Philosophy may be divided into three fields: physics (the study of the physical world), ethics (the study of morals), and logic (the study of logical principles). These fields may involve either "empirical" study of our experiences, or "pure" analysis of concepts. "Metaphysics" is the study of pure concepts as they relate to moral or physical experience.
People generally presume that moral principles must apply to all rational beings at all places and all times. Moral principles must therefore be based on concepts of reason, as opposed to particularities of culture or personality. The goal of the Grounding for the Metaphysics of Morals is to develop a clearer understanding of moral principles, so that people may better avert distractions.
Several general principles about moral duties may be advanced. First, actions are moral if and only if they are undertaken for the sake of morality alone (without any ulterior motive). Second, the moral quality of an action is judged not according to the action's consequences, but according to the motive that produced it. Third, actions are moral if and only if they are undertaken out of respect for the moral law (as opposed to some other motivation such as a need or desire).
Since specific interests, circumstances, and consequences cannot be considered, the moral "law" must be a general formula that is applicable in all situations. Rather than commanding specific actions, it must express the principle that actions should be undertaken with pure motives, without consideration of consequences, and out of pure reverence for the law. The formula that meets these criteria is the following: we should act in such a way that we could want the maxim (the motivating principle) of our action to become a universal law. People have a decent intuitive sense for this law. Still, it is helpful for philosophy to state the law clearly so that people can keep it in mind.
It is nearly impossible to find examples of pure moral actions. Nearly every action we observe can be attributed to some interest or motivation other than pure morality. Yet this should not discourage us, for moral principles come from reason, not from experience. Indeed, moral principles could not come from experience, for all experiences depend on particular circumstances, whereas moral principles must have absolute validity, independent of all circumstances.
Because it applies in all circumstances, reason's fundamental moral principle may be called the "categorical imperative." The categorical imperative may be expressed according to the same formula as the moral law: act only in such a way that you could want the maxim (the motivating principle) of your action to become a universal law. When people violate the categorical imperative, they apply a different standard to their own behavior than they would want applied to everyone else in the form of a universal law. This is a contradiction that violates principles of reason.
The categorical imperative may also be formulated as a requirement that we must not treat other rational beings as mere means to our own purposes. Rational beings have the capacity to pursue predetermined objectives ("ends") by means of their will, yet in pursuing their goals they never think of themselves as mere means to another purpose; they are themselves the purpose of their actions- -they are "ends in themselves. If we treat other rational beings as mere means, we contradict the fact that all rational beings are ends in themselves. In this case, our principles could not be universal laws, and we would violate the categorical imperative.
Another way of stating the point that rational beings are ends in themselves is to say that rational beings are simultaneously the authors and the subjects of the principles they execute through their will. The categorical imperative may also be formulated as a requirement that we act only according to principles that could be laws in a "kingdom of ends"--that is, a legal community in which all rational beings are at once the makers and subjects of all laws.
The argument so far has established what the moral law is, but has not demonstrated why we feel we should be moral. The basis for morality is the concept of freedom. Freedom is the ability to give your own law to your will. When we follow the demands of some need, desire, or circumstance, we are in a state of "heteronomy"; our will is determined by something outside of ourselves. When we follow the categorical imperative and chose maxims that could be universal laws, we are in a state of "autonomy"; we use reason to determine our own law for ourselves. In other words, we are free.
Freedom of the will can never be demonstrated by experience. It is a principle of reason that everything we understand may be explained on the basis of prior conditions. In other words, the world we observe and understand is a world governed by the principle that every event was caused by another event. Yet this world is nothing more than the picture that reason develops in making sense of "appearances." The world of "things in themselves"--the objects underlying appearances--may have different qualities, including freedom of the will. We can have no knowledge of things in themselves. Thus freedom of the will may be neither proven nor disproven. All that we may know is that we have a concept of freedom of the will, and that morality may be based on this concept.