De Anima

Now that these points have been determined, let us proceed to a general discussion of all sensation. As above remarked, sensation consists in being moved and acted upon, for it is held to be a species of qualitative change. Some add that like is in fact acted upon by like. How far this is possible or impossible we have explained in the general discussion of action and passivity. The question arises why there is no sensation of the senses themselves: that is, why they produce no sensation apart from external sensibles, though the senses contain fire, earth and the other elements, which are the objects of sensation either in themselves or through their attributes. Evidently it Sensation follows that the faculty of sensible perception exists not in activity, but only in potentiality. Hence it must be here as with the fuel which does not burn of and in itself without something to make it burn; otherwise it would kindle itself and would have no need of the fire which is actually existent. Now to have sensation has two meanings: we use the terms hearing and seeing "of that which has the capacity to hear and see, even though it be at the time asleep, just as we do of that which already actually hears and sees. And therefore sensation, too, will have two meanings: it may mean either potential or actual sensation. Similarly with having sensation, whether potential or actual.

Let us then first proceed on the assumption that to be acted Agent ana Upon or moved is identical with active operation. For movement is in fact active operation of some sort, though incomplete, as we have elsewhere explained. But in every case things are acted upon and moved by an agent in actual operation. It follows that in one sense what is acted upon is acted upon by what is like it, in another sense by what is unlike it, as we have explained. That is to say, while being acted upon it is unlike, after it has been acted upon it is like the agent.

We must also draw a distinction in regard to the terms potentiality and actuality: at present we are using them without qualification. For instance, we may use the term wise, firstly, in the sense in which we might speak of man as wise, because man is one of the genus of beings which are wise and have wisdom; secondly, in the sense in which we at once call the man wise who has learnt, say, grammar. Now of these two men each possesses the capacity, but in a different sense: the one because the genus to which he belongs, that is to say, his matter, is potentially wise; the other because he is capable, if he chose, of applying the wisdom he has acquired, provided there is nothing external to hinder. Whereas he who is at the moment exercising his wisdom is in actuality and is wise in the proper sense of the term: for example, he knows the A before him. Thus the first two are both potentially wise: the first becomes wise actually after he has undergone qualitative change through instruction and often after transition from the reverse condition; while in the latter case it is by another kind of transition that the man passes from the mere possession, without the use, of sensation or grammar to the use of it.

To suffer or be acted upon, too, is a term of more than one meaning. Sometimes it means a sort of destruction by the contrary, sometimes it is rather a preservation of what is potentially existent by what is actually existent and like it, so far as likeness holds of potentiality when compared with actuality. For it is by exercise of knowledge that the possessor of knowledge becomes such in actuality: and this either is no qualitative change (for the thing develops into its own nature and actuality), or else is qualitative change of a different sort. Hence it is not right to say that that which thinks undergoes change when it thinks, any more than that the builder undergoes change when he builds. That, then, which works the change from potential existence to actuality in a thinking and intelligent being should properly receive a different name and not be called instruction: while that which learns and is brought from potential to actual knowledge by that which is in actuality and capable of instructing should either not be said to suffer or be acted upon at

all, or else two modes of change should be assumed, one to the negative states and the other to the normal habits and the true nature.

In the sensitive subject the first change is due to the parent: once generated it possesses sensation exactly in the same sense as we possess knowledge. And to have actual sensation corresponds to exercise of knowledge. There is this difference, however, that in the one case the causes of the activity are external: as, for instance, the objects of sight, hearing and the other senses. The reason is that actual sensation is always of particulars, while knowledge is of universals: and these universals are, in a manner, in the soul itself. Hence it is in our power to think whenever we please, but sensation is not in our power: for the presence of the sensible object is necessary. It is much the same with the sciences which deal with sensible objects; and for the same reason, namely, that sensibles are particulars and are external.

But we shall have a further opportunity of making this clear hereafter. For the present let us be content to have established that of the two meanings of potentiality, the one according to which a child might be called potentially a general, and the other according to which a man of full age might be so called, it is the latter which applies to the faculty of sense-perception. But as this distinction has no word to mark it, although the fact and the nature of the distinction have been established, we are compelled to use the terms to suffer or be acted upon and to be qualitatively changed as if they were the proper terms. Now, as has been explained, the sensitive faculty is potentially such as the sensible object is in actuality. While it is being acted upon, it is not yet similar, but, when once it has been acted upon, it is assimilated and has the same character as the sensible object.