Ernst Mach and William James were personal friends and intellectual allies. Might there have been an American pragmatist influence on Logical Positivism via James’s influence on Mach? I explore the relationship between these two friends, arguing that, if anything, Mach’s instrumentalism about science actually influenced James more than James’s pragmatism influenced Mach. What is more, empirical and not philosophical issues dominated their intellectual exchanges, and I examine the three topics about which they most frequently engaged one another: the role of the semicircular canals in the perception of bodily orientation, the question of whether there is a distinctive 'feeling of effort' (Innervationsgefühl), and the nature of visual spatial perception. The debate over the Innervationsgefühl is particularly interesting because James apparently convinced Mach to reverse his position on the matter. In short, we remember Mach as a master experimentalist and James as a philosophical populariser, so it is a surprise to learn that the main philosophical influence apparently flowed from Mach to James, while the main influence when it comes to matters of empirical interest actually flowed the other way.