Plotinus on the Firmest Principle

In Ennead VI 5 [23], 1, Plotinus calls the «firmest principle» the «common notion» that says that «that which is one and identical in number is everywhere, and simultaneously whole».1 The expression «firmest principle» comes from Metaphysics Γ 3, where Aristotle uses it, along with the adjective «unhypothetical»,2 to describe the Principle of Non-Contradiction (PNC), i.e., the principle that «it is impossible for the same thing to belong and not to belong to the same thing, at the same time and in the same respect».3 By calling the PNC the «firmest principle» and «unhypothetical», Aristotle subtly transfers to it features that Plato had reserved for the Idea of the Good in Republic VI-VII, while engaging in the same breath with Plato’s own formulation of the PNC in Republic IV. This essay unpacks how Plotinus in Ennead VI 5 [23], 1 reacts to this Aristotelian twist to defend Platonism. We shall see that his defense is not, mechanically, a series of replies to Aristotle’s objections and a reaffirmation of Plato’s positions. Rather, through the development of both a distinctive interpretation of claims from the Parmenides and original accounts of the inborn notions of human rationality and of the causality of the intelligibles, Plotinus sets out a view whereby the firmest principle is the claim of the undivided omnipresence in the manifold of what is one and identical. This principle was destined to become a key axiom of Neoplatonist metaphysics.