What lies behind Aristotle’s declarations that an attribute or feature that is demonstrated to belong to a scientific subject is proper to that subject? The answer is found in APo. 2.8-10, if we understand these chapters as bearing not only on Aristotle theory of definition but also as clarifying the logical structure of demonstration in general. If we identify the basic subjects with what has no different cause, and demonstrable attributes with what do have ‘a different cause’, the definitions of (...) demonstrable attributes necessarily have the minor terms of the appropriate demonstrations in their definitions, for which reason the subjects and demonstrable attributes are coextensive. (shrink)