This paper offers the first systematic analysis of Aristotle’s discussion of ontological priority (what he calls “priority in substance” [proteron kata ousian] or “priority in nature” [proteron tēi phusei]) in Cat. 12 and 13. On my reading, Aristotle puts forward two non-reducible criteria for ontological priority that are individually sufficient and disjunctively necessary and sufficient for ontological priority. According to the first criterion, A is ontologically prior to B if necessarily B’s being implies A’s being but not necessarily A’s being implies B’s being. According to the second criterion, A is ontologically prior to B if necessarily A’s being and B’s being symmetrically imply each other and A’s being is in some way a cause (aitionpōs) of B’s being. I argue that in both criteria, “being” can be understood either existentially (“existing”) or veridically (“being true”) but never essentially (“being what it is”).

I argue that a case-by-case decision must be made regarding which reading is the most appropriate for a given passage. Ontological priority is captured by means of a conditional where “einai” (being) shows up in both the antecedent and the consequent. I show that it can be the case that “einai” should be read existentially in the antecedent and predicatively in the consequent, and vice versa. So, my reading of the passages on ontological priority in Aristotle does not fit naturally with the classification of the ontological priority readings as either existential or predicative (especially essentialist). In addition, I argue that these readings are more closely connected for Aristotle than they are for us, since he takes it that a predicative implication necessarily implies an existential implication.