The author analyses the role of the principle of plenitude (no genuine possibility remains forever unrealised) and its relations to the analysis of modal concepts in nineteenth century french philosophy. He shows that from maine de biran and ravaisson to renouvier, Boutroux and bergson, As well as in the philosophy of science of comte and cournot, There is a common thread in french philosophy, The metaphysical doctrine of "contingentism," according to which a radical contingency is at the root of being. This doctrine leads to the progressive rejection of the principle of plenitude.