Since an imperative with universal and intrinsic validity cannot include any circumstantial considerations, the only possible categorical imperative is that actions must conform to a requirement of universal validity. Thus the categorical imperative may be formulated as follows: act only in such a way that you could want the maxim (the motivating principle) of your action to become a universal law. This statement can also be given this formulation: act as if your action would establish its maxim as a universal law of nature.
Four examples demonstrate how common notions of duty conform to the categorical imperative. First, people have a duty not to commit suicide, because it clearly cannot be a law of nature for all people to kill themselves; if everyone died, nature itself would cease to exist. Second, people have a duty to borrow money only if they have the intention of paying it back, because if everyone failed to pay their debts no one would ever lend money. Third, people have a duty to cultivate their talents, because if everyone spent their life in idleness no one would benefit from human capacities. Fourth, people have a duty to assist others in need, because if all of us were heartless then none of us could find assistance in times of need.
In each of these cases as in all cases where people neglect their duties, individuals are involved in a contradiction: they accept the objective validity of the law, and yet they want an exception to be made for them.
Chapter 2 picks up where Chapter 1 left off. Kant again defends his efforts by noting that a clearer understanding of a priori principles can help to strengthen our moral sense. He reiterates that actions are moral if and only if they are performed out of a pure sense of duty. And he again emphasizes that pure moral principles must be grasped a priori.
Kant adds an important new twist to these by now familiar arguments, however. In Chapter 1 Kant argued that only a priori ideas could have the purity and universality that we expect of moral principles. All other ideas, he observed, are dependent on specific circumstances and situations. At the start of Chapter 2 Kant points out an important implication of this analysis: because all actions depend on specific circumstances, it would be impossible for us to derive a priori ideas from examples in our experience.
In order to understand this point, it may be helpful to recall Kant's arguments about causation that were summarized in the Context section. In the Critique of Pure Reason, Kant argues that it would be impossible for us to derive our notion of causation from our observations of the world. ##David Hume## argued that the notion of a cause- effect relationship is simply a manner of speaking that we use when we talk about events that regularly occur one after another. Kant countered Hume's view by arguing that our notion of causation is too fundamental to be simply a manner of speaking. In Kant's view, causation is an a priori concept; it is an idea that occurs to us automatically whenever we think about the world. Whether we are observing events or imagining possibilities, we cannot help but think in terms of causation.