Summary: Regardless of the type of theory or knowledge praxis one engages in, the question of the relation between theory and practice or theory and reality arises as a problematic one. To address this issue, this article explores the limits of the nominalist argument that views concepts as post res labels attached to concrete objects. Among other things, the reality to which the number zero (as a concept) corresponds constitutes a major difficulty for nominalism. Based on this difficulty, the article elaborates another epistemological view, a specific kind of rationalism or dialectical materialism that can be found in the work of Sigmund Freud and Jacques Lacan. This dialectical materialism emphasizes the specific ability of concepts to mark and realize negative features in reality, making it possible to effect changes in it. In this sense, both the question of the relation of theory to reality and that of change in psychoanalytic practice can be viewed from a different angle than that of a simple one-to-one correspondence. To put this view to the test, the article explores the extent to which it can get a grip on some of the slippery fundamental concepts of psychoanalysis, such as the unconscious and sexuality.