The Metaphysics of Morals by Immanuel Kant Chapter 2 Page 62

in which case whatever has reference to anything empirical is necessarily excluded; since if reason of itself alone determines the conduct (and it is the possibility of this that we are now investigating), it must necessarily do so a priori.

The will is conceived as a faculty of determining oneself to action in accordance with the conception of certain laws. And such a faculty can be found only in rational beings. Now that which serves the will as the objective ground of its self-determination is the end, and, if this is assigned by reason alone, it must hold for all rational beings. On the other hand, that which merely contains the ground of possibility of the action of which the effect is the end, this is called the means.

The subjective ground of the desire is the spring, the