Why did Kant feel the need to write a Critique of Practical Reason after he had published the Groundwork of the Metaphysics of Morals? Is it because he acknowledged, with the ‘fact of pure practical reason’, that his attempt to deduce the moral law in the Groundwork had failed? Or did he have other reasons to write a second Critique before publishing his Metaphysics of Morals? Obviously enough, Kant seeks to establish in the Critique of Practical Reason ‘that there is pure practical reason’. Kant does so with his critique of the ‘entire practical faculty’ that underlies the merely pure practical and the empirical practical application of reason. These two forms of the application of practical reason (roughly) correspond to the distinction between the ‘analytic of pure practical reason’ and the ‘dialectic of pure practical reason’.

In the Analytic, Kant shows ‘that pure reason can be practical – that is, can of itself, independently of anything empirical, determine the will – and it does so by a fact in which pure reason in us proves itself actually practical, namely autonomy in the principle of morality by which reason determines the will to deeds’ (CpV 5:42). In the Dialectic in contrast, Kant maintains that pure reason in its practical application gets into conflict with itself while determining the highest good, which runs counter to the facticity of pure practical reason and is in danger of undercutting the validity of the moral law: ‘If, therefore, the highest good is impossible in accordance with practical rules, then the moral law, which commands us to promote it, must be fantastic and directed to empty imaginary ends and must therefore in itself be false’ (CpV 5:114).