The Metaphysics of Morals by Immanuel Kant Chapter 1 Page 29

But it is soon clear to me that such a maxim will still only be based on the fear of consequences. Now it is a wholly different thing to be truthful from duty and to be so from apprehension of injurious consequences. In the first case, the very notion of the action already implies a law for me; in the second case, I must first look about elsewhere to see what results may be combined with it which would affect myself. For to deviate from the principle of duty is beyond all doubt wicked; but to be unfaithful to my maxim of prudence may often be very advantageous to me, although to abide by it is certainly safer. The shortest way, however, and an unerring one, to discover the answer to this question whether a lying promise is consistent with duty, is to ask myself, “Should I be content that my maxim (to extricate myself from difficulty by a false