I examine Aristotle’s reasons in DA I.3 for rejecting the claim that understanding (nous) is a magnitude (megethos), an idea Aristotle associates most explicitly with Plato, who describes nous as a self-moving circle in the Timaeus. Aristotle shows that his definition of soul, on which soul is not a magnitude or body of any kind, can explain perception, thought, and motion better than his predecessor’s materialist accounts. But unlike perception and motion, nous is not actualized through the body nor does it have a bodily organ, which makes nous a very different kind of soul capacity. Earlier thinkers, including Plato, already maintain that nous does not have a bodily organ, but they cannot explain how nous could operate or be a mover without being some sort of body itself. Even in the Timaeus, nous is described as being a kind of magnitude. But if nous were a magnitude of any kind, Aristotle claims it would not be able to think or reason. There is something about being a magnitude qua magnitude that makes reason impossible. His critique of Plato in I.3 prepares the way for his account of nous in DA III.4.