To proceed to closer details regarding the actual scheme of the laws of political revolutions as drawn out by Plato, we must first note that the primary cause of the decay of the ideal state is the general principle, common to the vegetable and animal worlds as well as to the world of history, that all created things are fated to decay—a principle which, though expressed in the terms of a mere metaphysical abstraction, is yet perhaps in its essence scientific. For we too must hold that a continuous redistribution of matter and motion is the inevitable result of the nominal persistence of Force, and that perfect equilibrium is as impossible in politics as it certainly is in physics. The secondary causes which mar the perfection of the Platonic ‘city of the sun’ are to be found in the intellectual decay of the race consequent on injudicious marriages and in the Philistine elevation of physical achievements over mental culture; while the hierarchical succession of Timocracy and Oligarchy, Democracy and Tyranny, is dwelt on at great length and its causes analysed in a very dramatic and psychological manner, if not in that sanctioned by the actual order of history. And indeed it is apparent at first sight that the Platonic succession of states represents rather the succession of ideas in the philosophic mind than any historical succession of time. Aristotle meets the whole simply by an appeal to facts. If the theory of the periodic decay of all created things, he urges, be scientific, it must be universal, and so true of all the other states as well as of the ideal. Besides, a state usually changes into its contrary and not to the form next to it; so the ideal state would not change into Timocracy; while Oligarchy, more often than Tyranny, succeeds Democracy. Plato, besides, says nothing of what a Tyranny would change to. According to the cycle theory it ought to pass into the ideal state again, but as a fact one Tyranny is changed into another as at Sicyon, or into a Democracy as at Syracuse, or into an Aristocracy as at Carthage. The example of Sicily, too, shows that an Oligarchy is often followed by a Tyranny, as at Leontini and Gela. Besides, it is absurd to represent greed as the chief motive of decay, or to talk of avarice as the root of Oligarchy, when in nearly all true oligarchies money-making is forbidden by law. And finally the Platonic theory neglects the different kinds of democracies and of tyrannies. Now nothing can be more important than this passage in Aristotle’s _Politics_ (_v._ 12.), which may he said to mark an era in the evolution of historical criticism. For there is nothing on which Aristotle insists so strongly as that the generalisations from facts ought to be added to the data of the _a priori_ method—a principle which we know to be true not merely of deductive speculative politics but of physics also: for are not the residual phenomena of chemists a valuable source of improvement in theory?