On the transcendental deduction in Kant’s Groundwork III

The purpose of the third section of Kant’s Groundwork is to prove the possibility of the categorical imperative. In the end of the second section, Kant establishes that a proof like this is necessary to show that morality is ‘something’ and ‘not a chimerical idea without any truth’ or a ‘phantom’ (1785: 445). Since the categorical imperative was established as a synthetic a priori practical proposition, in order to prove its possibility it is necessary ‘to go beyond cognition of objects to a critique of the subject, that is, of pure practical reason’ (1785: 440). Kant names this kind of proof a deduction. The present paper intends to (1) show the argument whose purpose is to justify the categorical imperative; (2) show that the argument is a transcendental deduction; (3) present the argument as it is reconstructed by Allison, and (4) show that, although it seems compelling, the position of the commentator could not be accepted by Kant himself.