Much attention has been devoted to Kant's famous doctrine of autonomy, the proposition that morality finds its source in the will's self-legislation, depending neither for the content of its principle nor for its motivating power on any source, natural or transcendent, outside the will and its power of self-rule. But Kant also advances another striking proposition about the will, that it is nothing but practical reason. Though less extensively investigated, this idea is at least as important, both in its own right and for the light it throws on other parts of his ethics, including his doctrine of autonomy, which can seem unduly voluntaristic if not appreciated in its practical-cognitivist setting. According to tradition, the will is rational desire. Kant too understands the will in terms of reason and desire, but his way of combining these notions in his conception of a practical application of reason accounts for much of what is distinctive in his moral philosophy.

This chapter examines Kant's mature conception of the will, as presented in the Introduction to the Metaphysics of Morals. Kant approaches this conception from a definition of the faculty of desire. But before doing that or indeed anything else, he makes a few remarks about the system of philosophical rational knowledge within which the metaphysics of ­morals is situated. Though somewhat fragmentary, these remarks recall the account he offered at the outset of the Groundwork of the Metaphysics of Morals – the work that, as its title announces, lays the ground for the metaphysics of morals.