This chapter discusses attempts made in the early twentieth century to give psychology a foundation in dialectical materialist philosophy. It argues that Lacan’s critical position on materialism was in part a response to this movement. The chapter begins by discussing the work of Lev Vygotsky, Georges Politzer, and Henri Wallon, three Marxist psychologists who thought that psychoanalysis would have an important role to play in the development of a dialectical materialist psychology. The chapter then makes a close reading of Lacan’s responses to Pavlovian reflexology, to illustrate Lacan’s critical approach to materialist theories of psychology, and to demonstrate why he thought psychoanalysis was incompatible with them. As the chapter aims to show, Lacan’s theory of the unconscious directly responded to errors he perceived in attempts to make psychology a materialist science. The chapter argues that this allowed Lacan to make a deeper argument, about flaws in the conceptual basis of materialism itself.