At the very centre of the Rights of Man, a fundamental right is formulated : that of freedom of conscience with its inseparable corollary of tolerance. An attempt is made here to follow the main stream of the progression of this ideal — from the 15th to the 16th centuries, from Nicolas of Cues to Locke — not in an ideal, abstract manner, but in showing how the thought of the writers quoted is connected with the upheavals and changes in the state of the world and society whose challenges are to be met. Three phases may thus be distinguished at the dawn of modern times hallmarked by the geographical and religious break-up of the unity of the mediaval world : a more specifically philosophical stage, a more purely ethical and religious period, finally the development — coinciding with the definition of a right seen as natural — of a political doctrine of tolerance in the full sense of the word.