 The Categories sets out important logical distinctions between different kinds of attribute, but it does not enter into a metaphysical analysis of substance itself. This takes place mainly in Metaphysics, Book Z. In the latter, the analysis of substances in terms of form and matter is developed, whereas these notions have no place in Categories. The distinction has led some commentators to talk of Aristotle’s ‘two systems’, containing two radically different conceptions of substance (Graham 1987). In the earlier, Categories, substances are simply individuals; in the later work they are complexes of form and matter. Whether this represents a change of view, or whether the purposes of the Categories simply did not require reference to the metaphysical analysis of substance is a moot point. It seems unlikely, however, given Aristotle’s Platonic background, that his early thought was oblivious to the role of form in substance. Whichever interpretation of the development of Aristotle’s thought is correct, the introduction of substantial form is what gives the fully developed Aristotelian account of substance.