The problem of reality as the foundation of the discourse on subject-matter in psychology originated in history of philosophy. Immanuel Kant’s critique of dogmatism disclosed a debate about the alternatives of realism and idealism. They are possible solutions for the problem, which means that they attend to the onto-gnoseological difference, i.e., the relationship between subject and object, representation and existence, or reason and being. To grasp these concepts, philosophy engages with gnoseology and metaphysics, discussing issues like the epistemic structure of affection or the order of categories. Further philosophical reflections reveal the interdependence of realism and idealism, initiating the search for a mediation, such as Kant’s transcendental idealism which espouses empirical realism. Albeit conceiving the problem, Kant himself did not satisfactorily solve it. His successors would advance the discourse and thereby transform the problem itself.