Kant's ethics of duty holds that only a rational being could have a moral duty because only a rational being could envision something beyond what he wants in the moment to satisfy himself or to do harm to others. Only a rational being could conceive of an ethical duty to act according to the consideration that all other human beings would act in the same way. No animal or irrational human being could conceive of such a concept, such a duty, such a way of life. The rationality required for moral duty leads the individual to recognize that "the moral worth of an action does not lie in the effect expected from it, nor in any principle of action which requires to borrow its motive from this expected effect" (Timmons 156). Only reason will act according to the worth of an action in and of itself, without consideration for the gain or loss of any effect.
The categorical imperative holds that "I am never to act otherwise than so that I could also will that my maxim should become a universal law" (Timmons 156). Kant allows no exceptions to this imperative. He apparently believes that once one brick is taken out of the wall--say, a justifiable lie or false promise is told because of the gains won or losses avoided--then the whole wall is in danger of immediately collapsing. This makes the moral duty "imperative" for him. It is not a matter of what is "prudent," for example, in the case of telling a lie to bring a positive result or avoid a negative result. It is only a matter of what is "right," based on the consideration that all human beings will act in the same way. If one person justifies a lie based on the effects, then everybody can justify lies based on the effects as they perceive them or hope them or fear them to be.
Kant gives a number of examples, such as the man contemplating borrowing money and making a promise to repay the loan when he knows he will not be able to do so. Clearly, if this false promise were made a universa...