The purpose of this lecture is to identify and investigate two things: (1) the centre of the intellectual activity described in its title as the defining and development of international law, and (2) the fundamental concept around which both defining and development revolve. The centre as indicated will be found to consist of (representatives of) what is to be called the three “legal archetypes”. The fundamental concept alluded to is the “normative concept of law”. The two notions are, of course, highly abstract as compared to the realities they are designed to help understand. They will be set forth as clearly as possible within the strict limits of this lecture, although it is realized that a truly satisfactory treatment cannot be imagined outside a full-size methodology. An apology beforehand is, therefore, in place. One particular question relating to the normative concept of law in international relations will be dealt with in some detail: it has to do with “tragedy in law” as resulting from the clash between competing normative concepts. As a final observation, the need for a maximum degree of flexibility in the international lawyer's way of thought will be stressed.