Sensation may be described as the most elementary cognitive reaction of an organism to its environment. The awareness of green, of warm, of sharp—when not accompanied by the awareness of something green, warm, or sharp—is a simple sensation; the awareness of a particular green leaf, on the other hand, is not a sensation but a perception. Pure perceptions occur in animals, e.g., a dog perceives a cat. But in man every perception is integrated into a more complex totality, involving the concept and the judgment. When someone sees a cat and makes the statement: "That is a cat," he expresses a judgment containing an affirmation, a concept (cat), a perception (being here and now aware of this cat), and a certain number of sensations (e.g., gray, soft, purring). Much of what is traditionally said about sensation, it should be noted, does not strictly apply to simple sensation but rather to sense knowledge or to perception.
This article outlines a philosophical view of sensation, based mainly on the teaching of St. thomas aquinas, and considered in the totality of cognitional experience.
The Thomistic teaching on sensation defies simple, brief presentation. When simplified it is easily misunderstood, and when presented in its totality, it is difficult and calls for considerable insight. It is best explained, therefore, by first giving a rough outline that can serve as an approximation, and then correcting this on points of specific detail.
Act of Sensation. Several stages may be distinguished in the act of sensation. In the physical stage, an outside stimulus (e.g., electromagnetic or sound waves) originating from some object impinges upon a sense organ. This is followed by a physiological stage, wherein a modification is produced by the stimulus in the sense organ. Since the organ is living and animated, its physiological modification is accompanied by another, or psychic modification, whose production constitutes the psychological stage. The result produced is called the impressed species or the impressed intentional form (see species, intentional). It is a substitute for the outside object within the sense power, by means of which the object becomes known. Up to this point the sense power can be regarded as passive or receptive. But knowledge is an activity. Hence there is a last stage, the active psychological stage, in which the sense power turns, as it were, toward the object, grasps it, and knows it. Only when this occurs is there real knowledge.
This elementary explanation of sense knowledge raises a number of questions that must now be considered.
Impressed Species and Object. Why do the senses know the object itself, and not simply its substitute, the impressed species? One answer is: because the species is a formal, not an objective, sign. Objective signs must be known first in themselves, before the thing they signify can be known; examples would be a photograph, a traffic light. Formal signs, on the other hand, are not known in themselves, but become the means whereby the signified object is known. An example of a formal sign is the retinal image, or picture of a perceived object on the retina of the eye; man never sees the image itself, but through it he sees the object.
In sensation, therefore, man knows the object itself, and not simply the impression made by the object. If he knew only the impression, he would forever be cut off from reality. He would not know that any impression was a faithful reproduction of the outside world, since, in that hypothesis, he could never be in contact with the outside world. He could not even be certain that there is a reality "outside" of himself. If man knew only impressions produced
on his senses, he would arrive at knowledge of reality only by inference, by reasoning that this impression must be the effect of some external cause and thus applying the principle of causality. To claim that the sense powers reason to the existence of an outside world in this way, however, is quite implausible.
What man knows, therefore, is not simply impressions made upon him by things, but things themselves. The problem of the passage from an outside world to an inner world is thus a pseudoproblem. There is no real passage—these two worlds unite in man. Through his body, he is part of the outside world, a material object among other material objects and continually influenced by them. The human soul, man's inner world, animates his body. Together body and soul constitute not a union, but a unity, the animated body.
Cartesian Dualism. To admit a passage from outer to inner world is to fall into the dualism of René descartes. Every time the body is affected, so is the soul, because what is affected is the animated or besouled body. Man's awareness of objects is neither merely subjective nor merely objective. He is aware neither of the impression alone, nor of the object alone; but of the object as it affects him, or of himself as affected by the object. External sensation may thus be described as the zone of consciousness that occurs at the common boundary of outside objects and animated corporeity.
The physicist and the psychologist are, of course, free to consider electromagnetic or sound waves as causes of sensation. But philosophers cannot be content with this type of explanation alone, lest it lead them into insuperable difficulties. To explain sensation by means of light or sound waves, themselves knowable to philosophers only through sensation, is to become involved in a vicious circle. Psychologists may be content to explain sensation by the stimulation of sense organs, nerves, and brain. Should philosophers do the same, they could fall into Cartesian dualism. For example, they might say that the stimulus coming from outside affects the organ, travels through the nerves, and is led into the brain center. But this explanation would tacitly assume that the soul is at the end of this circuit supposedly dwelling inside the body to listen, receive the message, and interpret it. Such a view is Cartesian. The organs, the nerves, and the brain center do not lead up to man, they are man. They are animated, besouled organs; as soon as they are affected, the human soul is affected.
These observations, inspired by Gabriel Marcel, explain why external sensation or perception does not require an image or substitute for the object. Thomists therefore deny the need of an expressed species in external sense knowledge; in their analysis, perception puts man directly in contact with reality, and not merely with some substitute for it.
Powers of Sensation. A distinction is usually made between external and internal sense faculties (see faculties of the soul). External sense faculties, the five senses commonly referred to, put man directly in contact with the outside world. Through the intermediary of the external senses, the internal senses also attain reality. Thus imagination is the internal faculty that provides representations of singular, material objects, in the absence of such objects. Memory also furnishes such representations, but further recognizes objects as formerly perceived. The two remaining internal senses are the central sense and the estimative power.
The central sense, or common sense, makes man aware of the objects and operations of the external senses. When a person sees a dog, he is aware of the dog and of seeing that dog. Such awareness is not an act of seeing, nor is it an intellectual operation. It stands between external sensation and thought, and is attributed to the central or common sense, which may be considered as the seat of sense consciousness.
The estimative power corresponds roughly to the cognitive aspect of instinct. Animals perceive not only colors and sounds, but also the usefulness or danger of things in their environment. Since this awareness cannot be attributed to reasoning, nor to the operation of the external senses, it is explained as the effect of a special internal sense. In animals this power is a lower analogate of prudence, through which animals know, with knowledge previous to any experience, that a particular thing is useful or harmful. In man the estimative power takes on new functions because of its connection with intelligence, and is called the cogitative power. It serves as a transition between the intellect with its universal ideas and the senses with their individual perceptions, and is the point of contact between sense and intellectual knowledge.
Functioning of the Sense Powers. Although it is useful to distinguish between external and internal senses, these powers are not completely independent of each other. True, the internal senses may operate without the actual cooperation of the external senses; but the opposite is not true. Whenever the external senses act, the internal senses act with them. One would be unable to hear a sentence or a melody, unless, at the end of that sentence or that melody, he somehow remembered the first words or the first bars. When the eyes take in a landscape or a large painting, they scan what is present before them; one sees the painting only if, throughout the whole process of perception, he keeps remembering what was perceived before.
Expressed Species. Imagination and memory know objects that are not actually exerting influence from the outside. Physiologically this supposes the reactivating of the traces of a previous action of such objects upon the brain. On the conscious level, one speaks of images of these objects; Thomists identify such images as expressed species. The expressed species is that "in which" one knows, imagines, or remembers an object. It differs from the impressed species in three ways: (1) man is conscious of the expressed species, whereas he is not conscious of the impressed species; (2) the expressed species is that "in which" he knows the object, while the impressed species that "by means of which" he knows the object; and (3) the production of the former is the act of knowing, whereas the production of the latter precedes knowledge.
Validity of Sensation. It has already been shown that sensation is not purely subjective. Man does not know the impression objects make, but rather the objects themselves. On the other hand, sense knowledge is not as objective as intellectual knowledge; through sensation one does not know things as they are in themselves. This explains why color-blind people perceive some objects differently than those with normal vision. The state of the sense organ affects the sensations, and renders sense knowledge to some extent relative. Man knows objects as they affect him, as they appear to him. To use a Kantian terminology, the senses give a phenomenal, not a noumenal knowledge of reality (see noumena; phenomena). It should be remembered, in this connection, that man never experiences pure sense knowledge. Human sensations are always accompanied by concepts and judgments, which assure an objective knowledge of reality.
Organic Bases of Sensation. Sensation is rooted in the animated body. Whereas the body is only a necessar condition of thinking, it is, together with the soul, a cause of sensation. This organic causality is exercised by specialized parts of the body, known as senses or sense organs. Traditionally five of these senses are mentioned: sight, hearing, smell, taste, and touch.
Modern psychology calls the fifth sense the somesthetic sense, and distinguishes within it four cutaneous and three intraorganic senses. The cutaneous senses are those of pressure, cold, warmth, and pain. They are considered distinct senses, because they consist of specialized nerve endings that, as a rule, react only to their specific stimuli. The senses of sight, hearing, smell, taste, pressure, cold, warmth, and pain are called exteroceptive senses. They give information about the exterior world.
The intraorganic senses inform man about his own body. These are divided into the proprioceptive and the interoceptive senses. Proprioceptive sensations are known also as kinesthetic sensations. Through them man is aware of the movements of his limbs in relation to each other (e.g., flexing the arm, stretching the leg), and of the resistance met by such movements. The organs of this sense are in the muscles and the tendons. The two interoceptive senses are the static and the visceral sense. The static sense, whose organ is located in the inner ear, tells man about the position of his body as a whole; it is the sense of balance and of equilibrium. Movement of the body as a whole, when nonuniform, may also be perceived by this sense. Visceral or organic sensations provide information about the state of the inner organs. Under this heading come such sensations as hunger, thirst, satiety, fatigue, and nausea. Such sensations gradually fade into feeling and drives. The general bodily feeling constituted by the totality of the visceral sensations is sometimes called coenesthesis. It is important for man's affective life, as an underlying basis of his moods, and seems to function also in his perception of time.
See Also: knowledge, process of; sense knowledge; sensibles.
Bibliography: thomas aquinas, Summa theologiae 1a, 78.3–4; 85.1. j. f. donceel, Philosophical Psychology (2d ed. rev. and enl. New York 1961). g. p. klubertanz, The Discursive Power (St. Louis 1952); Philosophy of Human Nature (New York 1953). j. e. royce, Man and His Nature (New York 1961). r. e. brennan, General Psychology (rev. ed. New York 1952); Thomistic Psychology (New York 1941).
[j. f. donceel]
Aristotle recognized only five senses — vision, hearing, taste, smell and touch, each associated with a particular sense organ (counting the skin as a sense organ). The term ‘special sense’ is still used for vision, hearing, taste and smell. In reality, there are many more sensory systems, including those that mediate the sensations of pain and temperature, those of the positions of parts of the body (proprioception), and those related to balance (conveyed by the vestibular apparatus). A more fundamental way of classifying sensory receptors is according to the types of physical stimuli to which they respond. It is remarkable that all the majesty of our sensory experiences depends only on sensitivity to light, and to mechanical, chemical, and thermal stimulation.
The experience of our own bodies, somatic sensation (from the Greek ‘soma’ meaning body), conventionally includes sensations arising from nerve endings in skin, muscle, bones, and joints. A different term, visceral sensation, is used to describe awareness (usually discomfort) arising from receptors in the internal organs (viscera). Such sensations are only vaguely localized compared to somatic sensations, and in some instances may be ‘referred’, that is, felt to be coming from a site quite different from where the excitation arises. For instance, the pain of angina, originating from sensory fibres in the muscle of the heart, is typically felt in the left shoulder and upper arm. Some visceral sensations, such as the feeling of a full bladder, are associated with normal functions; others signal abnormality, such as the pain of intestinal colic or a growing tumour.
Sensory experiences have distinct subjective qualities, such as colour, pitch, tickle, bitterness, and floral. These are sometimes referred to as ‘modalities’ or ‘sub-modalities’ of sensation, especially for sensations derived from the skin. Valiant, partially successful, attempts have been made to correlate each such mental property with a particular specialized type of sensory receptor, nerve pathway or region of the brain. In 1823, the British neurologist and anatomist, Sir Charles Bell, first described evidence for what subsequently became known as Müller's Law of ‘Specific Nerve Energies’ (after the German physiologist, Johannes Müller). Bell wrote: ‘every nerve of sense is limited in its experience, and can minister to certain perceptions only’. He and Müller cited examples of false sensations, elicited by inappropriate stimulation of particular nerves. For instance, pressing on the side of the eyeball with a finger, which causes direct, mechanical stimulation of the retina, gives rise to a ‘phosphene’ — a visual sensation in the form of a curious blob in the part of the visual field corresponding to that region of the retina. Certainly, many modalities of sensation can be identified with particular specialization of the sensory receptors involved. Touch, vibration, warmth, and coldness are indubitably associated with particular, highly specialized nerve endings in the skin.
Sensory receptors provide information about the quality of the stimuli that they detect, giving rise to what philosophers call the qualia of conscious perceptual experience. We see colour and brightness, hear pitch and timbre, and taste sweetness and sourness: we can distinguish whether an object touching the skin is sharp or smooth, hot or cold. But stimulus quality is not the only kind of information that sense organs provide. They also transmit data on the intensity, duration, and location of the stimulus. Generally speaking (though there are exceptions), the quality of a sensation is determined by which type of specialized nerve ending or receptor cell is stimulated, but the perceptual intensity (brightness, loudness, etc.) depends on how strongly the receptor is stimulated.
However, there is not perfect congruence between particular individual sensory receptors and the qualia – the units of subjective experience. For example, our retina contains just three types of cone receptor cell which have slightly different but overlapping spectral absorption. The large number of different colours that we can distinguish are somehow derived by the brain from the relative strength of the signals from these three basic detectors.
pain is usually thought of as rather different from other modalities, in that it bears no simple relationship to the physical world. It is, most simply, a perception resulting from a noxious stimulus — one associated with actual or potential tissue damage. But pain may be caused by excessive stimulation of receptors ordinarily involved with other modalities: for example heat on the skin is felt as heat, but excessive heat is felt also as pain.
Although, strictly, sensation refers only to conscious feelings, it is clear that much of the massive, unremitting inflow of information from sensory nerves does not enter consciousness. Even for the special senses, we are aware at any moment only of what we attend to, which is a tiny fraction of the information streaming in. Indeed, much sensory processing, essential for the regulation of the body, is entirely unconscious. The receptors in muscles, joints and tendons, some of which give rise to conscious proprioception, are involved in the constant, unconscious task of regulating posture and guiding movement. The heart, lungs, and major blood vessels have a variety of specialized sensory endings, conveying information to the brain about the pressure and composition of the blood, and about the stretching of the lungs. Within the brain itself, the hypothalamus and parts of the medulla contain sensory nerve cells that are specialized to detect such properties as the acidity of the blood, its temperature and the concentration of glucose, salt and other constituents. These unconscious sensory systems play an essential part in homeostasis — the maintenance of the internal environment of the body, by initiating reflex regulation of breathing, heart rate, blood vessel size, sweating and shivering, as well as in regulating essential behaviour such eating and drinking.
Psychologists and philosophers often draw a distinction between sensation (said to have a raw, unprocessed quality) and perception, the interpreted meaning of sensory activity. This view can be traced to Immanuel Kant's ‘transcendental’ philosophy, namely to the view that knowledge of the world arises from sensory experience, furnished by the mind with such ‘archetypal’ properties as space, time, relation, and causality.
The distinction between sensation and perception is hard to defend on the basis of what we now know about how sensory receptors and their associated brain areas work. At each stage, even at the sensory receptors themselves, the particular, selective characteristics of the nerve cells impose expectation and order on sensory signals. The detection of stimuli and their cognitive interpretation, to provide knowledge of the world, are inextricably linked within our sensory systems. Effortless though our perceptions seem, they involve immense ‘computational’ tasks. More than half of the human cerebral cortex is devoted to analysing sensory signals.
‘Common sense’, meaning native intelligence, derives from the medieval term sensus communis. This described the place in the fluid-filled chambers of the brain at which signals from all the sense organs were supposed to mix together, to provide the ingredients of imagination and rational thought. The near-miraculous process that intervenes between the irritation of the membrane of a sensory receptor and our perception of the world is arguably the most intelligent thing that we do.
See also cerebral cortex; cerebral ventricles; hearing; pain; perception; proprioception; sensory receptors; somatic sensation; taste and smell; vestibular system; visceral sensation; vision
sen·sa·tion / senˈsāshən/ • n. 1. a physical feeling or perception resulting from something that happens to or comes into contact with the body: a burning sensation in the middle of the chest. ∎ the capacity to have such feelings or perceptions: they had lost sensation in one or both forearms. ∎ an inexplicable awareness or impression: she had the eerie sensation that she was being watched. 2. a widespread reaction of interest and excitement: his arrest for poisoning caused a sensation. ∎ a person, object, or event that arouses such interest and excitement: she was a sensation, the talk of the evening.
Sensation ★★ 1994 (R)
A college coed agrees to participate in a professor's paranormal experiments in order to test her psychic abilities. Unfortunately what she senses is her prof involved in the unsolved sexmurder of a former student. 102m/C VHS . Kari Wuhrer, Eric Roberts, Ron Perlman, Ed Begley Jr., Paul LeMat, Claire Stansfield, Kieran Mulroney, Tracey Needham; D: Brian Grant; W: Doug Wallace; M: Arthur Kempel.