From Plotinus by Lloyd P Gerson:
Where a creation metaphysics such as that of Aquinas differs from Plotinian metaphysics is in its claim that the [ἀρχή] of all is the sole cause not of the existence but of the being of everything else, hence of existence and[i] essence. Accordingly, Aquinas must say that God is not just virtually all things but eminently all things as well. That is, every predicate that belongs to complexes belongs to their simple cause in a higher mode of being. In short, [οὐσία] cannot be a real [ἀρχή] for him. Were it so, this would compromise the omnipotence of God. By contrast, Plotinus is less concerned with preserving omnipotence than he is with preserving the unqualified simplicity of the first [ἀρχή]. One way to express the differences between the two in the matter of omnipotence is to say that although they agree that the [ἀρχή] of all of logical possibility is grounded in the second [ἀρχή], Intellect, whereas Aquinas will want to say that logical possibility and impossibility are ultimately to be accounted for by the first principle, God…
Plotinus certainly emphasizes the idea of supreme power in his characterizations of the One. What precisely is this power supposed to be? How is power to be analyzed? First, as the text in V.3.15.33-6 indicates, Plotinus takes over Aristotle’s distinction between a passive and an active power, identifying the latter with the One (see Metaphysics 9.1.1046a19ff.). Second, the power of the One is indicated by its results, namely, the existence of everything that can exist. But since the existence of everything that exists is not identical with the One, the One’s power is evidently that in virtue of which everything else exists. But this power is in no way really distinguishable from anything else in the One, else its perfect simplicity would be destroyed. The One is virtually everything else, including Intellect. Since, however, the One does not confer existence on a waiting recipient (since the recipient would then already[i] exist), the power of the One is not the power to bring about a substantial change like generation. It is something even more radical than this. It is the power to cause to exist everything that can exist, including eternal Intellect and Forms. With the causal power of the One even eternal truths would not exist.
The [i]’s denote italicized words in the original.