 It would seem that these properties belong to objects in different ways when they are part of a definition from when they are not. The issue is what constitutes the unity of the species or secondary substance: why is it not just a collection of properties and, if it is just such a collection, why is it so different from any other collection of properties? In order to begin to see how Aristotle tackled this problem we need the apparatus of form and matter, which does not appear in the Categories. (We will see when discussing contemporary theories in  section 3.1,  however, that it is not clear that even now can we provide a wholly non-circular account of substance.)