This change from Measure to Essential Being is one which Greek philosophy seems to exhibit in the step from Pythagoreanism to Platonism. Plato himself has noted the passage from what have been called the mathematical to the metaphysical categories, and insisted on the essential and higher truth to which mathematics only point. Mathematical terms give the supreme definiteness to the world of being; they show it as in its several compartments a world immanently ordered and measured. As in Greek Art, all seems to be fully brought to the surface: as the image suggests no further and deeper meaning, but affords an absolute identity of aspect and purport; so the natural and semi-popular philosophy of Greece was satisfied for its ethics with the proportionate, the becoming, the beautiful. Plato however passes beyond the surface, and reflects the apparent fact on a deeper permanent reality behind. That reality is still, in name, only the 'form' or 'shape'-- only the regular and permanent type--only the measure. But it is called the really real, the _ὄντως ὄν_,--the being of being. In it the truth is clear, transparent, one and systematic, which in the sensible or immediate world is obscure, confused, multiple. It is the key to explain the difficulties and irregularities of the first and visible scene. Yet even Plato never for a moment forgets the essential correspondence of the two realms, however he may insist upon their separation, and however hard he may find it to explain how being can be duplicated, how the one can be many and yet not cease to be one, how appearance has part in reality. This indeed is not a difficulty confined to Plato. It is, after all, the same antithesis as we found in the beginning: the Is which lapses into the Is not. It now becomes the play of positive and negative--of perpetual relativity: of a known dependent on an unknown, and an unknown interpreted by a known: an essence guaranteed by its show or seeming and a _Schein_ which supposes permanent _Sein._ How can a thing be, and yet not be true? How can pleasures seem and not be real? Aristotle, taking up the Platonic antithesis of true and apparent being, carries it on into greater detail. Matter and form: possibility and actuality: are amongst his cardinal pairs of correlatives. But he is anxious to maintain their essential relativity: to show that reality only is and maintains itself as the unity of the two poles of universal and particular, reason and sense, or as a syllogism and a development. So far as he succeeds in doing this, Aristotle rises above the correlational view of reality into the comprehension of it as a unity, which carries itself through difference into self-realisation. [Footnote 1: What he did show was that these Ideas were not objects in the vulgar sense of reality, or things.] [Footnote 2: Cf. the controversy between Schiller and Goethe as to idea and observation, quoted by Whewell, _Scientific Ideas_, i. 36.]