This mythical account shows, however, the inherent dualism in Plato’s doctrine. The Idea never fully realizes itself in corporeal things, and Plato was called on to explain the cause of the evil and imperfection of the physical world. Moreover, the imperfection of the physical world got new emphasis in the influence upon him of the Pythagorean doctrine, which had set the perfect and imperfect worlds in opposition. What prevents the Idea from fully appearing in phenomena? The more Plato conceived the world of Ideas as ethical Ideals and a kingdom of pure worth, and the more teleological the Ideas became, the less could he regard the Ideas as the cause of imperfection in nature. Ideas are Being, and the essence of perfection. The cause of imperfection must therefore be that which has no being whatsoever. The physical world as “becoming” has participation, not only in that which has Being (Ideas), but in that which has no Being (empty space). The physical world has a composite character. It has sprung from the union of the Ideas and an absolutely negative factor, which Plato calls empty space. This eternal negative is formless and unfashioned, but it is capable of taking on all possible forms. The physical universe is therefore neither Ideas simply, nor matter simply, but a composition of the two. This non-Being is not like the matter, “unformed stuff,” of Aristotle, from which all sensible things are made; but it is that in which Ideas have to appear. The Ideas are plunged into this empty non-Being, which they take on as a veil. And just this is the origin of imperfection; non-Being withholds the Ideas from perfect expression. Non-Being, or empty space, is an indispensable auxiliary to the Ideas, for without it no physical universe would be possible. But at the same time it is the eternal foe and obstruction of the Ideas. Its coöperation with the Ideas is at the same time a resistance to them. It is the perpetual negation of Being, and the primary cause of imperfection, change, and instability. On this account the universe can never be like the Ideas, but it can approximate them. The soul of the world, for example,――which was regarded by Plato in Pythagorean fashion as number subjecting chaotic space to harmony,――is the most perfect reproduction of the Idea of the Good. The existence of matter detracts from the perfection of the world, but it does not detract from the majesty of the Ideas. =Plato’s Conception of Man.= Plato needed a psychology of another sort from that developed by the Cosmologists. His analysis of the mental life of man stands or falls with his metaphysical theory of Ideas, but it has this importance: it is the first attempt to understand the psychical life from within.