red button in the White House, the future effects do not only include the dispatch of nuclear missiles, but also the fingerprint on the button, his trembling, the further depletion of his gin bottle, the recording of the button’s click on tape, he emission of light waves bearing the image of his action through the window, the warnings of the wave from the passage often signal current, and so on, and so on, and on.
Lewis relates this asymmetry of over-determination to the asymmetry of causation as follows. If we suppose the cause of a given effect to have been absent, then this implies the effect would have been absent too, since (apart from freaks like the lightning-shooting case) there will not be any other causes left to ‘fix’ the effect. By contrast, if we suppose a given effect of some cause to have been absent, this does not imply the cause would have been absent, for there are still all the other traces left to ’fix’ the causes. Lewis argues that these counterfactual considerations suffice to show why causes are different from effects.
Other philosophers appeal to a probabilistic variant of Lewis’s asymmetry. Following, the philosopher of science and probability theorists, Hans Reichenbach (1891-1953), they note that the different causes of any given type of effect are normally probabilistically independent of each other, by contrast, the different effects of any given type of cause are normally probabilistically correlated. For example, both obesity and high excitement can cause heart attacks, but this does not imply that fat people are more likely to get excited than thin ones: Its facts, that both lung cancer and nicotine-stained fingers can result from smoking does imply that lung cancer is more likely among people with nicotine-stained fingers. So this account distinguishes effects from causes by the fact that the former, but not the latter are probabilistically dependent on each other.
However, there is another course of thought in philosophy of science, the tradition of negative or eliminative induction. From the English statesman and philosopher Francis Bacon (1561-1626) and in modern time the philosopher of science Karl Raimund Popper (1902-1994), we have the idea of using logic to bring falsifying evidence to bear on hypotheses about what must universally be the case that many thinkers accept in essence his solution to the problem of demarcating proper science from its imitators, namely that the former results in genuinely falsifiable theories whereas the latter do not. Although falsely allowed many people’s objections to such ideologies as psychoanalysis and Marxism.
Hume was interested in the processes by which we acquire knowledge: The processes of perceiving and thinking, of feeling and reasoning. He recognized that much of what we claim to know derives from other people secondhand, thirdhand or worse: Moreover, our perceptions and judgements can be distorted by many factors - by w hat we are studying, as well as by the very act of study itself., the main reason, however, behind his emphasis on ‘probabilities and those other measures of evidence on which life and action entirely depend’ is this:
It is evident that all reasoning concerning ‘matter of fact’ are founded on the relation of cause and effect, and that we can never infer the existence of one object from another unless the are connected together, either mediately or immediately.
When we apparently observe a whole sequence, say of one ball hitting another, what exactly do we observe? And in the much commoner cases, when we wonder about the unobserved causes or effects of the events we observe, what precisely are we doing?
Hume recognized that a notion of ‘must’ or necessity is a peculiar feature of causal relation, inference and principles, and challenges us to explain and justify the notion. He argued that there is no observable feature of events, nothing like a physical bond, which can be properly labelled the ‘necessary connection’ between a given cause and its effect: Events simply are, they merely occur, and there is in ‘must’ or ‘ought’ about therm. However, repeated experience of pairs of events sets up the habit of expectation in us, such that when one of the pair occurs we inescapably expect the other. This expectation makes us infer the unobserved cause or unobserved effect of the observed event, and we mistakenly project this mental inference on to the events themselves. There is no necessity observable in causal relations; all that can be observed is regular sequence, here is necessity in causal inferences, but only in the mind. Once we realize that causation is a relation between pairs of events. We also realize that often we are not present for the whole sequence e which we want to divide into ‘cause’ and ‘effect’. Our understanding of the casual relation is thus intimately linked with the role of the causal inference cause only causal inferences entitle us to ‘go beyond what is immediately present to the senses’. But now two very important assumptions emerge behind the causal inference: The assumptions that ‘like causes, in like circumstances, will always produce like effects’, and the assumption that ‘the course of nature will continue uniformly the same’ - or, briefly that the future will resemble the past. Unfortunately, this last assumption lacks either empirical or a priori proof, that is, it can be conclusively established neither by experience nor by thought alone.
Hume frequently endorsed a standard seventeenth-century view that all our ideas are ultimately traceable, by analysis, to sensory impressions of an internal or external kind. Accordingly, he claimed that all his theses are based on ‘experience’, understood as sensory awareness together with memory, since only experience establishes matters of fact. But is our belief that the future will resemble the past properly construed as a belief concerning only a mater of fact? As the English philosopher Bertrand Russell (1872-1970) remarked, earlier this century, the real problem that Hume raises is whether future futures will resemble future pasts, in the way that past futures really did resemble past pasts. Hume declares that ‘if . . . the past may be no rule for the future, all experience become useless and can give rise to inference or conclusion. And yet, he held, the supposition cannot stem from innate ideas, since there are no innate ideas in his view nor can it stem from any abstract formal reasoning. For one thing, the future can surprise us, and no formal reasoning seems able to embrace such contingencies: For another, even animals and unthinkable people conduct their lives as if they assume the future resembles the past: Dogs return for buried bones, children avoid a painful fire, and so forth. Hume is not deploring the fact that we have to conduct our lives on the basis of probabilities, and he is not saying that inductive reasoning could or should be avoided or rejected. Rather, he accepted inductive reasoning but tried to show that whereas formal reasoning of the kind associated with mathematics cannot establish or prove matters of fact, factual or inductive reasoning lacks the ‘necessity’ and ‘certainty’ associated with mathematics. His position, therefore clear; because ‘every effect is a distinct event from its cause’, only investigation can settle whether any two particular events are causally related: Causal inferences cannot be drawn with the force of logical necessity familiar to us from a priori reasoning, but, although they lack such force, they should not be discarded. In the context of causation, inductive inferences are inescapable and invaluable. What, then, makes ‘past experience’ the standard of our future judgement? The answer is ‘custom’, it is a brute psychological fact, without which even animal life of a simple kind would be more or less impossible. ‘We are determined by custom to suppose the future conformable to the past’ (Hume, 1978), nevertheless, whenever we need to calculate likely events we must supplement and correct such custom by self-conscious reasoning.
Nonetheless, the problem that the causal theory of reference will fail once it is recognized that all representations must occur under some aspect or that the extentionality of causal relations is inadequate to capture the aspectual character of reference. The only kind of causation that could be adequate to the task of reference is intentional causal or mental causation, but the causal theory of reference cannot concede that ultimately reference is achieved by some met device, since the whole approach behind the causal theory was to try to eliminate the traditional mentalism of theories of reference and meaning in favour of objective causal relations in the world, though it is at present by far the most influential theory of reference, will prove to be a failure for these reasons.
If mental states are identical with physical states, presumably the relevant physical states are various sorts of neural states. Our concepts of mental states such as thinking, sensing, and feeling are of course, different from our concepts of neural states, of whatever sort. But that is no problem for the identity theory. As J.J.C. Smart (1962), who first argue for the identity theory, emphasized, the requisite identities do not depend on understanding concepts of mental states or the meanings of mental terms. For ‘a’ to be the identical with ‘b’, ‘a’, and ‘b’ must have exactly the same properties, but the terms ‘a’ and ‘b’ need not mean the same. Its principal means by measure can be accorded within the indiscernibility of identicals, in that, if ‘A’ is identical with ‘B’, then every property that ‘A’ has ’B’, and vice versa. This is, sometimes known as Leibniz’ s Law.
But a problem does seem to arise about the properties of mental states. Suppose pain is identical with a certain firing of c-fibres. Although a particular pain is the very same as a neural-firing, we identify that state in two different ways: As a pain and as neural-firing. that the state will therefore have certain properties in virtue of which wee identify it as pain and others in virtue of which we identify it as an excitability of neural firings. The properties in virtue of which we identify it as a pain will be mental properties, whereas those in virtue of which ewe identify it as neural excitability firing, will be physical properties. This has seemed to many to lead to a kind of dualism at the level of the properties of mental states, even if we reject dualism of substances and take people simply to be physical organisms, those organisms still have both mental and physical states. Similarly, even if we identify those mental states with certain physical states, those states will, nonetheless have both mental and physical properties. So disallowing dualism with respect to substances and their states simply es to its reappearance at the level of the properties of those states.
There are two broad categories of mental property. Mental states such as thoughts and desires, often called ‘propositional altitudes’, have ‘content’ that can be de scribed by ‘that’ clauses. For example, one can have a thought, or desire, that it will rain. These states are said to have intentional properties, or ‘intentionality sensations’, such as pains and sense impressions, lack intentional content, and have instead qualitative properties of various sorts.
The problem about mental properties is widely thought to be most pressing for sensations, since the painful qualities of pains and the red quality of visual sensations seem to be irretrievably non-physical. And if mental states do actually have non-physical properties, the identity of mental states generate to physical states as they would not sustain a thoroughgoing mind-body materialism.
The Cartesian doctrine that the mental is in some way non-physical is so pervasive that even advocates of the identity theory sometimes accepted it, for the ideas that the mental is non-physical underlies, for example, the insistence by some identity theorists that mental properties are really neural as between being mental or physical. To be neural is in this way, a property would have to be neutral as to whether its mental at all. Only if one thought that being meant being non-physical would one hold that defending materialism required showing the ostensible mental properties are neutral as regards whether or not they’re mental.
But holding that mental properties are non-physical has a cost that is usually not noticed. A phenomenon is mental only if it has some distinctively mental property. So, strictly speaking, a materialist who claims that mental properties are non-physical phenomena exist. This is the ‘Eliminative-Materialist position advanced by the American philosopher and critic Richard Rorty (1979).
According to Rorty (1931-) ‘mental’ and ‘physical’ are incompatible terms. Nothing can be both mental and physical, so mental states cannot be identical with bodily states. Rorty traces this incompatibly to our views about incorrigibility: ‘Mental’ and ‘physical’ are incorrigible reports of one’s own mental states, but not reports of physical occurrences, but he also argues that we can imagine a people who describe themselves and each other using terms just like our mental vocabulary, except that those people do not take the reports made with that vocabulary to be incorrigible. Since Rorty takes a state to be a mental state only if one’s reports about it are taken to be incorrigible, his imaginary people do not ascribe mental states to themselves or each other. Nonetheless, the only difference between their language and ours is that we take as incorrigible certain reports which they do not. So their language as no less descriptive or explanatory power than ours. Rorty concludes that our mental vocabulary is idle, and that there are no distinctively mental phenomena.
This argument hinges on building incorrigibly into the meaning of the term ‘mental’. If we do not, the way is open to interpret Rorty’s imaginary people as simply having a different theory of mind from ours, on which reports of one’s own mental stares are not incorrigible. Their reports would this be about mental states, as construed by their theory. Rorty’s thought experiment would then provide to conclude not that our terminology is idle, but only that this alternative theory of mental phenomena is correct. His thought experiment would thus sustain the non-eliminativist view that mental states are bodily states. Whether Rorty’s argument supports his eliminativist conclusion or the standard identity theory, therefore, depends solely on whether or not one holds that the mental is in some way non-physical.
Paul M. Churchlands (1981) advances a different argument for eliminative materialism. According to Churchlands, the common-sense concepts of mental states contained in our present folk psychology are, from a scientific point of view, radically defective. But we can expect that eventually a more sophisticated theoretical account will relace those folk-psychological concepts, showing that mental phenomena, as described by current folk psychology, do not exist. Since, that account would be integrated into the rest of science, we would have a thoroughgoing materialist treatment of all phenomena, unlike Rorty’s, does not rely of assuming that the mental is non-physical.
But even if current folk psychology is mistaken, that does not show that mental phenomena does not exist, but only that they are of the way folk psychology described them as being. We could conclude they do not exist only if the folk-psychological claims that turn out to be mistaken actually define what it is for a phenomena to be mental. Otherwise, the new theory would be about mental phenomena, and would help show that they’re identical with physical phenomena. Churchlands argument, like Rorty’s, depends on a special way of defining the mental, which we need not adopt, its likely that any argument for Eliminative materialism will require some such definition, without which the argument would instead support the identity theory.
Despite initial appearances, the distinctive properties of sensations are neutral as between being mental or physical, in that borrowed from the English philosopher and classicist Gilbert Ryle (1900-76), they are topic neutral: My having a sensation of red consists in my being in a state that is similar, in respect that we need not specify, even so, to something that occurs in me when I am in the presence of certain stimuli. Because the respect of similarity is not specified, the property is neither distinctively mental nor distinctively physical. But everything is similar to everything else in some respect or other. So leaving the respect of similarity unspecified makes this account too weak to capture the distinguishing properties of sensation.
A more sophisticated reply to the difficultly about mental properties is due independently to forthright Australian David Malet Armstrong (1926-) and American philosopher David Lewis (1941-2002), who argued that for a state to be a particular sort of intentional state or sensation is for that state to bear characteristic causal relations to other particular occurrences. The properties in virtue of which e identify states as thoughts or sensations will still be neural as between being mental or physical, since anything can bear a causal relation to anything else. But causal connections have a better chance than similarity in some unspecified respect to capturing the distinguishing properties of sensations and thought.
This casual theory is appealing, but is misguided to attempt to construe the distinctive properties of mental states as being neutral as between being mental; or physical. To be neutral as regards being mental or physical is to be neither distinctively mental nor distinctively physical. But since thoughts and sensations are distinctively mental states, for a state to be a thought or a sensation is perforce for it to have some characteristically mental property. We inevitably lose the distinctively mental if we construe these properties as being neither mental nor physical.
Not only is the topic-neutral construal misguided: The problem it was designed to solve is equally so, only to say, that problem stemmed from the idea that mental must have some non-physical aspects. If not at the level of people or their mental states, then at the level of the distinctively mental properties of those states. However, it should ne mentioned, that properties can be more complicated, for example, in the sentence, ‘John is married to Mary’, we are attributing to John the property of being married, and unlike the property of John is bald. Consider the sentence: John is bearded. The word ‘John’ in this sentence is a bit of language - a name of some individual human being - and more some would be tempted to confuse the word with what it names. Consider the expression ‘is bald’, this too is a bit of language - philosopher call it a ‘predicate’ - and it brings to our attention some property or feature which, if the sentence is true,. Is possessed by John. Understood in this ay, a property is not its self linguist though it is expressed, or conveyed by something that is, namely a predicate. What might be said that a property is a real feature of the word, and that it should be contrasted just as sharply with any predicates we use to express it as the name ‘John’ is contrasted with the person himself. Controversially, just what sort of ontological status should be accorded to properties by describing ‘anomalous monism’, - while its conceivably given to a better understanding the similarity with the American philosopher Herbert Donald Davidson (1917-2003wherefore he adopts a position that explicitly repudiates reductive physicalism, yet purports to be a version of materialism, nonetheless, Davidson holds that although token mental event nd states are identical to those of physical events and states - mental ‘types’ - i.e., kinds, and/or properties - are neither to, nor nomically co-existensive with, physical types. In other words, his argument for this position relies largely on the contention that the correct assignment of mental a actionable properties to a person is always a holistic matter, involving a global, temporally diachronic, ‘intentional interpretation’ of the person. But as many philosophers have in effect pointed out, accommodating claims of materialism evidently requires more than just repercussions of mental/physical identities. Mentalistic explanation presupposes not merely that metal events are causes but also that they have causal/explanatory relevance as mental - i.e., relevance insofar as they fall under metal kinds or types. Nonetheless, Davidson’s position, which denies there are strict psychological or psychological laws, can accommodate the causal/explanation relevance of the mental quo mental: If to ‘epiphenomenalism’ with respect to mental properties.
But the idea that the mental is in some respect non-physical cannot be assumed without argument. Plainly, the distinctively mental properties of the mental states are unlikely any other properties we know about. Only mental states have properties that are at all like the qualitative properties that anything like the intentional properties of thoughts and desires. Bu t this does not show that the mental properties are not physical properties, not all physical properties like the standard states: So, mental properties might still be special kinds of physical properties. Its question beginning to assume otherwise. The doctrine that the mental properties is simply an expression of the Cartesian doctrine that the mental is automatically non-physical.
Its sometimes held that properties should count as physical properties inly if they can be defined using the terms of physics. This to far to restrictively. Nobody would hold that to reduce biology to physics, for example, we must define all biological properties using only terms that occur in physics. And even putting ‘reduction’ aside, I certain biological properties could have been defined, that would not mean that those properties were in n way non-physical. The sense of ‘physical’ that is relevant, that is of its situation it must be broad enough to include not only biological properties, but also most common-sense, macroscopic properties. Bodily states are uncontroversially physical in the relevant way. So, we can recast the identity theory as asserting that mental states are identical with bodily state.
In the course of reaching conclusions about the origin and limits of knowledge, Locke had occasioned concern himself with topics which are of philosophical interest in themselves. On of these is the question of identity, which includes, more specifically, the question of personal identity: What are the criteria by which a person at one time is numerically the same person as a person encountering of time? Locke points out whether ‘this is what was here before, it matters what kind of thing ‘this’ is meant to be. If ‘this’ is meant as a mass of matter then it is what was before so long as it consists of the same material panicles, but if it is meant as a living body then its considering of the same particles does mot matter and the case is different. ‘A colt grown up to a horse, sometimes fat, sometimes lean, is all the while the same horse though . . . there may be a manifest change of the parts. So, when we think about personal identity, we need to be clear about a distinction between two things which ‘the ordinary way of speaking runs together’ - the idea of ‘man’ and the idea of ‘person’. As with any other animal, the identity of a man consists ‘in nothing but a participation of the same continued life, by constantly fleeting particles of matter, in succession initially united to the same organized body, however, the idea of a person is not that of a living body of a certain kind. A person is a ‘thinking’. ‘intelligent being, that has son and reflection and such a being ‘will be the same self as far as the same consciousness can extend to action past or to come’ . Locke is at pains to argue that this continuity of delf-consciousness does not necessarily involve the continuity of some immaterial substance, ion the way that Descartes had held, fo we all know, says Locke, consciousness and thought may be powers which can be possessed by ‘systems of matter fitly disposed’, and even if this is not so the question of the identity of person is not the same as the question of the identity of an ‘immaterial; substance’. For just as the identity of as horse can be preserved through changes of matter and depends not on the identity of a continued material substance of its unity of one continued life. So the identity of a person does not depend on the continuity of a immaterial; substance. The unity of one continued consciousness does not depend on its being ‘annexed’ only to one individual substance, [and not] . . . continued in a succession of several substances. For Lock e, then, personal identity consists in an identity of consciousness, and not in the identity of some substance whose essence it is to be conscious
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