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STEP - St. Thomas Education Project


July 12-16, 2005


Thomistic Understanding of Natural Law

as the Foundation of Positive Law



Natural Law and Ius Gentium in Thomas Aquinas



Fulvio Di Blasi

Collegio Universitario ARCES - Thomas International, Italy





Contemporary international law does not seem to have much to do with its ancestor (Roman law concept of) ius gentium. Students, today (and not without reasons), trace international law back to the early modern world—particularly, to a strong legal and moral development mainly attributed to the ground-breaking work of Hugo Grotius.


Confronting these two notions, “international law” and “ius gentium,” we can adopt, roughly, two main attitudes. On the one hand, we can assume that they should be considered by now as totally discontinuous with each other. Consequently, we can be excused if we limit our attention to one of the two at the time according that our interests are more history related or contemporary legal and political issues related. On the other hand, we can assume that there is a useful continuity between the two relevant notions that still deserves attention. In this paper, I adopt the second attitude.


Thomas Aquinas looked closely at the Roman legal concept of ius gentium, and dealt with it in the context of his natural law theory. The main problem with his treatment of ius gentium is that, in different passages of his work, he seems to consider it at the same time as part of natural law and as part of human law. This sounds like a contradiction, and it’s going to be the main theoretical focus of my discussion.


In this paper, (1) I’ll first give a sketch of what we mean today by “international law” and of what Aquinas (and Roman lawyers) meant by ius gentium. (2) Then, I’ll explain why Aquinas considered ius gentium as part of natural law. (3) Third, I’ll explain why Aquinas considered ius gentium as part of human law. The explanation of these points, will shed not little light on Aquinas’ concept of natural law, and, I hope, will indicate some interesting ways in which his idea of ius gentium, even if not up to date anymore, can still reveal itself useful to international lawyers and to everybody who is concerned about international political and moral issues.