When we proceed to apply the law of the three stages to the general course of historical development, we are met at the outset by the difficulty that the advance in all the domains of activity is not simultaneous. If at a given period thought and opinions are partly in the theological, partly in the metaphysical, and partly in the scientific state, how is the law to be applied to general development? One class of ideas, Comte says, must be selected as the criterion, and this class must be that of social and moral ideas, for two reasons. In the first place, social science occupies the highest rank in the hierarchy of sciences, on which he laid great stress. [Footnote: Cours de phil. pos. v. 267. Law of consensus: op. cit. iv. 347 sqq., 364, 505, 721, 735.] In the second, those ideas play the principal part for the majority of men, and the most ordinary phenomena are the most important to consider. When, in other classes of ideas, the advance is at any time more rapid, this only means an indispensable preparation for the ensuing period. The movement of history is due to the deeply rooted though complex instinct which pushes man to ameliorate his condition incessantly, to develop in all ways the sum of his physical, moral, and intellectual life. And all the phenomena of his social life are closely cohesive, as Saint-Simon had pointed out. By virtue of this cohesion, political, moral, and intellectual progress are inseparable from material progress, and so we find that the phases of his material development correspond to intellectual changes. The principle of consensus or "solidarity," which secures harmony and order in the development, is as important as the principle of the three stages which governs the onward movement. This movement, however, is not in a right line, but displays a series of oscillations, unequal and variable, round a mean motion which tends to prevail. The three general causes of variation, according to Comte, are race, climate, and deliberate political action (such as the retrograde policies of Julian the Apostate or Napoleon). But while they cause deflections and oscillation, their power is strictly limited; they may accelerate or retard the movement, but they cannot invert its order; they may affect the intensity of the tendencies in a given situation, but cannot change their nature. 3.