Plato and Aristotle give different answers to the question ‘What are the substances (ousiai)?’. One way Aristotle defends his answer is by arguing that his candidate substances – particulars such as Socrates or Callias – better satisfy the criteria for substance than do Plato's candidates – eternal, unchanging, nonsensible universals called ‘Forms’. This defense goes along with another. For Aristotle disagrees with Plato, not only about the candidates, but also about the criteria, for substance: one reason Plato fastens on to the wrong candidates is that he focuses on some of the wrong criteria.
Aristotle mounts his defense in different ways in the Categories and Metaphysics. In both works he defends the priority of particulars. In the Cat., however, their nature is left unanalysed; and their priority is defended largely by appeal to unPlatonic criteria. In the Met., by contrast, Aristotle analyzes particulars into compound, form, and matter. Socrates, for example, may be viewed as a compound of his form (his soul) and his matter (his body); or he may be viewed as his form or soul. Further, Aristotle now invokes additional, Platonic criteria for substance; and this leads him to argue that it is Socrates as form that counts as primary substance; the primary substances are individual forms.