General Principles of the Metaphysics of Morals in the treatment of Pure Ethics

First. A duty can have only a single ground of obligation; and if two or more proof of it are adduced, this is a certain mark that either no valid proof has yet been given, or that there are several distinct duties which have been regarded as one.

For all moral proofs, being philosophical, can only be drawn by means of rational knowledge from concepts, not like mathematics, through the construction of concepts. The latter science admits a variety of proofs of one and the same theorem; because in intuition a priori there may be several properties of an object, all of which lead back to the very same principle. If, for instance, to prove the duty of veracity, an argument is drawn first from the harm that a lie