The aim of the paper is, on the one hand, to investigate the Greek philosophical origin of Ulpian's definitions of justice (iustitia est perpetua et constans voluntas ius suum cuique tribuendi) and the fundamental three legal precepts (iuris praecepta sunt haec: honeste vivere, alterum non laedere, suum cuique tribuere). On the other hand, an attempt was made to discover the extent to which these ancient definitions were or are an important constitutive element of the European legal tradition. Regarding the Greek philosophical foundations of Ulpian's definitions, it should be mentioned that the majority of modern and contemporary philosophers and legal scholars believe that the sources of these definitions are the writings of Stoic philosophers. According to our understanding, Ulpian's definitions have their oldest philosophical origins in Plato's conception of justice. While the foundations of Ulpian's definition of justice can be found in the Platonic Definitions, the foundations of his three praeceptorum iuris may be traced to the first book of Plato’s Politeia and specifically to the maieutic dialogue between Socrates and his collocutors on certain fundamental ethical precepts in the context of the virtue of justice, where all three precepts are to be found together for the first time in the same context. Ulpian’s definitions of justice and the three fundamental legal precepts retained their extraordinary significance in the medieval and modern European legal tradition. Thus, for example, the principle of alterum non laedere as a concise formulation of commutative justice (iustitia commutativa) and the principle of suum cuique tribuere as a concise articulation of distributive justice (iustitia distributiva), form an indispensable foundation of modern legislation, doctrine and practice in several of the most important legal branches.