The unity of opposites was the crux of ancient thinkers in the age of Plato: How could one thing be or become another? That substances have attributes was implied in common language; that heat and cold, day and night, pass into one another was a matter of experience 'on a level with the cobbler's understanding' (Theat.). But how could philosophy explain the connexion of ideas, how justify the passing of them into one another? The abstractions of one, other, being, not-being, rest, motion, individual, universal, which successive generations of philosophers had recently discovered, seemed to be beyond the reach of human thought, like stars shining in a distant heaven. They were the symbols of different schools of philosophy: but in what relation did they stand to one another and to the world of sense? It was hardly conceivable that one could be other, or the same different. Yet without some reconciliation of these elementary ideas thought was impossible. There was no distinction between truth and falsehood, between the Sophist and the philosopher. Everything could be predicated of everything, or nothing of anything. To these difficulties Plato finds what to us appears to be the answer of common sense--that Not-being is the relative or other of Being, the defining and distinguishing principle, and that some ideas combine with others, but not all with all. It is remarkable however that he offers this obvious reply only as the result of a long and tedious enquiry; by a great effort he is able to look down as 'from a height' on the 'friends of the ideas' as well as on the pre-Socratic philosophies. Yet he is merely asserting principles which no one who could be made to understand them would deny. The Platonic unity of differences or opposites is the beginning of the modern view that all knowledge is of relations; it also anticipates the doctrine of Spinoza that all determination is negation. Plato takes or gives so much of either of these theories as was necessary or possible in the age in which he lived. In the Sophist, as in the Cratylus, he is opposed to the Heracleitean flux and equally to the Megarian and Cynic denial of predication, because he regards both of them as making knowledge impossible. He does not assert that everything is and is not, or that the same thing can be affected in the same and in opposite ways at the same time and in respect of the same part of itself. The law of contradiction is as clearly laid down by him in the Republic, as by Aristotle in his Organon. Yet he is aware that in the negative there is also a positive element, and that oppositions may be only differences. And in the Parmenides he deduces the many from the one and Not-being from Being, and yet shows that the many are included in the one, and that Not-being returns to Being.