Giving a false promise is an example of an action that violates this moral law. Some people might reason that they should be permitted to lie in order to escape a difficult situation. Conversely, some people might reason that they should not lie because in doing so they might create still greater difficulties for themselves in the future. In both cases, the motivating consideration is a fear of consequences, not pure respect for duty. Applying the moral law reveals that lying can never be a universal law. If everyone were to give false promises, then there would be no such thing as a promise.
Although most people are not aware of the moral law in any conscious sense, even untrained minds show a remarkable ability to abide by it in practice. People's intuitive sense for theoretical matters is generally poor. By contrast, their intuitions in the field of practical reason--in other words, their intuitions about morality--are generally correct. For instance, people generally recognize that moral concerns should not include physical ("sensuous") motivations. Nevertheless, a philosophical understanding of morals is important, because untrained minds may be deceived and distracted by non-moral needs, concerns, and desires.
Since Kant's argument in this chapter is complex, it may be helpful to paraphrase it in a more compressed form. Kant starts from the presumption that an action is moral if and only if it is intrinsically good--good "in itself," as he puts it. This view has two main implications. First, moral actions cannot have impure motivations. Otherwise, the action would be based on some secondary motivation, and not on the intrinsic goodness of the action. Second, moral actions cannot be based on consideration of possible outcomes. Otherwise, the action would not be good in itself, but would instead be good in that it brought about a particular outcome.
If we can consider neither motivating circumstances nor intended outcomes, then we need to find a principle with universal validity--a principle that is valid no matter what issue we are considering. The only principles that fit this criterion are the a priori principles of reason--that is, the principles of logic that we have to follow if our statements are to make sense.
One fundamental principle of logic is the principle of non-contradiction: statements don't make sense if they contradict themselves. Kant's moral law is based on this principle of non-contradiction. In order for your action to be moral, he argues, it must be good in itself. In order for it be good in itself, it must make sense in pure logical terms. In order for it to make sense, it must not contradict itself. If you lie but expect other people to believe you, you contradict yourself. Your motivation lacks universal validity and is therefore immoral.
At the end of the chapter, Kant argues that his analysis of the moral law amounts, in effect, to a formalization of a moral sense that we already use intuitively. He argues that a more conscious understanding of the principles of our moral sense can help us to behave more morally. Given the complexity of his argument, it may seem surprising that he believes he is only teaching us what we already know. His claim may seem less surprising if we recognize that his moral law is fundamentally the same as the Biblical teaching that we should "do unto others as we would have done unto us." Kant argues that we violate rational principles of morality when we contradict ourselves, and that we contradict ourselves when we act in a way that we would not want others to imitate. In practice, his doctrine amounts to a doctrine of respect for others.