This article is the fifth in a series of papers on the International Law Association's assertion that the principle of prevention (i.e. the obligation of states in international law to prevent transboundary harm, also known as the no-harm rule) properly belongs to the corpus of international climate change law. I have been the only participant in the debate so far to refute the ILA's position. Here, I respond to the fourth article in the series, by Benoit Mayer. While Mayer has produced a lucid and helpful argument, he commits several errors in the process of defending and elaborating the ILA thesis. I address them under this essay's broad title because they are essentially errors of method.