Legal rules come and go. Methods of producing law may also flow and ebb. The authors of the call for papers in connection with this online Agora suggest that there is possible evidence that treaty as a method of producing international law is ebbing, and may be dying. I see no such evidence at present; rather, I argue here that the dying source of international law is not treaty but custom. In the more distant future, however, treaty, too, may become obsolete or at least less salient.

There are four categories of tools of international social cooperation: (i) international law produced through treaty (referred to herein simply as treaty); (ii) international law produced through custom, known as customary international law (CIL); (iii) international law produced through international organization decision-making (international legislation); and (iv) non-legal cooperative institutions (soft law). One of the signal characteristics of treaty is that no state is bound that has not explicitly and specifically consented. In order to sharpen the difference between treaty and international legislation, let us focus on international legislation produced by majority voting, which today is rare outside the European Union, but which may bind states without their specific consent.