Kant: Groundwork of the Metaphysics of Morals

The concept of freedom is the key to the explanation of the autonomy of the will

A will is a kind of causality of living beings in so far as they are rational, and freedom would be that property of such a causality, as it can be efficient independently of alien causes determining it; just as natural necessity is the property of the causality of all non-rational beings to be determined to activity by the influence of alien causes.

The explication of freedom stated above is negative and therefore unfruitful for gaining insight into its essence; but there flows from it a positive concept of freedom, which is so much the richer and more fruitful. Since the concept of causality carries with it that of laws according to which, by something that we call a cause, something else, namely the consequence, must be posited: freedom, though it is not a property of the will according to natural laws, is not lawless because of that at all, but must rather be a causality according to immutable laws, but of a special kind; for otherwise a free will would be an absurdity. Natural necessity was a heteronomy of efficient causes; for every effect was possible only according to the law that something else determines the efficient cause to causality; what else, then, can 4:447 freedom of the will be, but autonomy, i.e. the property of the will of being a law to itself? But the proposition: the will is in all actions a law to itself, designates only the principle of acting on no maxim other than that which can also have itself as its object as a universal law. But this is just the formula of the categorical imperative and the principle of morality: thus a free will and a will under moral laws are one and the same.