This work poses the problematic of the law in Montesquieu's thought. It is about the realism of the Enlightenment which supposes that political freedom depends ineluctably on the law. In this respect, we have summoned the philosophers of the Enlightenment, and we have shown that this philosophy has greatly changed the way we learn about the law and society. We have also shown the necessity of the separation of powers. We have exposed the variables that condition the political freedom.