Aristotle is sometimes committed to a pattern of inference that moves from complexity offunctioning to complexity in the entity's metaphysical structure. This article argues that Aristotle rejects this inference in the case of the basic essence, the ultimate differentia that determines the kind to which the entity belongs. Specifically, the functional difference between active and passive reasoning in humans is not matched in the structure of the basic human essence. The basic essence is an immediate unity in the strong sense (...) that it is wholly without structural parts. Complex rational functioning emerges from a metaphysically simple basic essence. (shrink)