As a contribution to the commemorations of Spinoza's death (1677), this article describes in a few pages the significance of Spinoza in the evolution of Western political thought. Especially in his (unachieved) Political Treatise, Spinoza attempted to elaborate a «scientific» theory of political life, i.e. a closing deductive theory based upon a «true knowledge of the causes and natural bases» of human actions and passions. In his view it can be proved with a rational necessity that democracy - defined as Spinoza defines it - is the best political regime. He strongly emphasizes that democracy and, consequently, a well organized and efficiently functioning life in common, is impossible without real freedom of opinion and speech and a political «neutralization» of religion.