Book excerpt: The main objective of this Element is to reconstruct Aristotle's view on the nature of ontological priority in the Categories. Over the last three decades, investigations into ontological dependence and priority have become a major concern in contemporary metaphysics. Many see Aristotle as the originator of these discussions and, as a consequence, there is considerable interest in his own account of ontological dependence. In light of the renewed interest in Aristotelian metaphysics, it will be worthwhile - both historically and systematically - to return to Aristotle himself and to see how he himself conceived of ontological priority (what he calls 'priority in substance' [proteron kata ousian] or 'priority in nature' [proteron tēi phusei]), which is to be understood as a form of asymmetric ontological dependence.