The Metaphysics of Morals by Immanuel Kant Chapter 2 Page 75

conception of duty. But we could not prove independently that there are practical propositions which command categorically, nor can it be proved in this section; one thing, however, could be done, namely, to indicate in the imperative itself, by some determinate expression, that in the case of volition from duty all interest is renounced, which is the specific criterion of categorical as distinguished from hypothetical imperatives.

This is done in the present (third) formula of the principle, namely, in the idea of the will of every rational being as a universally legislating will.

For although a will which is subject to laws may be attached to this law by means of an interest, yet a will which is itself a supreme lawgiver so far as it is such cannot possibly depend on any