The Metaphysics of Morals by Immanuel Kant Chapter 2 Page 76

interest, since a will so dependent would itself still need another law restricting the interest of its self-love by the condition that it should be valid as universal law.

Thus the principle that every human will is a will which in all its maxims gives universal laws, * provided it be otherwise justified, would be very well adapted to be the categorical imperative, in this respect, namely, that just because of the idea of universal legislation it is not based on interest, and therefore it alone among all possible imperatives can be unconditional.

Or still better, converting the proposition, if there is a categorical imperative (i.e., a law for the will of every rational being), it can only command that everything be done from maxims of one's will regarded as a will which