New principles of order and unity were needed to replace the principles which rationalism had discredited.

Just as the advance of science depended on the postulate that physical phenomena are subject to invariable laws, so if any conclusions were to be drawn from history some similar postulate as to social phenomena was required.
It was thus in harmony with the general movement of thought that about the middle of the eighteenth century new lines of investigation were opened leading to sociology, the history of civilisation, and the philosophy of history.

Montesquieu's De l'esprit des lois, which may claim to be the parent work of modern social science, Voltaire's Essai sur les moeurs, and Turgot's plan of a Histoire universelle begin a new era in man's vision of the past.
1.
Montesquieu was not among the apostles of the idea of Progress.

It never secured any hold upon his mind.

But he had grown up in the same intellectual climate in which that idea was produced; he had been nurtured both on the dissolving, dialectic of Bayle, and on the Cartesian enunciation of natural law.