Neutral monism is the view that both ‘mind’ and ‘matter’ are grounded in a more fundamental form of reality that is intrinsically neither mental nor material. It has often been treated as an odd fringe theory deserving of at most a footnote in the broader philosophical debates. Yet such attitudes do a grave disservice to its sophistications and significance for late nineteenth and early twentieth-century philosophy of mind and psychology. This paper sheds light on this neglected view by situating it (...) within broader historical monist debates about the mind and bringing attention to one of its central internal disputes regarding ‘mental chemistry’. By taking a closer look at how Ernst Mach, William James, and Bertrand Russell address the question of whether and how our mental episodes are composed of more basic elements, it highlights deep differences among their conceptions of the fundamental ‘neutral stuff’ and its relations to ‘mind’ and ‘matter’. (shrink)