Full context:   Rational persons regard themselves as belonging to both the world of understanding and the world of sense. As a member of the world of understanding, a person's actions would always conform to the autonomy of the will. As a part of the world of sense, he would necessarily fall under the natural law of desires and inclinations. However, since the world of understanding contains the ground of the world of sense, and thus of its laws, his actions ought to conform to the autonomy of the will, and this categorical "ought" represents a synthetic proposition a priori.