The Critique of Practical Reason

By the matter of the faculty of desire I mean an object the realization of which is desired. Now, if the desire for this object precedes the practical rule and is the condition of our making it a principle, then I say (in the first place) this principle is in that case wholly empirical, for then what determines the choice is the idea of an object and that relation of this idea to the subject by which its faculty of desire is determined to its realization. Such a relation to the subject is called the pleasure in the realization of an object. This, then, must be presupposed as a condition of the possibility of determination of the will. But it is impossible to know a priori of any idea of an object whether it will be connected with pleasure or pain, or be indifferent. In such cases, therefore, the determining principle of the choice must be empirical and, therefore, also the practical material principle which presupposes it as a condition.

In the second place, since susceptibility to a pleasure or pain can be known only empirically and cannot hold in the same degree for all rational beings, a principle which is based on this subjective condition may serve indeed as a maxim for the subject which possesses this susceptibility, but not as a law even to him (because it is wanting in objective necessity, which must be recognized a priori); it follows, therefore, that such a principle can never furnish a practical law.