There is a principle which Tocqueville never wearies of explaining, and which has been subsumed by Mr. Herbert Spencer under that general law common to all organic bodies which we call the Instability of the Homogeneous. The various manifestations of this law, as shown in the normal, regular revolutions and evolutions of the different forms of government, {193a} are expounded with great clearness by Polybius, who claimed for his theory in the Thucydidean spirit, that it is a [Greek], not a mere [Greek], and that a knowledge of it will enable the impartial observer {193b} to discover at any time what period of its constitutional evolution any particular state has already reached and into what form it will be next differentiated, though possibly the exact time of the changes may be more or less uncertain. {193c} Now in this necessarily incomplete account of the laws of political revolutions as expounded by Polybius enough perhaps has been said to show what is his true position in the rational development of the 'Idea' which I have called the Philosophy of History, because it is the unifying of history. Seen darkly as it is through the glass of religion in the pages of Herodotus, more metaphysical than scientific with Thucydides, Plato strove to seize it by the eagle-flight of speculation, to reach it with the eager grasp of a soul impatient of those slower and surer inductive methods which Aristotle, in his trenchant criticism of his great master, showed were more brilliant than any vague theory, if the test of brilliancy is truth. What then is the position of Polybius? Does any new method remain for him? Polybius was one of those many men who are born too late to be original. To Thucydides belongs the honour of being the first in the history of Greek thought to discern the supreme calm of law and order underlying the fitful storms of life, and Plato and Aristotle each represents a great new principle. To Polybius belongs the office--how noble an office he made it his writings show--of making more explicit the ideas which were implicit in his predecessors, of showing that they were of wider applicability and perhaps of deeper meaning than they had seemed before, of examining with more minuteness the laws which they had discovered, and finally of pointing out more clearly than any one had done the range of science and the means it offered for analysing the present and predicting what was to come. His office thus was to gather up what they had left, to give their principles new life by a wider application.