But that has not been, nor ever will be, lost time which is expended in the preliminary and lateral analysis of complex facts. To the methodical division of labor we owe precise learning, that is to say, the mass of knowledge passed into the sieve, systematized, without which social history would always be wandering in a purely abstract domain, in questions of form and terminology. The separate study of the historico-social factors has served, like any other empirical study which does not transcend the apparent movement of things, to improve the instrument of observation and to permit us to find again in the facts themselves, which have been artificially abstracted, the keystones which bind them into the social _complexus_. The different disciplines which are considered as isolated and independent in the hypotheses of the concurrent factors in the formation of history, both by reason of the degree of development which they have reached, the materials which they have gathered, and the methods which they have elaborated, have to-day become quite indispensable for us, if one desires to reconstruct any portion out of past times. Where would our historic science be without the one-sidedness of philology, which is the fundamental instrument of all research, and where should we have found the guiding thread of a history of juridical institutions, which returns again from itself to so many other facts and to so many other combinations, without the obstinate faith of the Romanists in the universal excellence of the Roman law, which engendered with generalized law and with the philosophy of law so many problems which serve as points of departure for sociology? It is thus, after all, that the historic factors, of which so many speak, and which are mentioned in so many works, indicate something which is much less than the truth, but much more than simple error, in the ordinary sense of a blunder, of an illusion. They are the necessary product of a knowledge which is in the course of development and formation. They arise from the necessity of finding a point of departure in the confused spectacle which human events present to him who wishes to narrate them; and they serve thenceforth, so to speak, as a title, category or index to that inevitable division of labor, by the extension of which the historico-social material has, up to this time, been theoretically elaborated. In this domain of knowledge, as well as in that of the natural sciences, the unity of real principle and the unity of formal treatment are never found at the first start, but only after a long and troublous road. So that again from this point of view the analogy affirmed by Engels between the discovery of historical materialism and that of the conservation of energy appears to us excellent.