Having traced mind's essence, operation, and objects, Aristotle needs to give more attention to thinking's initiation. Possession of knowledge puts universals in the soul that provide for thinking at wish. But what can provoke such wish, and in general how does dispositional knowledge lead to actual thinking? This is the role of phantasmata. This has been hinted at previously (see 403a8–10, 427b16, and perhaps 430a24–25), but he now needs to offer fuller consideration. He should not merely resort to phantasmata as a desperate assumption but argue for their role in thinking. In this chapter Aristotle begins with an obvious case, practical thinking and its initiation by phantasia, and then goes on to other types of intellect. His guide is the way sense perception of the pleasant or painful causes desire and pursuit or avoidance. As sense has desire and motion concatenated with it, so practical thinking has phantasmata, desire, and action linking with it. He then expands upon this case by further comparison of mind with sense. If sense is a unity, because (1) the action on the medium affects the sense and causes desire and pursuit or avoidance and (2) the five senses lead to a central sense and thus are subfaculties of the sense faculty, mind also is a unity requiring phantasmata along with its thinking and the mind is composed of subfaculties for practical thinking, theoretical thinking, and mathematical thinking. What applies to practical thinking will apply analogously to these other sorts of thinking.