When the will acts morally, it does not submit to a law imposed from outside — not by God, not by social convention, not by the desire for happiness. It acts on a law it has given itself through reason. This self-legislation is what Kant means by autonomy. The moral law is not alien to the rational will; it is its own expression.
Kant argues that every false principle of morality results from heteronomy — from the will seeking its law outside itself. Whether the external source is happiness, divine command, social approval, or moral sentiment, the result is always hypothetical imperatives rather than categorical ones. A will that is governed by something other than its own rational legislation cannot produce genuine moral obligation.
Autonomy is not merely the mechanism of moral action — it is the source of human dignity. Because rational beings are capable of self-legislation, they are ends in themselves. They cannot be assigned a price. Kant sees the recognition of autonomy as what unifies his three formulations of the categorical imperative: to act on universalisable maxims, to respect humanity, and to legislate in a kingdom of ends are all expressions of the will's autonomy.
Autonomy of the will is introduced as the supreme principle of morality near the end of the Second Section, and its connection to freedom is worked out in the Third Section.
