In the Transcendental Dialectic, Kant searched for a universal concept of reason different from the understanding and offered the short formula “the faculty of principles”. I will argue that this is only one and not the most pertinent and general mark of the concept of reason. There are more compelling short expressions in Kant’s Reflexionen, the third Critique and/or in the reception of Kant’s works: “the faculty of ideas” or reason in the narrower sense. The latter narrows down the logical (...) sphere of the concept of rational faculties, and the former contains reason’s most basic mark: ideas. The first part of this article will focus on preliminary remarks on Kant’s philosophical methodology and conceptual analysis. The second part will analyze the division of the logical sphere of the concept of reason by nine necessary and coherent marks. These marks are centered around the concept of ideas, which allows for an ideas-first understanding of reason and preference for the formulas the faculty of ideas and reason in the narrower sense. The article will end with an “imperfect definition” of reason based on those nine marks. (shrink)