Philosopher, Diderot is not an economist, but numerous remarks in his work, allow us to locate him amongst the different thinking streams of his time. The Prospectus as the Opening speech of Encyclopaedia locate economy into the knowlege tree. Branch of philosophy, and raising of ethic, economy is concerned, for its field, by the government of men as the administration of goods. Like the biology, economy is using an experimental method connecting together observation, hypothesis and verification. In his political struggle, Diderot assigns to economy and to the « Despote éclairé » precise objectives relating to numerous fields (agriculture, tax system, trade, markets, education, justice...).