What useful role (if any) could legal positivism play in the study or advancement of international law? For most of those who remember this once fashionable term at all, "international legal positivism" is redolent of the early years of the twentieth century--of Lassa Oppenheim (1) at best, and at worst of his model, John Austin, who famously denied that international law is or ever could be genuine law at all, "properly so called." (2) "Positive" law in its central and most usual sense is law "set by a sovereign individual or a sovereign body ... to a person or persons in a state of subjection to its author," (3) and legal "positivism" is the doctrine that there is and can be no law but positive law. (4) Seen in this way, international law, which rests substantially on opinio juris and "the general principles of law recognized by civilized nations," (5) will never fit the positivist paradigm. (6)
So why revive international legal positivism? I would suggest that despite its longstanding antipathy toward international legality and its radical failure as a general theory of law, legal positivism has always played a useful role in the progressive development of international legal institutions. Positive law has been part of international law from the beginning and will become increasingly important as international institutions become stronger. Legal positivism is a powerful tool for bringing greater clarity into international law. The better and more just the constitution of international society becomes, the more important and useful international legal positivism will be.
The traditional understanding of international law was well-stated by Henry Wheaton when John Austin made his first positivist attack on the law of nations: "International law, as understood among civilized nations, may be defined as consisting of those rules of conduct which reason deduces, as consonant to justice, from the nature of the society existing among independent nations; with such definitions and modifications as may be established by general consent." (7) The basis of international law rested for Wheaton, as for the earlier great publicists Hugo Grotius (8) and Emer de Vattel, (9) on reason and human nature, subject to specification by agreement. The clarifying provision of "definitions and modifications" by consent is the basis of "positive" international law.
Modern international law began with Hugo Grotius, whose turn to reason and the common good supplanted earlier sectarian conceptions of the law of nations. "All of this would still be true," Grotius insisted, "even if there were no God." (10) Grotius found international law in the social nature and mutual obligations of humanity, applied to states. (11) Emer de Vattel, who revised and restated in accessible French the Latin doctrines of Grotius and Christian Wolff, (12) asserted that precisely because international society has no authority capable of commanding its members and forcing obedience, (13) states are bound first and foremost by "necessary" laws that arise from the natural society of human beings. (14)
Vattel identified four varieties of international law. The "necessary" law of nations arises directly from reason, applied to the nature of men and states. (15) The "voluntary" law of nations protects the zone of liberty and independence within which states may depart from reason, provided they do no injury to others or the common good. (16) The "conventional" law of nations concerns the obligations that nations assume by treaty, binding only on the contracting parties. (17) And the "customary" law of nations is founded on tacit agreement among those nations who choose to observe it. (18) Nations may be presumed to consent to the voluntary law of nations, making it in a sense "positive" law, but treaty law and customary law, while both positive law as to those who choose to accept them, are also "arbitrary," and therefore subordinate to the necessary and voluntary law of nations. …