The ontological theory of the body is incompatible with a Cartesian type of dualism. How is it then that Maine de Biran, whose philosophical effort resulted in building the most profound theory of the body, the one most in conformity with the requirements of a phenomenological ontology given us by tradition, would remain in many respects duped by such a dualism? The revolution which he effected was so complete that we will, doubtless, have to wait for history to understand little by little its full meaning. So strong was the opposition of Maine de Biran to his century that his philosophy necessarily bore within it heterogeneous elements which, though they fit in with the thought of the period, were in fact foreign to his own outlook. The determination to grasp the profound intuition of Biranianism and to remain faithful thereto, therefore, implies the rejection of everything which, in Biranianism, does not really belong to it, but rather belongs to philosophical positions against which it was gradually constituted without always succeeding in eliminating them completely.