De Anima

And now let us sum up what has been said concerning the soul by repeating that in a manner the soul is all existent things. For they are all either objects of sensation or objects of thought; and knowledge and sensation are in a manner identical with their respective objects. How this is so requires to be explained. Knowledge and sensation, then, are subdivided to correspond to the things. Potential knowledge and sensation answer to things which are potential, actual knowledge and sensation to things which are actual, while the sensitive and the cognitive faculties in the soul are potentially these objects; I mean, object of sensation and object of cognition respectively. It follows that the faculties must be identical, if not with the things themselves, then with their forms. The things themselves they are not, for it is not the stone which is in the soul, but the form of the stone. So that there is an analogy between the soul and the hand; for, as the hand is the instrument of instruments, so the intellect is the form of forms and sensation the form of sensibles. But, since, apart from sensible magnitudes there is nothing, as it would seem, independently existent, it is in the sensible forms that the intelligible forms exist, both the abstractions of mathematics, as they are called, and all the qualities and attributes of sensible things. And for this reason, as without sensation a man would not learn or understand anything, so at the very time when he is actually thinking he must have an image before him¹. For mental images are like present sensations, except that they are immaterial. Imagination, however, is distinct from affirmation and negation, for it needs a combination of notions to constitute truth or falsehood. But, it may be asked, how will the simplest notions differ in character from mental images? I reply that neither these nor the rest of our notions are images, but that they cannot dispense with images.