What was the value of the achievements of science, and the improvement of the arts of life, if life itself could not be ameliorated?
Was not some radical reconstruction possible, in the social fabric, corresponding to the radical reconstruction inaugurated by Descartes in the principles of science and in the methods of thought?
Year by year the obscurantism of the ruling powers became more glaring, and the most gifted thinkers, towards the middle of the century, began to concentrate their brains on the problems of social science and to turn the light of reason on the nature of man and the roots of society.
They wrought with unscrupulous resolution and with far-reaching effects.
With the extension of rationalism into the social domain, it came about naturally that the idea of intellectual progress was enlarged into the idea of the general Progress of man.

The transition was easy.

If it could be proved that social evils were due neither to innate and incorrigible disabilities of the human being nor to the nature of things, but simply to ignorance and prejudices, then the improvement of his state, and ultimately the attainment of felicity, would be only a matter of illuminating ignorance and removing errors, of increasing knowledge and diffusing light.

The growth of the "universal human reason"-- a Cartesian phrase, which had figured in the philosophy of Malebranche--must assure a happy destiny to humanity.
Between 1690 and 1740 the conception of an indefinite progress of enlightenment had been making its way in French intellectual circles, and must often have been a topic of discussion in the salons, for instance, of Madame de Lambert, Madame de Tencin, and Madame Dupin, where Fontenelle was one of the most conspicuous guests.

To the same circle belonged his friend the Abbe de Saint-Pierre, and it is in his writings that we first find the theory widened in its compass to embrace progress towards social perfection.