Is there any one element which communicates the decisive impulse to all the rest,--any predominating agency in the course of social evolution? The answer is that all the other parts of social existence are associated with, and drawn along by, the contemporary condition of intellectual development. The Reason is the superior and preponderant element which settles the direction in which all the other faculties shall expand. 'It is only through the more and more marked influence of the reason over the general conduct of man and of society that the gradual march of our race has attained that regularity and persevering continuity which distinguish it so radically from the desultory and barren expansion of even the highest animal orders, which share, and with enhanced strength, the appetites, the passions, and even the primary sentiments of man.' The history of intellectual development, therefore, is the key to social evolution, and the key to the history of intellectual development is the Law of the Three States. Among other central thoughts in Comte's explanation of history are these:--The displacement of theological by positive conceptions has been accompanied by a gradual rise of an industrial _régime_ out of the military _régime;_--the great permanent contribution of Catholicism was the separation which it set up between the temporal and the spiritual powers;--the progress of the race consists in the increasing preponderance of the distinctively human elements over the animal elements;--the absolute tendency of ordinary social theories will be replaced by an unfailing adherence to the relative point of view, and from this it follows that the social state, regarded as a whole, has been as perfect in each period as the co-existing condition of humanity and its environment would allow. The elaboration of these ideas in relation to the history of the civilisation of the most advanced portion of the human race occupies two of the volumes of the _Positive Philosophy_, and has been accepted by competent persons of very different schools as a masterpiece of rich, luminous, and far-reaching suggestion. Whatever additions it may receive, and whatever corrections it may require, this analysis of social evolution will continue to be regarded as one of the great achievements of human intellect. The demand for the first of Comte's two works has gone on increasing in a significant degree. It was completed, as we have said, in 1842. A second edition was published in 1864; a third some years afterwards; and while we write (1876) a fourth is in the press. Three editions within twelve years of a work of abstract philosophy in six considerable volumes are the measure of a very striking influence. On the whole, we may suspect that no part of Comte's works has had so much to do with this marked success as his survey and review of the course of history.