To explain how the Nous could be identical with a number of distinct ideas was a difficult problem. We shall have to show at a more advanced stage of our exposition how Plotinus endeavoured to solve it with the help of Plato’s _Sophist_. In the essay where his theory is first put forward, he cuts the knot by asserting that each idea virtually contains every other, while each in its actual and separate existence is, so to speak, an independent Nous. But correlation is not identity; and to say that each idea thinks itself is not to explain how the same subject can think, and in thinking be identical with all. The personal identity of the thinking subject still stands in unreconciled opposition to the multitude of thoughts which it entertains, whether successively or in a single intuition. Of two things one: either the unity of the Nous or the diversity of its ideas must be sacrificed. Plotinus evades the alternative by a kind of three-card trick. Sometimes his ideal unity is to be found under the notion of convergence to a common centre, sometimes under the notion of participation in a common property, sometimes under the notion of mutual equivalence. The confusion was partly inherited from Aristotle. When discussing the psychology of that philosopher, we showed that his active Nous is no other than the idea of which we are at any moment actually conscious. Our own reason is the passive Nous, whose identity is lost in the multiplicity of objects with which it becomes identified in turn. But Aristotle was careful not to let the personality of God, or the supreme Nous, be endangered by resolving it into the totality of substantial forms which constitute Nature. God is self-conscious in the strictest sense. He thinks nothing but himself. Again, the subjective starting-point of Plotinus may have affected his conception of the universal Nous. A single individual may isolate himself from his fellows in so far as he is a sentient being; he cannot do so in so far as he is a rational being. His reason always addresses itself to the reason of some one else—a fact nowhere brought out so clearly as in the dialectic philosophy of Socrates and Plato. Then, when an agreement has been established, their minds, before so sharply divided, seem to be, after all, only different personifications of the same universal spirit. Hence reason, no less than its objects, comes to be conceived as both many and one. And this synthesis of contradictories meets us in modern German as well as in ancient Greek philosophy.