These sciences are the only ones whose consideration is of consequence to the end which Comte has in view. For why does he need a classification of the sciences? It is in order to study the ascent of the positive spirit through the successive orders of phenomena. For this, he has no occasion to consider the applied or concrete sciences, which receive their principles from the theoretical and abstract sciences. It suffices for him to be concerned with these. It is in the methods and the progress of these sciences that the characteristic efforts of the human mind have been manifested; and it is therefore here that we can grasp the laws of its evolution. In order to classify the fundamental sciences, Comte will conform to the principles of the positive method. He will be guided by the rational classifications of which the model is to be found in the natural sciences. The classification must spring from the very study of the objects which are to be classified, and must be determined by the real affinities and the series of connected links which they present, in such a way that this classification may itself be the expression of the most general truth, made manifest by the searching comparison of the objects which it embraces. Comte will not therefore stop to consider the classifications which have preceded his own. In the first place, when they appeared, the rational method of classification was not established. Further, how could anyone have united the whole of the sciences into an encyclopædic conception, when some had already reached the positive state, while others remained in the theological or metaphysical states? How could anyone rationally arrange heterogenous conceptions in a single system? Those premature attempts were doomed to failure. In order that the undertaking might succeed, it was necessary that _all_ our conceptions, relating to the various orders of phenomena should have reached the positive form. Here again, the creation of sociology has been the decisive event, for it has allowed the series of fundamental sciences to be made complete. The discovery of the law of the three States has founded sociology, and at the same time it has accomplished the homogeneity of human knowledge. In its time, this homogeneity renders possible the rational classification of the sciences. II.