Plato's difficulty seems to begin in the region of ideas. He cannot understand how an absolute unity, such as the Eleatic Being, can be broken up into a number of individuals, or be in and out of them at once. Philosophy had so deepened or intensified the nature of one or Being, by the thoughts of successive generations, that the mind could no longer imagine 'Being' as in a state of change or division. To say that the verb of existence is the copula, or that unity is a mere unit, is to us easy; but to the Greek in a particular stage of thought such an analysis involved the same kind of difficulty as the conception of God existing both in and out of the world would to ourselves. Nor was he assisted by the analogy of sensible objects. The sphere of mind was dark and mysterious to him; but instead of being illustrated by sense, the greatest light appeared to be thrown on the nature of ideas when they were contrasted with sense. Both here and in the Parmenides, where similar difficulties are raised, Plato seems prepared to desert his ancient ground. He cannot tell the relation in which abstract ideas stand to one another, and therefore he transfers the one and many out of his transcendental world, and proceeds to lay down practical rules for their application to different branches of knowledge. As in the Republic he supposes the philosopher to proceed by regular steps, until he arrives at the idea of good; as in the Sophist and Politicus he insists that in dividing the whole into its parts we should bisect in the middle in the hope of finding species; as in the Phaedrus (see above) he would have 'no limb broken' of the organism of knowledge;--so in the Philebus he urges the necessity of filling up all the intermediate links which occur (compare Bacon's 'media axiomata') in the passage from unity to infinity. With him the idea of science may be said to anticipate science; at a time when the sciences were not yet divided, he wants to impress upon us the importance of classification; neither neglecting the many individuals, nor attempting to count them all, but finding the genera and species under which they naturally fall. Here, then, and in the parallel passages of the Phaedrus and of the Sophist, is found the germ of the most fruitful notion of modern science.