Their work was a calculated campaign to transform the principles and the spirit of governments and to destroy sacerdotalism.
The problem for the human race being to reach a state of felicity by its own powers, these thinkers believed that it was soluble by the gradual triumph of reason over prejudice and knowledge over ignorance.

Violent revolution was far from their thoughts; by the diffusion of knowledge they hoped to create a public opinion which would compel governments to change the tenor of their laws and administration and make the happiness of the people their guiding principle.

The optimistic confidence that man is perfectible, which means capable of indefinite improvement, inspired the movement as a whole, however greatly particular thinkers might differ in their views.
Belief in Progress was their sustaining faith, although, occupied by the immediate problems of amelioration, they left it rather vague and ill-defined.

The word itself is seldom pronounced in their writings.

The idea is treated as subordinate to the other ideas in the midst of which it had grown up: Reason, Nature, Humanity, Illumination (lumieres).