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.
