Critical realism is a frequently mentioned, but not very well-known, late nineteenth-/early twentieth-century philosophical tradition. Having its roots in Kantian epistemology, critical realism is best characterized as a revisionist approach toward the original Kantian doctrine. Its most outstanding thesis is the idea that Kantian things-in-themselves are knowable. This idea was—at least implicitly—suggested by thinkers such as Alois Riehl, Wilhelm Wundt, and Oswald Külpe. Interestingly enough, the philosophical position of the early Moritz Schlick stands in the critical realist tradition as well. As will be outlined in the course of this paper, both Schlick’s magnum opus General Theory of Knowledge (1918) and his seminal Space and Time in Contemporary Physics (1917) are based on the assumption that the objects of science are relations and that relations have the status of Kantian things-in-themselves. By way of conclusion, I shall point out that this— more or less directly—leads to the current debate over ‘structural’ realism.