The supremacy of reason shook the thrones from which authority and tradition had tyrannised over the brains of men.
Cartesianism was equivalent to a declaration of the Independence of Man.
It was in the atmosphere of the Cartesian spirit that a theory of Progress was to take shape.
1.
Let us look back.

We saw that all the remarks of philosophers prior to the seventeenth century, which have been claimed as enunciations of the idea of Progress, amount merely to recognitions of the obvious fact that in the course of the past history of men there have been advances and improvements in knowledge and arts, or that we may look for some improvements in the future.

There is not one of them that adumbrates a theory that can be called a theory of Progress.

We have seen several reasons why the idea could not emerge in the ancient or in the Middle Ages.

Nor could it have easily appeared in the period of the Renaissance.