De Anima

Now actual knowledge is identical with the thing known. But potential knowledge is prior in time in the individual, and yet not universally prior even in time. For it is from something actually existent that all which comes into being is derived. And manifestly the sensible object simply brings the faculty of sense which was potential into active exercise: in this transition, in fact, the sense is not acted upon or qualitatively changed. Consequently this must be a different species of motion. For motion is, as we saw, an activity of that which is imperfect; but activity in the absolute sense, that is, activity of that which has reached perfection, is quite distinct.

Sensation, then, is analogous to simple assertion or simple apprehension by thought and, when the sensible thing is pleasant or painful, the pursuit or avoidance of it by the soul is a sort of affirmation or negation. In fact, to feel pleasure or pain is precisely to function with the sensitive mean, acting upon good or evil as such. It is in this that actual avoidance and actual appetition consist: nor is the appetitive faculty distinct from the faculty of avoidance, nor either from the sensitive faculty; though logically they are different.

But to the thinking soul images serve as present sensations: and when it affirms or denies good or evil, it avoids or pursues (this is why the soul never thinks without an image). To give an illustration: the air impresses a certain quality on the pupil of the eye, and this in turn upon something else, and so also with the organ of hearing, while the last thing to be impressed is one and is a single mean, though with a plurality of distinct aspects.

What that is by which the soul judges that sweet is different from warm has been explained above, but must be restated here. It is a unity, but one in the same sense as a boundary point, and its object, the unity by analogy of these two sensibles or their numerical unity, is related to each of the two in turn as they, taken separately, are to each other. For what difference does it make whether we ask how we judge the sensibles that do not fall under the same genus, or the contraries which do, like white and black? Suppose, then, that as A, the white, is to B, the black, so C is to D [that is, as those sensibles are to one another]. It follows, convertendo, that A is to C as B to D. If, then, C and D are attributes of a single subject, the relation between them, like that between A and B, will be that they are one and the same, though the aspects they present are distinct: and so, too, of their single subject. The same would hold, supposing A were the sweet and B the white.

Thus it is the forms which the faculty of thought thinks in mental images. And, as in the region of sense the objects of pursuit and avoidance have been defined for it, so also outside sensation, when engaged with images, it is moved to action: as, for instance, you perceive a beacon and say "That is fire"; and then [by the central sense], seeing it in motion, you recognise that it signals the approach of an enemy. But at other times under the influence of the images or thoughts in the soul you calculate as though you had the objects before your eyes and deliberate about the future in the light of the present. And when you pronounce, just as there in sensation you affirm the pleasant or the painful, here in thought you pursue or avoid: and so in action generally. And, further, what is unrelated to action, as truth and falsehood, is in the same class with the good and the evil. Yet in this, at any rate, they differ, that the former are absolute, the latter relative to some one concerned.

But the abstractions of mathematics, as they are called, the mind thinks as it might conceive the snub-nosed; qua snub-nosed, it would not be conceived apart from flesh, whereas qua hollow, if anyone ever had actually so conceived it, he would have conceived it without the flesh in which the hollowness resides. So, too, when we think of mathematical objects, we conceive them, though not in fact separate from matter, as though they were separate. And, speaking generally, mind in active operation is its objects [when it thinks them]. The question, whether it is possible for the mind to think anything which is unextended without being itself unextended, must for the present be postponed.