This paper aims to conduct a comparative analysis of Miguel Reale s tridimensional theory and José Pedro Galvão de Sousa s classical natural law theory. The first chapter analyzes Reale s tridimensional theory, seeking to highlight its context of emergence, its distinction from abstract tridimensionalism, its dialectical, ontognoseological, and historical-axiological foundations, and its relationship with natural law. In the second chapter, the natural law in Galvão de Sousa s work will be examined, including its introduction with the critique of positivism, the foundations of natural law rooted in Aristotelian-Thomistic thought, and its relationship with modern jusnaturalism. This chapter will also explore the application of a historically-positive natural law in response to the legal consequences arising from the modern state. Finally, the third chapter aims to compare the theories in terms of legal positivism, the debate surrounding natural law, and the necessity for the norm to reflect historical reality. In conclusion, the paper will analyze the points of convergence and essential divergences between the two theories.