The Law
Kant believes that for moral judgments to be valid they need to be based on a law, and to be formulated as a law. The law is a crucial concept in all of Kant's work, and he frequently uses law-related images and language, especially that of judging. The appeal of the law for Kant is its ability to be universal, and therefore to bring us out of the problem of solipsism and relativism—namely, the belief that our moral judgments are relevant only for us, and that we have no right to hold others to the same standards. For Kant, the law is the means by which human minds connect themselves to one another by seeing themselves as part of a community of rational thinkers.
Duty
For Kant, duty is the opposite of inclination. Duties are those things we feel that we must do—often in spite of the fact that we don't want to do them. The ability to feel the pull of a duty whose legitimacy we recognize— even though we might feel inclined to do otherwise—is for Kant the basic fact of our experience that lets there be a moral philosophy at all. The Groundwork for the Metaphysics of Morals works backwards from this fact of our experience to determine by what "right" we feel these duties and choose to follow them. The concept has an unmistakably Christian inflection—the archetypical duty Kant mentions Christ's command that we love our enemies.
Freedom
For Kant, freedom and morality are reciprocal concepts. We are free from the tyranny of our instincts and inclinations precisely because we are able to make moral judgments. Freedom consists in resisting the authority of anything other than our own mind—including our own bodies and emotions—to set laws for us. Therefore, reason is the only possible basis for freedom, and the only possible basis for morality. This notion of freedom would be crucial for the Enlightenment—namely, that freedom does not consist in being free from the law, which would be chaos, but rather in being able to legislate laws for oneself.
Autonomy
Central to Kant's anthropology (his account of man) is the notion that man's freedom and his dignity consists in his autonomy—his ability to give laws to himself. Considering the perceived coldness of the categorical imperative and Kant's emphasis on duty, it is important to note how quietly revolutionary this point is. Only man can be the proper source of his own moral authority and of his own freedom. The church, the state, the family: these are all simply forms of heteronomy, of outside tyranny, that can never serve as the legitimate basis for moral judgment. Kant's categorical imperative can therefore be seen as a tool for man to recover his autonomy from these outside powers. Only by embracing this autonomy does man properly become man.
Ends
Much of the argumentation in the second half of the Groundwork centers around the question of means and ends. Kant sees human beings as possessing reason, in other words, as being in the unique position of positing their own ends, not just for themselves as individuals, but for humanity as such. In this, Kant echoes the French philosopher Jean-Jacques Rousseau, who argues that self-perfectibility is the unique quality that distinguishes human beings from animals. The notion that morality consists of treating man as an end in himself goes hand in hand with Kant's methodology, namely, that we use our minds to investigate our own minds. Building on Rousseau, Kant sees man as the only creature whose goal can be the discovery of himself and the recovery of his dignity.
The Categorical Imperative
The categorical imperative is without a doubt the concept for which Kant is most famous. The principle of the categorical imperative is that when willing an action, we imagine our willing it to be the basis for a universal law. Can we imagine this law being a principle on which every individual should act? If so, then—and only then—can we consider it just. In effect, it is a kind of thought experiment that is meant to reveal to us the influence of our own inclinations, and to preserve our autonomy as the only legitimate authority that can legislate moral laws, without at the same time falling into the solipsism that would make morality completely impossible.
The Kingdom of Ends
Kant argues that when we use the categorical imperative to guide our moral actions, we imagine ourselves as belonging to a community of like-minded people who embrace their role as law-givers, and understand themselves as subjects to the moral laws of others, whose legitimacy we accept with our own reason. The kingdom of ends reveals the democratic and communitarian side of Kant's thinking, which is the flip side of his emphasis on autonomy. Reason is what lets us acknowledge the dignity of other human beings, because reason is by its nature universal; all people who use reason must imagine their judgments as being legitimate for all other rational beings. The kingdom of ends has a distinctly democratic flavor, since in it each person is both ruler and subject, but it also stresses the extent to which Kant understands the process of thought and reason as one that is social and political, that takes place in the company of others and has ramifications for them as well. If human beings did not have a collective life, there would be no basis for morality, and no need for it.
