The fundamental ideas of philosophical conceptions of Alexander I. Vvedensky (1856–1925) and Vasily E. Sesemann (1884–1963), philosophers representing the Russian variant of neo-Kantianism and post-neo-Kantianism, are discussing in this article. Particular attention is focused on the theory of “Logism” and the problem of what is irrational. According to Vvedensky, only logic, which is a part of epistemology, answers the crucial question to Criticism of the limits of knowledge. The Russian neo-Kantian philosopher negates the possibility of scientific metaphysics and at the same time he deems the metaphysical truths as a matter of faith; metaphysics as “morally founded belief” is an essential component of a harmonious world outlook. Sesemann, as a Russian post-neo-Kantian, stresses the limitations of natural-scientific model of cognition and a logical approach, and also assigns new meaning to the concepts of what rational and irrational.