The Metaphysics of Morals by Immanuel Kant Chapter 2 Page 30

purpose to be attained by it. This imperative is categorical. It concerns not the matter of the action, or its intended result, but its form and the principle of which it is itself a result; and what is essentially good in it consists in the mental disposition, let the consequence be what it may. This imperative may be called that of morality.

There is a marked distinction also between the volitions on these three sorts of principles in the dissimilarity of the obligation of the will. In order to mark this difference more clearly, I think they would be most suitably named in their order if we said they are either rules of skill, or counsels of prudence, or commands (laws) of morality.

For it is law only that involves the conception of an unconditional and objective necessity,