I examine the division of labor between the Metaphysical Deduction (MD) and the Transcendental Deduction (TD). Against a common reading, I argue that the MD is insufficient to prove the a priori origin of the categories. For both Kant and his main opponent, namely Hume, the question of whether the categories have an a priori origin in the pure understanding is inseparable from the question of whether they have objective validity. Since the MD does not establish the objective validity of the categories, it cannot establish their a priori origin either. The MD is nevertheless an indispensable part of Kant’s project because it lays the argumentative groundwork for the proof structure of the TD and because it provides the systematic plan for the future metaphysics of experience.
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