In The Metaphysics of Morals, Kant argues that there is a highest good that all humans should aim to reach and that ethics is not determined by the individual but innate due to human reason. It is human’s unique rationality that leads us to function well. Human well-functioning is the highest good, an innate good, true of all people. The goal of well-functioning is not optional and ought to be respected. What, then, is a rational life? How do we live rationally? Kant says a rational life is free, as freedom allows one’s rationality to work best. Kant equates rationality with freedom and aims to teach us how to be free. Freedom is self-determination, the ability to be who we are. Because we are rational beings, the more rational we are, the freer we become, as reason allows us to solve problems to achieve our ends; our goals and ambitions. There are two types of freedom: external and internal. This delineation forms the two halves of The Metaphysics of Morals. In The Doctrine of Right, Kant addresses external freedom regarding one’s means or external ability to achieve their goal. Then he moves on to The Doctrine of Virtue, which addresses internal freedom and specific ends we ought to take on. These doctrines' order reflects their application to an individual soul. The Doctrine of Right must come first because the available means determine the ends you can pursue. Your means and freedom are connected; if you are not free–if you have not the means–to pursue a goal, then that end is necessarily off the table. Intrapersonal freedom means little when one lacks the interpersonal. One must be free from the challenge of other people. Once external freedom is established, then internal freedom becomes most important. The Doctrine of Virtue determines which ends we should seek and how we ought to improve ourselves and others. One must build themselves up against the challenge of the non-rational aspects of one's mind.

The Doctrine of Right outlines what is right and wrong and the types of rights people have. Determining right and wrong is not the final judgment of an action. A right action may still be morally wrong but this doctrine focuses on whether an action interferes with another’s means. An action is wrong if one dictates how another’s means are to be used. For example, in regards to a person’s body (the most essential means), if I break a person’s leg, I have not wronged them because I have limited their means, but because I have chosen how their means are used. The Body is our primary means by which we achieve all our ends, so we hold a natural right to our bodies–freedom from external interference. The innate right to one’s body is an example of a private right as it does not pertain to the public sphere. Public rights are those that arise by living in a community. In the Prostitution of Public Rights, Kant states that when we cannot avoid living side by side, we ought to leave the state of nature and live together in a rightful condition. We should live together under a civil authority that will help us respect rights. Public rights are those such as the right to acquisition, which allows for ownership. While in the state of nature, all things are ownable, everything can be owned–claimed as one’s property–and men have a natural right to all things. There is conflict as not all men own all things. The Universal Principle of Right states that any action is right so long it can exist, along with everyone’s freedom, under universal law. The effect is that while all things are ownable and can be claimed, ownership must respect others' freedom and what is already owned.

Obligatory ends lay out the conditions under which our ends must meet to be worthy of pursuit. We are morally required to pursue obligatory ends; the Supreme Principle of the Doctrine of Virtue states that one must act according to a maxim of ends that can be a universal law for everyone to have. This Supreme Principle guides us as we discover the duties of the human experience. Duties are ends we must have despite being tempted otherwise. Kant lays out two obligatory ends, which are duties: perfection of one’s self and the happiness of others. If rationality leads us to a well-functioning life, then perfection follows naturally, as perfect rationality would lead to the most well-functioning. Firstly, moral perfection is the development of the good. A will is a capacity for rational action, to will is to act because you think it is good to do. A good will is the most competent and improved version; a good will acts because it knows it is good to do so. So, moral perfection forms a perfect will, but how do we create one? Natural perfection is developing one's power in support of the will. We should cultivate our powers so that we may realize any ends that we may encounter. If we have found perfection to be the obligatory end for oneself, why is this not the end for others? Could we not make it our end to make others perfect? In a sense, this is still our goal, but we cannot be direct because of the very nature of ends. One cannot force an end on another, it must always come from one's own willing will. Even when made to work for another’s end, this end does not become one’s own until personally accepted. Though we cannot determine another’s end, we guide them to perfection through happiness. Once we meet our fundamental needs, we tend to look for challenges that make us happy. So the activities we naturally do to realize our happiness more or less are the same that make us perfect. We are attracted to activities where we can see improvement over time, which is the perfection of our abilities. So, we facilitate happiness to help others realize perfection. Its important to note we cannot remove all obstacles from their path; this would undermine their pursuit of perfection, but we can enable them to discover that there is more to life than happiness by helping them achieve happiness.