Starting from two radically distinct points of view: the latency of the legal definition of the French service public between 1789 and 1880 on the one hand, and the ambiguity of the construction ex post of a legal definition of the public firm between 1930 and 1960 on the other hand, this paper presents the way in which lawyers represent of the public sphere and its most basic. It results in considering that legal theory develops rather like an engineering of justification or rationalization of the public policies than as a conceptual or normative framework of the latter.