The article aims at reconstructing the defense of utilitarianism, a philosophical doctrine being the basis for legal positivism (which is the foundation of the constitutional concept of sources of law), undertaken by H.L.A. Hart. Hart took up this defense in the face of a significant increase in the interest of legal theorists in concepts related to natural law. Discussing the views of his master, J. Bentham, Hart also expresses his own deep doubts about the ideology of natural law, the adoption of which leads to the rejection of legal positivism deeply rooted in utilitarianism. Presented more than four decades ago, Hart’s remarks remain relevant today; modern thinkers still search an appropriate, other than referring to natural law, philosophical justification for a specific code of fundamental human rights.