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Those features or aspects of reality that can be perceived by the senses; in scholastic terminology, the proper objects of the various senses. Aristotle and the scholastics distinguish between the "proper" and the "common" sensibles, whereas most modern thinkers since the time of J. locke regard the sensibles as "primary" and "secondary" qualities. Sense qualities that can be perceived by a single external sense, such as color, sound, taste, smell and tactile sensations, are the proper sensibles or secondary qualities, while qualities that are perceived by more than one sense, such as extended surface, shape, volume, number, rest and motion, are the common sensibles or primary qualities.

Perception. The common sensibles are not apprehended in the abstract by any joint action of the senses. The individual, concrete data from which the intellect abstracts such thought objects as magnitude or three-dimensional extension are complex data gleaned from more than one external sense. These composite sense data do not contain any sense element beyond the proper sensibles contributed by the separate senses in cooperation. Thus the perception of a common sensible involves the conscious coordination of the proper objects of vision and of tactile, muscular and motor sensations. Yet common sensibles are real objects of sense awareness, sensibilia per se, unified by the integrating internal sense faculty known as the central sense, the sensus communis of the Aristotelian-Thomistic tradition. Both the common and the proper sensibles, being direct data of sense perception, are percepts, not concepts. While the senses reveal the concrete complex of perceived qualities, the intellect apprehends the knowable object as a real substance having a specific nature or essence as determined by the perceived qualities.

The sensory and intellectual activities involved in perceptual and conceptual processes cannot be isolated as though sense perception in the human adult were prior to and independent of intellection in any simple and unqualified sense. The mind as a principle of intelligence possesses the tendency to form sensory data into some kind of perceptual whole or pattern, to render the extra-mental environment intelligible. The distinction between primary and secondary qualities should therefore not be pushed to excess, as though any sensible quality can be perceived independently of all relations to the senses. However, to attain a philosophical analysis of these relations, one must strive for some knowledge of the absolute terms that are related.

Objectivity. Thinkers in the Aristotelian-Thomistic tradition consider both primary and secondary qualities to have objective existence in extramental reality. galileo, Locke and many modern critical realists, on the other hand, hold that primary qualities have objective existence but that secondary qualities do not.

To the realist, the resistance one directly encounters from external objects is something proper to the objects themselves. Granted that vibrations of an atmospheric medium may be quite unlike the sensations of sight and hearing, it is nonetheless by the senses that the vibrations are discovered. The senses do not judge, they merely report the presence of certain sense impressions. Reason judges whether anything objective in nature corresponds to the sensations and perceptions. Sense knowledge is thus not intended as an objective report on the constitution of material reality. Its function is to offer an awareness of reality, a stimulus to intellection.

Phenomena or appearances presuppose that which appears. Thus they constitute the object of the external senses as a reality, a being, regardless of what kind; if they did not, one would be forced to conclude that perception terminates in absolutely nothing. One can know the sense object as a reality, a being, whether its esse is its percipi or not, whether it is internal or external, whether the one perceiving has caused it or not.

The material object of every sense is a being; by every sense man knows being, even though not precisely as being. By sight he knows a being as colored, by hearing as sounding. What man sees is a colored being. Thus the object of the external senses has absolute epistemological value for the intellect under the aspect of being.

All forms of phenomenalism deny that the object has any epistemological value whatever. Objective or immediate realism attributes absolute value to the object, because of itself the object satisfies man's desire to know the real-as-it-is. Critical or mediate realism holds that the object has only relative epistemological value as a means by which one may know the real-as-it-is, though of itself it cannot satisfy this appetite.

Critical Realism. Is the object that appears red to the human knower "really" red? If by red is meant the quality exactly as it is perceived, critical realists would answer in the negative. If by "red" is meant that quality really in an object whose very nature it is to be seen as red by a human being in the condition of conscious awareness, the answer would be in the affirmative. The red that is sensed does not exist apart from the human knower exactly as he senses it. Independently of his perception, however, it exists materiallyinseparably combined with the light energy of which it is only a distinguishable aspect. Thus it is not true that red exists only in the knower, with no objective quality of redness in the lighted object. In the very atomic-molecular structure of the object is the quality of objective color, such that, when illuminated and perceived by a sensing subject, the object can be said to be colored.

Several basic objective aspects are revealed in all incidents of general perceptual response. For example, such a response presupposes an interaction of the end organ and its immediate environment that involves an exchange of energy. The specific reaction occurring in the end organ is not initiated by any and every energy influence in the environment, but only by those within a rather well defined range of energies. Each of the different possible energies falling within this range can be said to possess the common property of being able to interact with the sensory organ in question. These factors, it would seem, are sufficient to ground the objectivity of external sensation.

According to critical realists, the sensation "red" is the same as the red appropriate to material things but in a derivative sense. The sensation "red" is analogous to what the physicist calls objective red. Although red exists in the human perceiver in a different way from that in which it is found in a red flower, there is a real resemblancethe experience red has an objective foundation. There is an analogous relationship between the flower's redness and the redness in the human perceiver. Reality is thus neither a univocal cause nor an equivocal cause; it is an analogous cause of human sensation. Objective color, sound, tactile, olfactory and gustatory qualities are translated into perceptual experiences so that intellect, the human power of generalizing thought, may possess knowledge of the nature of reality. Sensation is objective in that it attains, contacts and presents objectively existing reality; its "presentation," in turn, serves as an analogous cause of intellectual knowledge.

Only in experience can man's intellectual knowledge be resolved. The external senses are ultimate among his cognitive powers, and as such, are his only source of experiential knowledge; either he contacts real beings through them or he can never contact them at all. This resolution to things themselves is the unique source of certitude in the human order of knowing.

Quality. St. thomas aquinas ascribes to the physical cause of sensation a formal and qualitative aspect rather than a quantitative one; he does this on the basis of his metaphysics, not on the basis of an outmoded science. Only form or quality is act, and only act causes. Consequently, all the physical realities capable of stimulating human cognitive powers are referred to as sensible qualities. quantity conditions this causality but does not constitute it. Aquinas considers the common sensibles to be reducible to quantity, but he holds that the proper sensibles first affect the senses since they are the qualities that cause alteration. Since physical qualities exist only in quantified bodies, quantity conditions the efficacy of the qualities through such modifications as those of dimensions, space, motion and rest. Existing reality is always the cause of knowledge, but its causality is exerted through such quantitative modifications, and these in turn reveal the inner nature of reality in its dynamic and formal aspect.

The physical union of the human knower with the physical object through the instrumentality of quantified qualities is therefore the initial step in sense knowledge. Subject and object here become in some way identified and this even before the immanent act of sensing takes place.

See Also: sensation; sense knowledge; knowledge; perception.

Bibliography: l. m. rÉgis, Epistemology, tr. i. c. byrne (New York 1959). r. f. o'neill, Theories of Knowledge (Englewood Cliffs, N.J. 1960). d. m. armstrong, Perception and the Physical World (New York 1961). d. w. hamlyn, Sensation and Perception (New York 1961). w. f. sellars, Science, Perception and Reality (London 1963). w. von buddenbrock, The Senses, tr. f. gaynor (Ann Arbor 1958). f. a. geldard, The Human Senses (New York 1953).

[m. m. bach]