He starts by accepting the general principle that all things are fated to decay (which I noticed in the case of Plato), and that ’as iron produces rust and as wood breeds the animals that destroy it, so every state has in it the seeds of its own corruption.’  He is not, however, content to rest there, but proceeds to deal with the more immediate causes of revolutions, which he says are twofold in nature, either external or internal.  Now, the former, depending as they do on the synchronous conjunction of other events outside the sphere of scientific estimation, are from their very character incalculable; but the latter, though assuming many forms, always result from the over-great preponderance of any single element to the detriment of the others, the rational law lying at the base of all varieties of political changes being that stability can result only from the statical equilibrium produced by the counteraction of opposing parts, since the more simple a constitution is the more it is insecure.  Plato had pointed out before how the extreme liberty of a democracy always resulted in despotism, but Polybius analyses the law and shows the scientific principles on which it rests.

The doctrine of the instability of pure constitutions forms an important era in the philosophy of history.  Its special applicability to the politics of our own day has been illustrated in the rise of the great Napoleon, when the French state had lost those divisions of caste and prejudice, of landed aristocracy and moneyed interest, institutions in which the vulgar see only barriers to Liberty but which are indeed the only possible defences against the coming of that periodic Sirius of politics, the [Greek]

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.