The critical canon that the more a people has been interfered with, the more difficult it becomes to generalise the laws of its progress and to analyse the separate forces of its civilisation, is one the validity of which is now generally recognised by those who pretend to a scientific treatment of all history: and while we have seen that Aristotle anticipated it in a general formula, to Polybius belongs the honour of being the first to apply it explicitly in the sphere of history. I have shown how to this great scientific historian the motive of his work was essentially the search for causes; and true to his analytical spirit he is careful to examine what a cause really is and in what part of the antecedents of any consequent it is to be looked for. To give an illustration: As regards the origin of the war with Perseus, some assigned as causes the expulsion of Abrupolis by Perseus, the expedition of the latter to Delphi, the plot against Eumenes and the seizure of the ambassadors in Bœotia; of these incidents the two former, Polybius points out, were merely the pretexts, the two latter merely the occasions of the war. The war was really a legacy left to Perseus by his father, who was determined to fight it out with Rome. {78} Here as elsewhere he is not originating any new idea. Thucydides had pointed out the difference between the real and the alleged cause, and the Aristotelian dictum about revolutions, _οὐ περὶ μικρῶν ἀλλ’ ἐκ μικρῶν_, draws the distinction between cause and occasion with the brilliancy of an epigram. But the explicit and rational investigation of the difference between _αἰτία_, _ἀρχὴ_, and _πρόφασις_ was reserved for Polybius. No canon of historical criticism can be said to be of more real value than that involved in this distinction, and the overlooking of it has filled our histories with the contemptible accounts of the intrigues of courtiers and of kings and the petty plottings of backstairs influence—particulars interesting, no doubt, to those who would ascribe the Reformation to Anne Boleyn’s pretty face, the Persian war to the influence of a doctor or a curtain-lecture from Atossa, or the French Revolution to Madame de Maintenon, but without any value for those who aim at any scientific treatment of history. But the question of method, to which I am compelled always to return, is not yet exhausted. There is another aspect in which it may be regarded, and I shall now proceed to treat of it.