The Metaphysical Elements of Ethics by Immanuel Kant Chapter 2 Page 2

and these could not give any notion of duty; since this, the categorical ought, has its root in pure reason alone. Indeed, if the maxims were to be adopted in accordance with those ends (which are all selfish), we could not properly speak of the notion of duty at all. Hence in ethics the notion of duty must lead to ends, and must on moral principles give the foundation of maxims with respect to the ends which we ought to propose to ourselves.

Setting aside the question what sort of end that is which is in itself a duty, and how such an end is possible, it is here only necessary to show that a duty of this kind is called a duty of virtue, and why it is so called.

To every duty corresponds a right of action (facultas moralis generatim), but all duties do not imply a corresponding right