In the article, Anna Musioł, by referring to the assumptions of Marian Massonius’s doctoral dissertation and taking into account the assumptions of his several smaller works, considers Massonius’s approach to the Kantian system of critical philosophy. Analyzing, inter alia, the problem of analytical and synthetic judgments, and a priori synthetic judgments, Musioł addresses the issue of the possibility of pure mathematics. She considers the problem of time and space and analyzes the ways of presenting Kantian antinomies and the theory of cognition developed in the context of idealism and realism as well as the realism of time and space. Additionally, Musioł focuses on the problem of Massonius’s moderate agnosticism and his scientific approach to philosophy. Finally, she proposes an answer to the fundamental question, Why did Massonius, like the early neo-Kantist Liebmann in 1865, challenge a return to Kant (Zurück zu Kant!) and advocate as necessary the development of a critical formula of the a priori forms of the mind?