Kant: Fundamental Principles of the Metaphysics of Morals

We shall therefore have to investigate a priori the possibility of a categorical imperative, as we have not in this case the advantage of its reality... being given in experience, so that [the elucidation of] its possibility should be requisite only for its explanation, not for its establishment. In the meantime it may be discerned beforehand that the categorical imperative alone has the purport of a practical law.