Third Section — Transition from the Metaphysic of Morals to the Critique of Pure Practical Reason

The Concept of Freedom is the Key that explains the Autonomy of the Will

The will is a kind of causality belonging to living beings in so far as they are rational, and freedom would be this property of such causality that it can be efficient, independently of foreign causes determining it; just as physical necessity is the property that the causality of all irrational beings has of being determined to activity by the influence of foreign causes.