International law exists in the slippery zone between abstract speculation on binding principles and realistic deference to power. The position of Hugo Grotius as ‘father’ of international law, this article will suggest, results from the way later lawyers have appreciated his suggestion that when human beings enter that zone, they will discover a tendency to subordinate themselves to ‘rules’ that is lacking from other living creatures. Grotius then uses this assumed tendency to explain the trust and confidence with which members of good societies agree to live in peace and expect mutual benefits from cooperating with each other. The same tendency also entitles them to punish those who question the beneficial nature of these rules or lay down obstacles to their expansion. The importance of Grotius in the history of legal thought is highlighted by the manner in which the idea (though not the expression) of the ‘rule of law’ emerges in De iure belli ac pacis (1625) as a powerful justification of the government of a post-feudal, commercial state.