What one means by an external sense is obvious: one of the five means by which we are directly aware of the world around us. They are sight, hearing, smell, taste and touch. For Aquinas, the senses put in direct contact with external reality by making us aware of concrete individual qualities of material objects. The nature of the sense object defines and specifies each of the sense powers. For example, color defines sight, sound hearing, etc. Thus, sight just is that power by which we are aware of the colors of material things. (On the external sense, see also Aristotle. De anima II, 5; III, 2. Aquinas, ST I, 78, 3; In de anima II, lect.14; In de sensu at sensato.)
Objects of the external senses.
By the general object of a sense is meant that which in any way can be apprehended by a sense. It is called a sensible. The object of an external sense is called an external sensible.(See De anima II, 5 (418a 5-25); In de anima II, lect.13.)
- Per se (essential)
- Proper sensible: It directly refers to one sense. Color, e.g., is the proper sensible of sight.
- Common sensible: It is an essential sensible which can be sensed by more than one sense. Size, e.g, can be sensed by sight and touch. Common sensibles are not sensed immediately, but by means of a proper sensible. e.g., Peter’s size, by means of his color. Motion, rst, number, size, and shape are common sensibles. In modern philosophy they are called “primary qualities”. What we have called “proper sensibles”, modern philosophy (Galileo, Descartes,etc.) calls “secondary qualities”.
- Per accidens (accidental). The accidental (per accidens) sensible is not directly an object of the sense, but the object of another cognitive potency. But that potency is helped by that which the external sense grasps. We say, for example, that the sugar is sweet. Sugar is the object of the intellect, which is apprehended with the help of the external senses (which apprehend its sweetness and white color). The sugar is accidentally (per accidens) the object of the senses: per se (essentially), it is the object of the intellect.
The objects of the senses are accidents of beings, like color, sound, and so forth. These objects have to be immaterial because they are received without matter (i.e., without prime matter), though with some of the conditions of matter: “A sense can receive species without matter, although still under the conditions of matter; the intellect receives its species entirely purified of such conditions.”(De ver. 2, 2.) “For as things exist in sensation, they are free indeed from matter, but are not without their individuating material condition, nor apart from a bodily organ. For sensation is of objects in the particular, but intellection of objects universally.”(In de anima, n.284.)
Veracity of the external senses.
The senses deceive us, but not when they are concerned with their proper sensibles (e.g., sight and red color). They can deceive us regarding the common sensibles (like the size of a star) and the accidental sensibles (as when oleo is mistaken for the high priced spread). Consider the following statements of Aristotle and Aquinas:
Now I call that the proper object of each sense which does not fall within the ambit of another sense, and about which there can be no mistake, as sight is of color, and hearing of sound, and taste of savor, while touch has several different objects. Each particular sense can discern these proper objects without deception; thus sight errs, not as to colors, nor hearing as to sound; though it might err about what is colored or where it is, or about what is giving forth a sound. This, then, is what is meant by the proper objects of particular senses.Aristotle, De anima, II, 5 (418a 12-20).
But the senses can be deceived both about objects incidentally sensible and about objects common to several sense. Thus sight would prove fallible were one to attempt to judge by sight what a colored thing was or where it was; and hearing likewise if one tried to determine by hearing alone what was causing a sound.Aquinas,In de anima, II, lect.13, n.385.
And if someone raises the objection that error sometimes arises even with regard to proper sensible, his answer is that this is attributable not to the senses, but to the imagination; for when the imagination is subject to some sort of abnormality, it sometimes happens that the object apprehended by a sense enters the imagination in a different way than it was apprehended by the sense. This is evident, for example, in the case of madness, in anyone whose organ of imagination has been injured.Aquinas, In meta, n.693.
Sensation and the composite.
Sensation belongs neither to the soul, nor to the body, but to the composite. Therefore the sensitive power is in the composite as its subject. Some operations of the soul are performed by means of corporeal organs; as sight by the eyes, and hearing by the ears. Sense powers depend upon the body for their existence and operation.
Perception and sensation.
Sensation signifies solely an act of one of the senses, without that which is added to sensation by memory, intellect, and other senses. Perception includes everything which is added to a sensation. For example, if I hear a noise, I may interpret it as the sound of two cats. Two cats indicates my perception; the noise, my sensation.