For the restrictions under which Aristotle thought were not determined by his personality alone; they followed on the logical development of speculation, and would have imposed themselves on any other thinker equally capable of carrying that development to its predetermined goal. The Ionian search for a primary cause and substance of nature led to the distinction, made almost simultaneously, although from opposite points of view, by Parmenides and Heracleitus, between appearance and reality. From that distinction sprang the idea of mind, organised by Socrates into a systematic study of ethics and dialectics. Time and space, the necessary conditions of physical causality, were eliminated from a method having for its form the eternal relations of difference and resemblance, for its matter the present interests of humanity. Socrates taught that before enquiring whence things come we must first determine what it is they are. Hence he reduced science to the framing of exact definitions. Plato followed on the same track, and refused to answer a single question about anything until the subject of investigation had been clearly determined. But the form of causation had taken such a powerful hold on Greek thought, that it could not be immediately shaken off; and Plato, as he devoted more and more attention to the material universe, saw himself compelled, like the older philosophers, to explain its construction by tracing out the history of its growth. What is even more significant, he applied the same method to ethics and politics, finding it easier to describe how the various virtues and types of social union came into existence, than to analyse and classify them as fixed ideas without reference to time. Again, while taking up the Eleatic antithesis of reality and appearance, and re-interpreting it as a distinction between noumena and phenomena, ideas and sensations, spirit and matter, he was impelled by the necessity of explaining himself, and by the actual limitations of experience to assimilate the two opposing series, or, at least, to view the fleeting, superficial images as a reflection and adumbration of the being which they concealed. And of all material objects, it seemed as if the heavenly bodies, with their orderly, unchanging movements, their clear brilliant light, and their remoteness from earthly impurities, best represented the philosopher’s ideal. Thus, Plato, while on the one side he reaches back to the pre-Socratic age, on the other reaches forward to the Aristotelian system.