As Thomas Uebel has recently argued, some early logical positivists saw American pragmatism as a kindred form of scientific philosophy. They associated pragmatism with William James, whom they rightly saw as allied with Ernst Mach. But what apparently blocked sympathetic positivists from pursuing commonalities with American pragmatism was the concern that James advocated some form of psychologism, a view they thought could not do justice to the a priori. This paper argues that positivists were wrong to read James as offering a psychologistic account of the a priori. They had encountered James by reading Pragmatism as translated by the unabashedly psychologistic Wilhelm Jerusalem. But in more technical works, James had actually developed a form of conventionalism that anticipated the so-called “relativized” a priori positivists themselves would independently develop. While positivists arrived at conventionalism largely through reflection on the exact sciences, though, James’s account of the a priori grew from his reflections on the biological evolution of cognition, particularly in the context of his Darwin-inspired critique of Herbert Spencer.