So far for the conception of history.  Now for the groundwork.  As regards the character of the phenomena to be selected by the scientific investigator, Aristotle had laid down the general formula that nature should be studied in her normal manifestations.  Polybius, true to his character of applying explicitly the principles implicit in the work of others, follows out the doctrine of Aristotle, and lays particular stress on the rational and undisturbed character of the development of the Roman constitution as affording special facilities for the discovery of the laws of its progress.  Political revolutions result from causes either external or internal.  The former are mere disturbing forces which lie outside the sphere of scientific calculation.  It is the latter which are important for the establishing of principles and the elucidation of the sequences of rational evolution.

He thus may be said to have anticipated one of the most important truths of the modern methods of investigation:  I mean that principle which lays down that just as the study of physiology should precede the study of pathology, just as the laws of disease are best discovered by the phenomena presented in health, so the method of arriving at all great social and political truths is by the investigation of those cases where development has been normal, rational and undisturbed.

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 Boeotia; 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. {205}