The Metaphysics of Morals by Immanuel Kant Chapter 3 Page 21

conceive ourselves as free, we transfer ourselves into the world of understanding as members of it and recognise the autonomy of the will with its consequence, morality; whereas, if we conceive ourselves as under obligation, we consider ourselves as belonging to the world of sense and at the same time to the world of understanding.

How is a Categorical Imperative Possible?

Every rational being reckons himself qua intelligence as belonging to the world of understanding, and it is simply as an efficient cause belonging to that world that he calls his causality a will. On the other side he is also conscious of himself as a part of the world of sense in which his actions, which are mere appearances [phenomena] of that causality, are displayed; we cannot, however, discern how