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. (19)