In order that the idea of progress should reach its final form it was necessary, in the first place, that positive psychology should have put an end to the dreams of indefinite perfectibility. It was also necessary that the French Revolution should come to render the course of the history of humanity intelligible. Indeed, according to Comte, a “progression” cannot be understood, so long as we do not know at least three of its terms. Two terms do not suffice to define it. Now, up to the time of the French Revolution, several “progressions” or social series undoubtedly offered the required number of terms to scientific reflection; for instance, the evolution of such and such a science or of such and such an art. But, in sociology, the knowledge of secondary laws is subordinated to that of primary laws, and the advance of such and such a social series can only be understood if the development of society in general is known in its fundamental law. To discover this law then, we must possess at least, three terms of the general “progression.” Now, before the French Revolution two terms only were given: the _régime_ of the societies of antiquity, and the Christian _régime_ (that is to say, the one which attained its highest degree of perfection in the Catholic organisation of the Middle Ages.) The French Revolution came to furnish the third term. It brought the idea of a new _régime_. As Kant had said, in terms which were certainly unknown to Comte, it gave men the idea of a social organisation founded upon principles different from those of the existing societies. Henceforth the idea of progress could apply itself to the whole of the historical development of humanity. “It is to this salutary disturbance,” says Comte, “that we owe the strength and the audacity to conceive a notion upon which rests the whole of social science, and consequently the whole of positive philosophy, of which this final science alone could constitute the unity.”[268] This social science remained to be constructed. It will be the special work of Auguste Comte. According to him, the French Revolution only brought an imperfect idea of social progress. It helped to bring about the conception of the idea of a different _régime_, but without actually founding it. The functions of the new philosophy will be to realise the positive idea of social progress. In a word, the revolutionary impulse made this philosophy possible. It has not done away with its utility.[269] II.