Full Idea
Perception is of particular things, but knowledge is of universals, which are in a way in the soul itself. Thus a man may think whenever he wishes, but not perceive.
Gist of Idea
We may think when we wish, but not perceive, because universals are within the mind
Source
Aristotle (De Anima [c.329 BCE], 417b22)
Book Reference
Aristotle: 'De Anima (On the Soul)', ed/tr. Lawson-Tancred,H.C. [Penguin 1986], p.171