Philosophers

Montesquieu
He developed the idea of separation of powers (executive, legislature and judiciary) in his masterwork was “The spirit of the laws” (“De le spirit des lois”)

Rousseau
He defined the notion of “Social Contract” as a deal between all citizens recorded into a Constitution. He brought up the idea of national sovereignty expressed by suffrage.

Voltaire
He defended the necessity of a parliament that controlled royal power. He also believed in an equal tax system where the third estate was not the only taxpayer.