De Anima

As to the part of the soul with which it knows and understands, whether such part be separable spatially, or not separable spatially, but only in thought, we have to consider what is its distinctive character and how thinking comes about. Now, if thinking is analogous to perceiving, it will consist in a being acted upon by the object of thought or in something else of this kind. This part of the soul, then, must be impassive, but receptive of the form and potentially like this form, though not identical with it: and, as the faculty of sense is to sensible objects, so must intellect be related to intelligible objects. The mind, then, since it thinks all things, must needs, in the words of Anaxagoras, be unmixed with any, if it is to rule, that is, to know. For by intruding its own form it hinders and obstructs that which is alien to it; hence it has no other nature than this, that it is a capacity. Thus, then, the part of the soul which we call intellect (and by intellect I mean that whereby the soul thinks and conceives) is nothing at all actually before it thinks. Hence, too, we cannot reasonably conceive it to be mixed with the body: for in that case it would acquire some particular quality, cold or heat, or would even have some organ, as the perceptive faculty has. But as a matter of fact it has none. Therefore it has been well said that the soul is a place of forms or ideas: except that this is not true of the whole soul, but only of the soul which can think, and again that the forms are there not in actuality, but potentially. But that the impassivity of sense is different from that of intellect is clear if we look at the sense-organs and at sense. The sense loses its power to perceive, if the sensible object has been too intense: thus it cannot hear sound after very loud noises, and after too powerful colours and odours it can neither see nor smell. But the intellect, when it has been thinking on an object of intense thought, is not less, but even more, able to think of inferior objects. For the perceptive faculty is not independent of body, whereas intellect is separable. But when the intellect has thus become everything in the sense in which one who actually is a scholar is said to be so (which happens so soon as he can exercise his power of himself), even then it is still in one sense but a capacity: not, however, a capacity in the same sense as before it learned or discovered. And, moreover, at this stage intellect is capable of thinking itself.

Now, since magnitude is not the same as the quiddity of magnitude, nor water the same as the quiddity of water (and so also of many other things, though not of all, the thing and its quiddity being in some cases the same), we judge the quiddity of flesh and flesh itself either with different instruments or with the same instrument in different relations. For flesh is never found apart from matter, but, like "snub-nosed," it is a particular form in a particular matter. It is, then, with the faculty of sense that we discriminate heat and cold and all those qualities of which flesh is a certain proportion. But it is with another faculty, either separate from sense, or related to it as the bent line when it is straightened out is related to its former self, that we discriminate the quiddity of flesh. Again, when we come to the abstractions of mathematics, the straight answers to the quality "snub-nosed," being never found apart from extension. But the straightness of that which is straight, always supposing that the straight is not the same as straightness, is something distinct: we may, for instance, assume the definition of straightness to be duality. It is, then, with another instrument or with the same instrument in another relation that we judge it. In general, therefore, to the separation of the things from their matter corresponds a difference in the operations of the intellect.

The question might arise: assuming that the mind is something simple and impassive and, in the words of Anaxagoras, has nothing in common with anything else, how will it think, if to think is to be acted upon? For it is in so far as two things have something in common that the one of them is supposed to act and the other to be acted upon. Again, can mind itself be its own object? For then either its other objects will have mind in them, if it is not through something else, but in itself, that mind is capable of being thought, and if to be so capable is everywhere specifically one and the same; or else the mind will have some ingredient in its composition which makes it, like the rest, an object of thought. Or shall we recall our old distinction between two meanings of the phrase "to be acted upon in virtue of a" "common element," and say that the mind is in a manner potentially all objects of thought, but is actually none of them until it thinks: potentially in the same sense as in a tablet which has nothing actually written upon it the writing exists potentially? This is exactly the case with the mind. Moreover, the mind itself is included among the objects which can be thought. For where the objects are immaterial that which thinks and that which is thought are identical. Speculative knowledge and its object are identical. (We must, however, enquire why we do not think always.) On the other hand, in things containing matter each of the objects of thought is present potentially. Consequently material objects will not have mind in them, for the mind is the power of becoming such objects without their matter; whereas the mind will have the attribute of being its own object.