These factors, to express it in another way, arise in the mind as a sequence of the abstraction and generalization of the immediate aspects of the apparent movement, and they have an equal value with that of all other empirical concepts. Whatever be the domain of knowledge in which they arise, they persist until they are reduced and eliminated by a new experience, or until they are absorbed by a conception more general, genetic, evolutionary or dialectic. Was it not necessary that in the empirical analysis and in the immediate study of the causes and the effects of certain definite phenomena, for example the phenomena of heat, the mind should first stop at this presumption and this persuasion, that it could and should attribute them to a subject, which if it was never for any physicist a true and substantial entity, was certainly considered as a definite and specific force, namely, heat. Now we see that at a given moment, as a result of new experiences, this heat is resolved in given conditions into a certain quantity of motion. Still further, our thought is now on the way toward resolving all these physical factors into the flux of one universal energy, in which the hypotheses of the atoms, in the extent to which it is necessary, loses all residue of metaphysical survival. Was it not inevitable, as a first step of knowledge in what concerns the problem of life, to spend a considerable time in the separate study of the organs and to reduce them to systems? Without this anatomy, which seems too material and too gross, no progress in these studies would have been possible; and nevertheless, above the unknown genesis and co-ordination of such an analytic multiplicity, there were evolving, uncertain and vague, the generic conceptions of life, soul, etc. In these mental creations have long been seen that biological unity which has finally found its object in the certain beginning of the cell and in its _processus_ of immanent multiplication. More difficult certainly was the way which the thought had to traverse to reconstruct the genesis of all the facts of psychic life, from the most elementary successions up to the most complex derived products. Not only for reasons of theoretical difficulties, but in consequence of popular prejudices, the unity and continuity of psychic phenomena appeared, up to the time of Herbart, as separated and divided into so many factors, faculties of the soul. The interpretation of the historico-social _processus_ met the same difficulties; it also was obliged to stop at first in the provisional view of factors. And that being so, it is easy for us now to find again the first origin of that opinion in the necessity that the historians have of finding in the facts that they relate with more or less artistic talent and in different professional views, certain points of immediate orientation, such as may be offered by the study of the apparent movement of human events.