"[I]f there is anything uncontentious that can be said about Aristotle's metaphysics, it would seem to be that it is centrally concerned with the question of Being [as opposed to 'beings']. That being said, questions immediately start to multiply in many directions, not least as regards the translation of the most basic terms in which Aristotle addresses the question. A case--and a crucial case--in point is the translation of the pivotal term ousia. In his guide to The Greek Philosophical Vocabulary , J. O. Urmson notes that 'philosophically' ousia means 'nature, essence, substance, being', which, of course, begs the question as to what each of these terms means and how they are to be related to each other.
Are they synonyms, such that Being 'is' identical with nature, essence, or substance? That is... a question we can reformulate as follows: Is Being exhaustively knowable in its manifestation as nature, essence, and substance? Or, does the knowledge that we can attain of nature, essence, and substance give us a full and adequate knowledge of Being-Itself? The basic terms in which these questions are posed are already set out in Aristotle's Metaphysics, a text which would shape the way in which Christian theology itself developed its thinking about the Being of God, despite significant changes such as those resulting from the Christian emphasis on creation out of nothing." - George Pattison, God and Being, p. 39.