Book II, Chapter 12

In general, concerning all sense-perception, we must grasp that sense is that which is receptive of the sensible forms without the matter, as the wax receives the imprint of the signet-ring without the iron or the gold: it takes the golden or the bronze imprint, but not qua gold or bronze. And similarly the sense of each thing is affected by that which has colour or flavour or sound, yet not insofar as each of those is said to be what it is, but insofar as it is of such a quality, and in accordance with the ratio (logos).

The primary sense-organ is that in which such a potentiality resides. Now the organ and the potentiality are the same, but their being is different; for that which perceives would be a magnitude of some sort, whereas what it is to be a sense-faculty is not a magnitude, nor is the sense a magnitude, but a certain ratio and potentiality of that organ.

From this it is also evident why excesses among sensible objects destroy the sense-organs. For if the movement is stronger than the sense-organ, the ratio is dissolved — and this was just what the sense was — exactly as the concord and the pitch are destroyed when the strings are struck too violently.

It is evident too why plants do not perceive, although they have a certain part of soul and are affected in some way by tangible things: for they are both cooled and heated. The reason is that they do not possess a mean, nor such a principle as can receive the forms of sensible things, but they are affected together with the matter.

One might raise the difficulty whether that which is incapable of smelling could be affected in any way by an odour, or that which is unable to see by a colour; and similarly in the other cases. But if the object of smell is odour, then, if odour produces anything, it produces smelling. Hence none of the things incapable of smelling can be affected by odour; and the same account holds also in the other cases. Nor, indeed, can those that are capable be affected, except insofar as each is percipient.

At the same time this is clear in the following way too. For neither light and darkness, nor sound, nor odour has any effect upon bodies; rather it is the things in which they reside that do so — as the air that accompanies thunder splits the timber. But tangible things and flavours do act; for if not, by what would inanimate things be affected and altered? Will those others, then, also act? Or is it rather that not every body is capable of being affected by odour and sound, and that the things affected are indefinite and do not persist, as air does not? For air smells, as though it had been affected in some way. What, then, is smelling over and above being affected in some way? Or is it that to smell is to perceive, whereas the air, once affected, quickly becomes perceptible?