Sense

Plato treats the senses as a lower faculty whose objects cannot furnish knowledge in the strict sense. In the divided line of the Republic, the visible world stands to the intelligible world as shadow stands to substance. Sensible things are particular, changing, and perishable; the Forms are universal, changeless, and eternal. Sense perception, on this account, yields at best opinion (doxa), because its objects lack the stability that knowledge (episteme) requires. The cave allegory illustrates the same ranking: prisoners chained before a wall take flickering shadows for real things, and only the turn of the soul toward the sun, which is the Good, brings understanding.

In the Theaetetus Plato examines and rejects the thesis that knowledge is sense perception. Perception varies from person to person, depends on conditions, and cannot grasp general truths such as being, identity, difference, and number. These are the work of the soul itself, not of the eyes or ears. Plato does not deny that sensation occurs, nor that animals as well as men possess it. He denies that sensation by itself can deliver what the tradition later calls knowledge of the universal. The contrast between sensible and intelligible reality is treated more fully under the ideas of Being, Form, and Idea; the place of sense among the powers of the soul belongs also to the chapters on Mind and Soul.

Plato's ranking of intellect above sense is taken up by later writers in several ways. Descartes revives the concern that the senses may deceive and seeks certainty in clear and distinct ideas of pure thought. Aristotle, by contrast, makes sensation the starting point of all knowledge and denies that the Forms are separate from the sensible things from which the intellect abstracts. Plotinus will develop the Platonic hierarchy of powers still further, treating sense-perception as an act of the soul that is not a mere seal-impression on the body.

Aristotle treats sensation not as a defective mode of awareness but as the proper starting point of knowledge. Against the Platonic separation of sensible and intelligible worlds, he holds that the soul perceives through bodily organs and that perception is the reception of the sensible form without the matter. When the eye sees red, the organ takes on the form of redness without becoming the red object. Each of the five exterior senses has its proper object: sight has color, hearing sound, touch texture and temperature. Aristotle also distinguishes common sensibles (motion, rest, number, shape, magnitude) that can be perceived by more than one sense, and he assigns to an internal faculty, the common sense (koine aisthesis), the coordination of the deliverances of the exterior senses.

Sensation, on this view, is always of particulars; the intellect grasps universals. Yet the universals are abstracted from sensory experience, not discovered in a separate world of Forms. The maxim later summarized as "nothing is in the intellect that was not first in the senses" belongs to this line of thought. Aristotle further holds that the senses are normally reliable about their proper objects, while error arises mainly about incidental sensibles (as when one judges that the white thing in the distance is Socrates) or about common sensibles under poor conditions. Sleep, he observes, can occur only in living things that possess sense-perception, so that the power of sense marks animals off from plants as well as from inanimate bodies. The relation of sense to imagination and memory is taken up under those ideas; the distinction between animal and plant is treated under Animal and Life.

The tension Aristotle leaves between the reliability of the senses about proper objects and the readiness of error about incidental sensibles is pressed in different directions by later writers. Aquinas develops the anatomy of the interior senses and the distinction between natural and spiritual immutation. Descartes seizes on the possibility of sensory error and uses it to disqualify sensation as a foundation for certain knowledge. The problem of primary and common sensibles reappears, under other names, in Locke's distinction between primary and secondary qualities.

Lucretius gives the Epicurean account of sense as an entirely corporeal process. Living things consist of body and soul, but the soul differs from the body only in the fineness and mobility of the atoms that compose it. Sensation is kindled by the common motions of body and soul when external bodies act upon the organs. Either the external body itself strikes the sense, as in touch, or minute replicas (simulacra), shed as thin films of atoms from the surfaces of distant things, enter the eyes, ears, or nostrils and produce vision, hearing, or smell. There is no immaterial principle receiving forms, no divine illumination, and no recollection of a prior life. Imagination and memory are treated as further bodily reverberations of sensation, further motions in the atoms of the mind.

On this theory the senses are the primary criterion of truth and cannot themselves be refuted. If the reliability of sense is denied, nothing remains by which true and false can be distinguished. The senses always report accurately what strikes them; error arises only when the mind adds judgment to what the senses present. A square tower seen from a distance may look round: the sense accurately reports the impression, and the mistake lies in the judgment that the tower is round. The same line of argument is later taken up, in different vocabularies, by writers who hold that the senses never deceive in their own deliverances and that falsity resides only in the compounding of judgment. The materialist reduction of sensation to motion reappears in Hobbes and, with important qualifications, in the mechanical philosophy of the seventeenth century.

Lucretius thus stands with those who treat the motions of matter as adequate to explain the phenomena of sensing and thinking, and who tend to regard man as differing from other animals in degree rather than in kind. His insistence that the senses are never deceived, while judgment errs, is shared in substance by Aristotle on proper objects and by Descartes on the restriction of attention to what is immediately presented. The question whether thought itself reduces to trains of sensation and image is taken up under Mind and under Idea.

Aquinas develops Aristotle's account of sensation by a threefold ranking of the soul's operations. At one extreme stand vegetative functions, performed by a corporeal organ and by virtue of a corporeal quality such as heat. At the other stands intellectual understanding, which does not belong to a corporeal organ at all. Between these he places sensation and imagination: operations performed through a corporeal organ, but not through a corporeal quality. The distinction is further explained by contrasting natural and spiritual immutation. Natural immutation occurs when the form of the agent is received according to its natural existence, as heat is received into the heated thing. Spiritual immutation occurs when the form is received according to a spiritual mode of existence, as the form of color is received into the eye, which does not thereby become colored. Without spiritual immutation, Aquinas argues, all natural bodies would feel when they undergo alteration.

The exterior senses (sight, hearing, smell, taste, touch) are distinguished from the interior senses: the common sense, which unifies the data of the exterior senses; imagination, which retains sensible forms in the absence of objects; the estimative power, which grasps intentions not perceived by the exterior senses (as the sheep apprehends the wolf as dangerous); and memory. The intellect depends on the senses for its material: there is nothing in the intellect that was not first in sense. Yet sensation is not knowledge of the universal. The senses perceive particulars; the intellect abstracts universals from the phantasms that sensation and imagination provide. Man shares sense-perception, memory, and imagination with the brutes, and alone possesses the higher faculty. The difference between men and brutes is thus conceived as one of kind, not of degree, when the difference between the senses and the reason is also conceived as a difference in kind. These questions are treated further under Soul, Mind, and Animal.

Aquinas's framework requires that the cognitive act of sensing be something different in kind from the mere physical alteration of the organ. Descartes will treat sensation, at least in animals and in its corporeal aspect in man, as reducible to motion in the nerves and brain, and will reserve pure thought for an immaterial soul that can act independently of the body. Locke will retain dependence of the understanding on the senses while treating all mental activity as a reworking of original impressions, without the Scholastic distinction between natural and spiritual immutation.

Descartes renews the ancient suspicion of the senses within a new method of systematic doubt. The First Meditation catalogs ways in which perception can mislead: distant objects appear small, sticks look bent in water, and the dream argument raises the possibility that present experience is illusory. Even if the senses usually get things right, the fact that they sometimes deceive is held sufficient to disqualify them as foundations of certain knowledge. The cogito is discovered by pure thought, without aid from the senses. In the Second Meditation the analysis of the wax shows that what is known of the wax (extension, flexibility, mutability) is grasped by the intellect, not by sight or touch, since the wax can change all its sensible qualities and remain the same wax. Ideas of God and of the soul are offered as further cases of content in the mind that has no origin in sensation or fancy.

At the same time Descartes does not treat sensation as useless or as wholly unreal. In men as in animals the external senses, so far as they are parts of the body, perceive by passivity alone, as wax receives an impression from a seal. Fancy or imagination is likewise a genuine part of the body, and corporeal memory is not distinct from imagination. The mind can act independently of the brain in pure thought; the brain's use is for imagining and perceiving. In the Sixth Meditation the senses are rehabilitated for practical life: God has given them to preserve the body, and in that function they serve adequately. Their testimony about the external world is reliable enough for action, even if it cannot ground metaphysical certainty. Falsity, Descartes holds, does not reside in sensations themselves but in the judgments we are accustomed to pass about external things on the basis of sensations. The same dualism leads him to deny knowledge to brutes and to ascribe to them only fancy of a purely corporeal kind. The relation of mind to body is examined under Mind and Soul; the status of animals under Animal.

Descartes's sharp distinction between the faculties of sense and understanding places him with Plato, Aristotle, Aquinas, and Kant against those who reduce all thought to elaborations of sense. Yet for Descartes as for Plato the intellect in its own sphere is like the senses in theirs, able to behold its proper objects intuitively, whereas for Aristotle and Aquinas the intellect abstracts its ideas from the particulars of sense. Locke, Berkeley, and Hume will build programs that reverse the Cartesian ranking while still answering the questions Descartes posed about certainty, the external world, and the origin of ideas.

Hobbes reduces sense to bodily motion. The cause of sense is the external body or object, which presses the organ proper to each sense, either immediately, as in taste and touch, or mediately, as in seeing, hearing, and smelling. That pressure, by the mediation of nerves and other membranes, continues inward to the brain and heart and there causes a resistance or counter-pressure. The outward endeavor of this counter-pressure "seems to be some matter without," and this seeming or fancy is what men call sense. Colors, sounds, and smells have, in the object, no existence other than so many motions of the matter by which it presses the organs; neither in us who are pressed are they anything else but diverse motions, for motion produces nothing but motion. There is no immaterial soul receiving forms, no spiritual immutation, and no common sense in the Aristotelian sense. The entire Scholastic apparatus of faculties is set aside.

Hobbes extends the same analysis to imagination and memory. Imagination is decaying sense, the continuation of internal motions after the object is removed; memory is the awareness that the image is of something past. Using "thoughts" for the images derived from sense, he holds that besides sense, thoughts, and the train of thoughts, the mind of man has no other motion, though by the help of speech and method the same faculties may be improved so as to distinguish men from other living creatures. Only the use of words makes the difference in the exercise of the imagination that is generally called understanding, and that understanding is common to man and beast. Abstract ideas are denied: thoughts or images are no less particular than sensations, there being nothing in the world universal but names. On this view man differs from the brutes only in degree, through language, and all mental phenomena are manifestations of bodily motion. The relation of sense to language is treated further under Sign and Symbol; the question of mind in animals under Animal and Mind.

Hobbes stands with Lucretius against Aquinas on the question of matter and spirit in the activity of the senses. Berkeley and Hume will seem to agree with him that man has no abstract ideas and that the operations of thought are elaborations of original impressions of sense, while differing sharply on whether matter or bodies exist at all. Locke will try to preserve a distinction between primary qualities, which really belong to bodies, and secondary qualities, which are powers to produce sensations in us, without abandoning the dependence of all ideas on experience.

Locke makes sensation the first source of all ideas. The mind at birth is blank paper, and sensation writes the first characters upon it. Through the five senses the mind receives simple ideas of particular qualities (color, sound, warmth, hardness, sweetness). These ideas are simple and unanalyzable; the mind cannot create them or refuse them when the appropriate object is present. Reflection, the mind's observation of its own operations, is the second source of ideas, but even reflection works on materials originally supplied by the senses. Mental activity begins with the passive reception of the simple ideas of sense and the simple ideas of reflection, and it includes the formation of complex ideas by compounding simple ones, and even the act whereby we form abstract ideas, in doing which man, in Locke's opinion, is distinguished from brutes.

All these activities require soul as well as body, yet all are somehow a reworking of the original sensations passively received. In this last respect Locke accords with Hobbes, Berkeley, and Hume, though he differs from them on abstract ideas and in his theory of body and soul. On the maxim that there is nothing in the understanding which has not first been in the senses, he stands opposed to Descartes, who offers the ideas of God and of the soul as counterexamples. Locke's distinction between primary and secondary qualities shapes the subsequent discussion. Primary qualities (solidity, extension, figure, motion, number) really belong to bodies and produce ideas that resemble them. Secondary qualities (color, sound, taste, smell) are powers in objects to produce sensations in us, and the ideas they produce do not resemble anything in the object. The distinction parallels, under other names, Aristotle's contrast between common and proper sensibles, though Aristotle does not treat the so-called secondary qualities as entirely subjective. Locke defines sensitive knowledge as that which informs us of the existence of things actually present to our senses: less certain than intuition or demonstration, yet an assurance that deserves the name of knowledge. The origin of ideas is treated further under Idea and Knowledge; the status of qualities under Quality.

Locke's empiricism, grounded in sensation, provides the framework within which much later British philosophy works. The primary and secondary quality distinction provokes Berkeley's denial that matter exists apart from perception, and Hume's question whether the perceptions of the mind can be proved to be caused by external objects at all. Kant will later grant that the senses supply the matter of experience while insisting that the mind contributes the forms under which that matter is ordered.

Berkeley turns Locke's empiricism against the residual materialism still present in Locke. If all we ever perceive are our own ideas, and if the supposed primary qualities are as much sensory experiences as the secondary ones (extension is felt, figure is seen), then the notion of a material substance behind our sensations is without foundation. To be is to be perceived. There is no unperceived matter; what we call the physical world is a coherent set of ideas. Berkeley regards this conclusion as a defense of common sense rather than an attack upon it. The philosopher who posits an unknowable material substrate is, on this view, the one who multiplies mysteries; Berkeley trusts what perception gives and refuses a gap between appearance and a hidden material reality.

He uses "idea" for sense-impressions actually imprinted on the senses, for what is perceived by attending to the passions and operations of the mind, and for ideas formed by memory and imagination. The ideas of sense are stronger, livelier, and more distinct than those of the imagination, but they do not exhaust the objects of thought: he admits notions whereby we understand words such as "spirit" or "soul," referring to a substance of which we can form no idea. The soul passively receives its original impressions directly from God and actively forms the ideas it derives from them. The regularity of sensory experience (that fire burns and bread nourishes) is God's doing: God presents ideas to finite minds in orderly patterns, and these patterns are the laws of nature. Sensation thus gives us the world directly, with no intervening matter. Berkeley agrees with Hobbes that the operations of thought are elaborations of sense, while differing radically in denying that bodies exist as material causes of sensation. The question of the external world and of the cause of our perceptions is taken up under Knowledge, Being, and Mind.

Berkeley's argument leaves subsequent writers to say what, if anything, sensation gives us beyond our own ideas. Hume accepts the empiricist premise that the mind has present to it only perceptions, and draws skeptical rather than theological conclusions about the proof of external objects. Kant seeks a third path: the senses supply the matter of experience, while the mind contributes a priori forms of sensibility without which no object would be given.

Kant redraws the distinction between sense and understanding. Sensibility (Sinnlichkeit) is the faculty by which objects are given to us; understanding is the faculty by which they are thought. Neither alone produces knowledge. Sensibility contributes the matter of sensation (the raw manifold of colors, sounds, pressures) and two pure forms: space and time. Space is the form of outer sense; time is the form of inner sense. These are not properties of things in themselves, nor are they abstracted from experience. They are the conditions under which any experience is possible. We perceive everything in space and time because the mind structures all sensory input through these forms. The distinction between perception or intuition and judgment or reasoning is thus stated in transcendental terms: sense alone is a faculty of intuition, while the understanding is discursive, forming concepts and judgments about what is given.

Things as they are in themselves (noumena) remain beyond sensory reach; we know only appearances (phenomena), the world as shaped by our own faculties. Kant thereby mediates between rationalists who would have knowledge without the senses and empiricists who would have the senses deliver reality unformed. The senses are indispensable (without them we have no content), but they do not give us things as they are independently of the mind's contribution. In this respect he stands with Plato, Aristotle, Aquinas, Descartes, and Spinoza in sharply distinguishing the faculties of sense and understanding, yet he agrees with Aristotle against Descartes and Plato that sense alone, not intellect, is a faculty of intuition. The a priori forms of intuition are treated further under Space and Time; the relation of sense to judgment and knowledge under Knowledge and Judgment.

Kant's Transcendental Aesthetic shifts the traditional problem of sense. The question is no longer only whether the senses are reliable, but what the mind contributes to sensory experience before any empirical judgment occurs. Later psychology and philosophy of perception, including the work of William James, inherit this concern with the active structuring of the sensory field, even when they reject Kant's a priori forms or his distinction between phenomena and noumena.

James approaches sensation as a psychologist rather than as a metaphysician of mind and body. The newborn's sensory world is a "blooming, buzzing confusion," an undifferentiated flux from which experience gradually carves out stable objects through attention, habit, and interest. Sensation is not, on this account, the passive reception of distinct impressions of the sort Hume described, nor the mere filling of a priori forms of the sort Kant described. It is an active process shaped by the organism's needs and history. We do not first sense and then attend; attention is woven into sensing from the start. Two people in the same room may perceive different things because they attend to different features. James also rejects the "psychologist's fallacy" of assuming that the object as known by the psychologist is the same as the object as experienced by the perceiver. The sensation of a baby or an animal is not a degraded version of adult human perception; it is a different kind of experience.

He is no less attentive to physiology. Sensation depends on nerve currents reaching the brain, and the same stimulus can produce different sensations according to the state of the nervous system. Against those who treat sensational consciousness as quasi-material and hardly cognitive while regarding rational consciousness as a deeper mystery, James holds that we can correlate consciousness with the brain's workings only empirically, and that no explanation of how brains give rise to knowing consciousness is yet in sight. Sensation aware of mere qualities involves that mystery as much as thought aware of complex systems. He places sensation, perception, memory, and imagination on one side, and conception, judgment, and reasoning on the other, not as distinct faculties or powers, but as different functions which one and the same mind performs. All forms of consciousness are somehow functions of the brain; yet percept and concept remain, for him, radically distinct forms of consciousness. Perception differs from sensation by the consciousness of further facts associated with the object of sensation; both are varieties of the association of ideas rather than, as some psychologists suggested, species of unconscious reasoning. The relation of sense to memory and imagination is treated under those ideas; the problem of consciousness and the brain under Mind and Soul.

James's account of the stream of experience and the role of attention bridges the philosophical tradition on sense with experimental psychology. His distinction between sensation and perception reworks, in psychological vocabulary, points that Aristotle had made about proper objects and incidental sensibles. The emphasis on the active, interested character of sensing feeds into later phenomenology, Gestalt psychology, and the cognitive sciences, even where those movements abandon his particular dualism of percept and concept.