Thomas Aquinas, Commentary on Aristotle's De Anima

NOW, RECAPITULATING WHAT WE HAVE SAID about the soul, let us repeat that the soul is somehow all that exists; for things are either sensible or intelligible; and knowledge is in some way the knowable, and sensation is the sense object. But how this is so we must enquire. For knowledge and sensation are divided into realities: the potential answers to things that are really in potency, the actual to things really in act. In the soul the sensitive faculty and that which can know are these [things] in potency; the latter [faculty] the understandable, the former the sensible.§§ 787-8

Now they must be the things themselves or their forms. But they are certainly not the things themselves: no stone is in the soul, but only its form. Thus the soul is like a hand: the hand is the instrument that includes other instruments, and the intellect is the form that includes other forms, and sense the form that includes sensible things.§§ 789-90

But since there are no real things apart from things sensible and extended (so it would seem), then in the sensible species are the intelligible, both what are predicated as a result of abstraction and whatever qualities and habits are found in sensible things. And on this account, what does not perceive by sensation acquires no knowledge or understanding at all; and when thinking occurs there must be at the same time a phantasm as its object; for phantasms are as sense objects save that they are without matter.§§ 791-2

Imagination is other than affirmation and negation: for the true and the false are a combining of intellectual concepts.§ 793

What difference have the primary concepts that they should not be phantasms? But neither are the others phantasms, though they do not exist apart from phantasms.§ 794