The aim of this paper is to review the conception of justice of Book IV in Plato's The Republic. We will start with some general considerations about the work that justify the choice for this particular Book. Justice is approached by the author from a double perspective. We will develop an understanding separately, of its social (based on the πóλις) and individual (own of the ψυχη). In both cases, we will place special emphasis on the particular methodological strategy used by the author, consisting on the tripartition of the polis and the soul. On this point, we will propose a rereading of the relationship between the two kinds of justice and we will clarify the underlined internal character of individual justice. Finally, and once justice is fully characterized, we will concentrate our reflections on the role of the law on the platonic state.