Plato suggested the idea (later much expanded upon by Plotinus) that a single ineffable Good is the highest principle of all things. The Good was characterized as hyperousia, or “beyond ousia“, where ousia is the same word Aristotle glossed as “what it was to have been” a thing, later misleadingly translated into Latin as substantia or substance. In discussions of neoplatonism, hyperousia used to be often loosely understood as “beyond being”, which is confusing and engendered all sorts of arguments. The problem is that modern people tend to think of being primarily in terms of what is really a kind of brute existence, whereas Plato and Aristotle were more concerned with intelligibility. Even existence in its Greek root has more to do with being able to be picked out than just being there indiscriminately. At any rate, Plato and Aristotle both considered ousia something definable (“intelligible being”, if you will), and they both agreed that the Good as such is undefinable, while drawing different conclusions.
The Platonic Good is the archetype of what Aristotle called an end. Plato held fast to the notion that there should be a single idea of the Good, even if we cannot comprehend or define it. He gave it a quasi-definition as that at which all things aim. Aristotle agreed that all things aim at some good, but pointed out that “good” is used equivocally when we say this. He preferred to say that each thing has its own good that is in principle intelligible. To say something is intelligible for Aristotle still does not mean all details are determined in advance. As Brandom has also emphasized, purpose and contingency are deeply interwoven.
Putting aside this difference between Plato and Aristotle for the moment, I want to suggest that for both of them, a consideration of ends and of what ought to be (and thus of ethics and meta-ethics) implicitly comes first in the order of explanation, before any ontology or any putative facts about what is. Kant made this more explicit as what he called the primacy of practical reason. Plato’s first principle is the Good. Aristotle’s nominally “First cause” of pure actuality or at-work-ness is a generalized end implicit in the ends and proper activities of particular things or kinds.