The Categories is Aristotle’s first attempt to outline a theory of being, in addition to the work’s central focus, which is to provide an account of the ways in which we think about being, and beings. In total, there are ten categories of thought about being, but the core of his theory of being begins with the first category. This is what he called “substance”. This essay will summarise Aristotle’s conception of substance as he presents it in The Categories, briefly explain what distinguishes substance from the other categories, and offer some additional thoughts about the metaphysics of being, in relation to Aristotle’s mentor, Plato.