In book 3, chapter 3 of his On the Soul, Aristotle gives several arguments meant to demonstrate the type non-identity of belief and imagination. Each of these arguments rewards study, but this discussion will focus on one in particular, perhaps the most puzzling. The argument concerns the relation between truth and control. Belief is connected with truth and falsehood in a way that imagination is not, and that in turn means that we can control what we imagine in a way that we cannot control what we believe. Here is Aristotle’s argument in full:
(1) It’s clear as well that it [sc. imagination] is not the same thing as belief. For that particular affection is up to us, whenever we want (since it’s possible to produce [sc. images] before one’s eyes, like those forming mental images and putting them into mnemonic systems), while forming a belief is not up to us: for it must either be true or false. (De Anima 3.3.427b16-20)
Aristotle’s reasoning is, to put it mildly, lacunose. The reason (gar) that belief is not under our control (up to us, or eph’ êmin) is that it must either be true or false. He declines, however, to say what the connection is between truth and control. My project in what follows is to try to make sense of this argument. There is a family of interpretation which I call the “traditional reading.” On this reading, belief is not up to us in virtue of its simply having a truth value. This interpretation proves inadequate, and I propose in its place that belief, for Aristotle, has a normative connection to truth and falsehood. This connection imposes restrictions on belief formation which render belief not up to us.
Flora, Ian McCready, "Aristotle on [Part of] the Difference between Belief and Imagination" (2009). The Society for Ancient Greek Philosophy Newsletter. 345.