In this paper Andrea Staiti presents Rickert’s foundation of transcendental idealism in his major work Der Gegenstand der Erkenntnis (1892 - 1929). First, he introduces the premises of Rickert’s argument for transcendental idealism: (1) knowledge requires a transcendent criterion of truth; (2) the theory of knowledge must aim at presuppositionlessness; (3) the theory of knowledge must start with the standpoint of immanence. Second, he introduces the meaning of transcendental idealism for Rickert and the arguments that are meant to establish its validity. Rickert draws on the history of philosophy and blends together idealism in the Platonic sense (the existence of an ideal domain beyond the sensible world) and idealism in the modern Cartesian sense (the primacy of ideés as occurrent episodes of thinking in consciousness). Rickert shows that arguments that are meant to establish the existence of a transcendent reality and that in order to salvage the idea of transcendence, which is required by the phenomenon of knowledge, we need to focus on the phenomenon of judgment. When we judge, we acknowledge a transcendent ought that breaks the circle of consciousness and points toward a domain of value. In the third section of the paper Andrea Staiti addresses a few lines of criticism that have been leveled at Rickert and discuss his replies.