 The division between being said of and being in—that is, between substance concepts and other properties—seems intuitively clear enough until one remembers that substance concepts are complex and are definable in terms of other properties. Thus, ‘man’ is defined as ‘featherless biped’ or ‘rational animal’, and it would seem that ‘featherless’, ‘two-footed’ or ‘rational’ are properties and hence the kinds of things that are in objects. Aristotle denies that this is so when they enter into the definition of a substance. The features that specifically make an object the kind of substance that it is, are called differentiae, and Aristotle says