In the Critique of Pure Reason Kant introduced both transcendental idealism and transcendental arguments into philosophy. Transcendental arguments in general aim to establish conditions necessary for our having self-conscious experience at all. Transcendental idealism holds that such conditions do not hold independently of human subjects; those conditions obtain or are satisfied because they are generated or fulfilled by the structure or functioning of the subject’s cognitive capacities. Is transcendental idealism the only possible explanation of such conditions? I pursue this question by exploring a widely neglected issue, the transcendental affinity of the manifold of intuition. I argue the following: (1) This issue remains vital to the second edition of the Critique, even though many passages on the topic were omitted from that edition. (2) Kant’s link between transcendental idealism and transcendental arguments is substantive, not methodological. (3) Kant’s views on transcendental affinity show that there are non-subjective, transcendental material conditions for the possibility of unified self-conscious experience. (4) These conditions and Kant’s arguments for them directly undermine Kant’s own arguments for transcendental idealism. This criticism of Kant’s arguments for transcendental idealism is entirely internal to the first Critique. (5) These points reveal some serious flaws in Henry Allison’s defense of Kant’s idealism. (6) Realists, naturalists, and pragmatists have much to learn and to borrow from Kant’s transcendental analysis of the conditions of unified self-conscious experience.