We saw, in treating of the Eleatics, that for them the absolute Being contained no not-being, and the absolute One no multiplicity. And it was just because they denied all not-being and multiplicity of the absolute reality that they were unable to explain the world of existence, and were forced to deny it altogether. The same problem arises for Plato. Is Being absolutely excludent of not-being? Is the Absolute an abstract One, utterly exclusive of the many? Is his philosophy a pure monism? Is it a pluralism? Or is it a combination of the two? These questions are discussed in the "Sophist" and the "Parmenides." {196} Plato investigates the relations of the One and the many, Being and not-being, quite in the abstract. He decides the principles involved, and leaves it to the reader to apply them to the theory of Ideas. Whether the Absolute is one or many, Being or not-being, can be decided independently of any particular theory of the nature of the Absolute, and therefore independently of Plato's own theory, which was that the Absolute consists of Ideas. Plato does not accept the Eleatic abstraction. The One cannot be simply one, for every unity must necessarily be a multiplicity. The many and the One are correlative ideas which involve each other. Neither is thinkable without the other. A One which is not many is as absurd an abstraction as a whole which has no parts. For the One can only be defined as that which is not many, and the many can only be defined as the not-one. The One is unthinkable except as standing out against a background of the many. The idea of the One therefore involves the idea of the many, and cannot be thought without it. Moreover, an abstract One is unthinkable and unknowable, because all thought and knowledge consist in applying predicates to subjects, and all predication involves the duality of its subject.