Enneads V.6: The Superessential Principle Does Not Think; Which is the First
The First Thinking Principle Is the Second Principle
Therefore, the First principle (the One) does not think; the second (Intelligence) is the first thinking principle; the third (the Soul) is the second thinking principle. If the first Principle thought, it would possess an attribute; consequently, instead of occupying the first rank, it would occupy only the second; instead of being One, it would be manifold, and would be all the things that it thought; for it would already be manifold, even if it limited itself to thinking itself.
THE FIRST MUST BE ONE EXCLUSIVELY, WHICH WOULD MAKE THOUGHT IMPOSSIBLE.
3. It might be objected that nothing (in all this) would hinder the first Principle from being both single and manifold. We will answer that the manifold needs a single subject. The manifold cannot exist without the One from which it comes, and in which it is; without the One which is counted the first outside of other things, and which must be considered only in itself. Even on the supposition that it co-exists with other things, it must, none the less, while being taken with the other things with which it is supposed to co-exist, be considered as different from them. Consequently, it must not be considered as co-existing with other things, but be considered as their subject (or, substrate), and as existing in itself, instead of co-existing with the other things of which it is the subject.