The Metaphysics of Morals by Immanuel Kant Chapter 2 Page 91

Such then is the formula of an absolutely good will.

Rational nature is distinguished from the rest of nature by this, that it sets before itself an end. This end would be the matter of every good will. But since in the idea of a will that is absolutely good without being limited by any condition (of attaining this or that end) we must abstract wholly from every end to be effected (since this would make every will only relatively good), it follows that in this case the end must be conceived, not as an end to be effected, but as an independently existing end. Consequently it is conceived only negatively, i.e., as that which we must never act against and which, therefore, must never be regarded merely as means, but must in every volition be esteemed as an end likewise.