One of the great difficulties with Kant's moral philosophy is that it seems to imply that our moral obligations leave us powerless in the face of evil. Kant's theory sets a high ideal of conduct and tells us to live up to that ideal regardless of what other persons are doing. The results may be very bad. But Kant says that the law “remains in full force, because it commands categorically” (G 438–39). The most well-known example of this “rigorism,” as it is sometimes called, concerns Kant's views on our duty to tell the truth.

In two passages in his ethical writings, Kant seems to endorse the following pair of claims about this duty: first, one must never under any circumstances or for any purpose tell a lie; second, if one does tell a lie one is responsible for all the consequences that ensue, even if they were completely unforeseeable.

One of the two passages appears in the Metaphysical Principles of Virtue. There Kant classifies lying as a violation of a perfect duty to oneself. In one of the casuistical questions, a servant, under instructions, tells a visitor the lie that his master is not at home. His master, meanwhile, sneaks off and commits a crime, which would have been prevented by the watchman sent to arrest him. Kant says:

Upon whom … does the blame fall? To be sure, also upon the servant, who here violated a duty to himself by lying, the consequence of which will now be imputed to him by his own conscience.