Based on two intertwined distinctions (I-Archetype / I-Rule and categorial / systematic / ideal reading of the connection between Analytic and Dialectic), both the status and the function of transcendental illusion are reconstructed within the framework of Kant’s regulative use of reason. It is shown that in Kant’s philosophy multi-layered a priori structures and different claims are connected with each other. In contrast to the constitutive use of reason, the regulative one has merely a certain degree of objective validity. Nevertheless, regulative ideas are not mere conventions, but pragmatic principles through which science is possible at all. This differentiation makes it possible to situate Kant’s approach in the context of the debates on the dynamization and relativization of the a priori in the long 19th century.