Philosophy in eighteenth-century France was not a subject matter. It was rather, to judge by the texts of the philosophes, a form of action; it was not a noun, but a verb. The classical Enlightenment historians (Peter Gay, Ernst Cassirer, etc.) reflect a rereading of the French Enlightenment from the point of view of the very different German idealist philosophical tradition, but this point of view hides its distinctive character. The French Enlightenment was in effect a plurivocal debate, a long and convoluted conversation, a descendant of the Republic of Letters, a proof that literacy is not a state, but an activity creating culturally dense communities.

In the French Enlightenment philosophical activity was neither homogeneous nor monolithic. There are two complementary ways to cut across the dozens if not hundreds of authors and voices throughout nearly a hundred years: a chronological one – they sort rather cleanly into three generations; and a conceptual one – they separate into three strains of conceptualising the activity and consequences of knowing.

First, the question of generations. Voltaire lived through almost the entire period that we now consider the Enlightenment. How can we attribute him to a single generation? Because what is determining is what the members of each generation had in common with each other and the relationship that each generation understood itself to have to the others – an identity explicitly and consciously assumed by each, an attribution whose consequences were strategic: each generation served with respect to its successor as its conceptual foundation, but also as the source of its philosophical authority.