Nor can the scientific investigator of history isolate the particular elements, which he desires to examine, from disturbing and extraneous causes, as the experimental chemist can do (though sometimes, as in the case of lunatic asylums and prisons, he is enabled to observe phenomena in a certain degree of isolation). So he is compelled either to use the deductive mode of arguing from general laws or to employ the method of abstraction which gives a fictitious isolation to phenomena never so isolated in actual existence. And this is exactly what Polybius has done as well as Thucydides. For, as has been well remarked, there is in the works of these two writers a certain plastic unity of type and motive; whatever they write is penetrated through and through with a specific quality, a singleness and concentration of purpose, which we may contrast with the more comprehensive width as manifested not merely in the modern mind, but also in Herodotus. Thucydides, regarding society as influenced entirely by political motives, took no account of forces of a different nature, and consequently his results, like those of most modern political economists, have to be modified largely {207} before they come to correspond with what we know was the actual state of fact. Similarly, Polybius will deal only with those forces which tended to bring the civilised world under the dominion of Rome (ix. 1), and in the Thucydidean spirit points out the want of picturesqueness and romance in his pages which is the result of the abstract method ([Greek]), being careful also to tell us that his rejection of all other forces is essentially deliberate and the result of a preconceived theory and by no means due to carelessness of any kind. Now, of the general value of the abstract method and the legality of its employment in the sphere of history, this is perhaps not the suitable occasion for any discussion. It is, however, in all ways worthy of note that Polybius is not merely conscious of, but dwells with particular weight on, the fact which is usually urged as the strongest objection to the employment of the abstract method--I mean the conception of a society as a sort of human organism whose parts are indissolubly connected with one another and all affected when one member is in any way agitated. This conception of the organic nature of society appears first in Plato and Aristotle, who apply it to cities. Polybius, as his wont is, expands it to be a general characteristic of all history. It is an idea of the very highest importance, especially to a man like Polybius whose thoughts are continually turned towards the essential unity of history and the impossibility of isolation.