Perhaps we may say that with him the philosophy of history is partly in the metaphysical stage, and see, in the progress of this idea from Herodotus to Polybius, the exemplification of the Comtian Law of the three stages of thought, the theological, the metaphysical, and the scientific: for truly out of the vagueness of theological mysticism this conception which we call the Philosophy of History was raised to a scientific principle, according to which the past was explained and the future predicted by reference to general laws. Now, just as the earliest account of the nature of the progress of humanity is to be found in Plato, so in him we find the first explicit attempt to found a universal philosophy of history upon wide rational grounds. Having created an ideally perfect state, the philosopher proceeds to give an elaborate theory of the complex causes which produce revolutions, of the moral effects of various forms of government and education, of the rise of the criminal classes and their connection with pauperism, and, in a word, to create history by the deductive method and to proceed from _a priori_ psychological principles to discover the governing laws of the apparent chaos of political life. There have been many attempts since Plato to deduce from a single philosophical principle all the phenomena which experience subsequently verifies for us. Fichte thought he could predict the world-plan from the idea of universal time. Hegel dreamed he had found the key to the mysteries of life in the development of freedom, and Krause in the categories of being. But the one scientific basis on which the true philosophy of history must rest is the complete knowledge of the laws of human nature in all its wants, its aspirations, its powers and its tendencies: and this great truth, which Thucydides may be said in some measure to have apprehended, was given to us first by Plato. Now, it cannot be accurately said of this philosopher that either his philosophy or his history is entirely and simply _a priori_. _On est de son siècle même quand on y proteste_, and so we find in him continual references to the Spartan mode of life, the Pythagorean system, the general characteristics of Greek tyrannies and Greek democracies. For while, in his account of the method of forming an ideal state, he says that the political artist is indeed to fix his gaze on the sun of abstract truth in the heavens of the pure reason, but is sometimes to turn to the realisation of the ideals on earth: yet, after all, the general character of the Platonic method, which is what we are specially concerned with, is essentially deductive and _a priori_. And he himself, in the building up of his Nephelococcygia, certainly starts with a _καθαρὸς πίναξ_, making a clean sweep of all history and all experience; and it was essentially as an _a priori_ theorist that he is criticised by Aristotle, as we shall see later.