The Metaphysics of Morals by Immanuel Kant Chapter 3 Page 3

principle: “To act on no other maxim than that which can also have as an object itself as a universal law.” Now this is precisely the formula of the categorical imperative and is the principle of morality, so that a free will and a will subject to moral laws are one and the same.

On the hypothesis, then, of freedom of the will, morality together with its principle follows from it by mere analysis of the conception. However, the latter is a synthetic proposition; viz., an absolutely good will is that whose maxim can always include itself regarded as a universal law; for this property of its maxim can never be discovered by analysing the conception of an absolutely good will.

Now such synthetic propositions are only possible in this way: that the two cognitions are connected