The progress of thought from the philosopher of the Academe to the Arcadian historian may be best illustrated by a comparison of the method by which each of the three writers, whom I have selected as the highest expression of the rationalism of his respective age, attained to his ideal state: for the latter conception may be in a measure regarded as representing the most spiritual principle which they could discern in history. Now, Plato created his on _a priori_ principles; Aristotle formed his by an analysis of existing constitutions; Polybius found his realised for him in the actual world of fact. Aristotle criticised the deductive speculations of Plato by means of inductive negative instances, but Polybius will not take the ‘Cloud City’ of the _Republic_ into account at all. He compares it to an athlete who has never run on ‘Constitution Hill,’ to a statue so beautiful that it is entirely removed from the ordinary conditions of humanity, and consequently from the canons of criticism. The Roman state had attained in his eyes, by means of the mutual counteraction of three opposing forces, {59} that stable equilibrium in politics which was the ideal of all the theoretical writers of antiquity. And in connection with this point it will be convenient to notice here how much truth there is contained in the accusation often brought against the ancients that they knew nothing of the idea of Progress, for the meaning of many of their speculations will be hidden from us if we do not try and comprehend first what their aim was, and secondly why it was so.