One classic question in the philosophy of mind is: What is the relation between mental and physical phenomena? Contemporary attempts at a solution to this problem typically reject substance dualism and tend towards monism, though this does not always mean reductive materialism. The problem of explaining how we are to account for mental states and their relations to physical phenomena (if there are such relations) or eliminate them altogether plays a central role and the options are many. We owe one the first comprehensive discussion of reductionism to the logical positivists of the Vienna Circle. But the philosophers of the Lvov-Warsaw School made significant contributions to this debate as well. Their own views, sadly enough, were not influential. Nonetheless, their analyses pertain to philosophical discussions that were lively and important at the time.