Aristotle thought we should be ethically committed to the idea that becoming or process is in principle intelligible. An often misunderstood part of his program for showing this was to emphasize that our very talk about change presupposes that we can pick out relative stability or persistence somewhere in the context.
This is a careful, minimalist assertion of moments of weak unity or stable points of attraction within the flux, intended only to deny Plato’s strong pessimistic denial of the knowability of any such points of attraction. It has nothing to do with some direct incarnation of metaphysically given essences. (See Aristotelian Identity; Identity, Isomorphism; Equivocal Determination.)
Plato recommends an ethic of quasi-skeptical honesty about the epistemic difficulties involved in any practical judgment or view of the world. Aristotle deeply respects the intellectual honesty promoted by Plato about what we do not know in life, while putting a higher value on things subject to becoming.
Ousia (traditionally “substance”, or more accurately “what it was to have been” a thing) is Aristotle’s preferred alternative to talking about Being (either as utterly general or as utterly unique). It redirects our attention away from these sterile extremes toward a fertile middle ground where conceptual articulation is possible. In the Metaphysics, it undergoes a major dialectical development through many senses, including a division into actuality and potentiality. (See also Abstract and Concrete; Being, Existence; Aristotelian Dialectic; Free Will and Determinism.)
Later authors developed increasingly rigidified reinterpretations of Aristotelian substance, such as the Latin medieval notion of substantial form. This laid the basis for early modern redefinitions of substance in terms of some kind of logical identity.