The Metaphysics of Morals by Immanuel Kant Chapter 3 Page 12

ought to detach ourselves from these interests, i.e., to consider ourselves as free in action and yet as subject to certain laws, so as to find a worth simply in our own person which can compensate us for the loss of everything that gives worth to our condition; this we are not yet able to discern in this way, nor do we see how it is possible so to act- in other words, whence the moral law derives its obligation.

It must be freely admitted that there is a sort of circle here from which it seems impossible to escape.

In the order of efficient causes we assume ourselves free, in order that in the order of ends we may conceive ourselves as subject to moral laws: and we afterwards conceive ourselves as subject to these laws, because we have attributed to ourselves freedom of will: for