In a not very well known essay, Hans Reiner claims that there are two main theories which have considered the phenomenon of moral obligation: on the one hand, the theory which acknowledges the validity of such phenomenon and tries to found it either on the religiousmetaphysical presupposition of the faith in God an on His commandments or in the metaphysics of an “ontological structure” or “essence” of human being; on the other hand, the theories which deny the character of original phenomenon to obligation. This kind of theories offer just a geneticcausal analysis of obligation. Phenomenological method would appear, according to Reiner, as a “third way”. The aim of this paper is to criticize this last statement by Reiner and offer a general view of the main elements of this method which can be especially relevant for Philosophy of Law.