 In the Categories, Aristotle was concerned with subjects of predication: what are the things we talk about, and ascribe properties to? In the Physics, his concern is with subjects of change: what is it that bears (at different times) contrary predicates and persists through a process of change? But there is an obvious connection between these conceptions of a subject, since a subject of change must have one predicate belonging to it at one time that does not belong to it at another time. Subjects of change, that is, are also subjects of predication. (The converse is not true: numbers are subjects of predication—six is even, seven is prime—but not of change.)