I read this book for a graduate seminar on Ethics. In "The Groundwork of the Metaphysics of Morals," Kant astutely observes how ordinary people speak about morality. He argues, ordinary people's views are presupposed about morality, that there is one supreme moral principle it is the "Categorical Imperative" which is discussed in section two of the book. In section one, he talks about value, and special regard or esteem we have for someone who does the right things. Sometimes, people do the right things for wrong reasons. He is interested in what has to be true for an action to have moral worth. He has a kind of criticism of Utilitarians. Utilitarians say you can talk about what is good, i.e., happiness, before talking about what is right or moral. For Kantians "right" comes prior to the question of what is good. One must bring morality in before talking about the good. Talent and ability is good if put to good use, it can also be bad; for example computer hackers creating "viruses." Only one thing is good in and of itself unconditionally, which is a good "will" which means the will of a person who wants to do the right thing. Even if the plan doesn't work out they still have good will. They desire to do the right thing because it is the right thing.
Kant argues that action has moral worth only if it is done out of respect for duty. For example, if a shopkeeper is honest in an effort to look good to customers he did the right thing, but only in "conformity with duty." He acted out of inclination. If the shopkeeper is honest out of being nice or likes kids then his action is still done out of inclination because he "likes to do it," but his moral worth is less in the action. The shopkeeper who has moral worth is the one who is honest because it was the right thing to do.
Kant's 2nd proposition is that an action gets its moral worth from its "maxim." Maxim is a technical term for Kant; maxim is a kind of principle that explains why someone does something. Kant thinks that whenever we act on an action there always is some maxim that we are acting on. So you can think of a maxim as having the form: I will do A (some kind of action) in C (some set of circumstances) for P. (for some purpose). Now it is not as if normally when you act you formulate to yourself here is my maxim, here is what I am acting on. However, Kant thinks that when you do something there is some maxim that describes your choice. Therefore, Kant thinks there is an underlying maxim there, and it is this maxim Kant thinks that is the real decider about whether your action has moral worth or not. Only actions with the right maxim he thinks have moral worth.
Kant's3rd proposition is that duty is the necessity of acting out of respect for law, (not government law). Kant thinks that actions get there moral worth from being done out of respect for a "universal moral law" that is binding on all rational beings. This is the real clincher for Kant in the first section of his book. That actions have moral worth when the person who did the action did it because he or she thought that there is a moral law that commands them to do the action. For example, "I must obey that law, it is necessary; I have no choice but to obey the law." That notion of following the universal moral law is what gives the action, Kant thinks, its worth that is what makes it worthy of the special esteem he thinks we give actions when people have done them just because they thought they were right.
This is the setup for Kant's all important and famous "categorical imperative which he argues applies to everyone. This is all in Section II. We can deduce many rules from the categorical imperative. The categorical imperative is the only one fundamental principle of morality, but it can be formulated in a variety of different ways. Kant had three formulas of the categorical imperative. All three formulas are a different way of wording the categorical imperative. The categorical imperative is a moral law that has to apply to all rational beings, regardless of what ends they have.
The 1st formula is the "Universal Law Formula," which Kant said that every action has a maxim. Whenever you do anything there is some maxim, some subjective principle you are acting on and that we shouldn't act on any maxim that we couldn't choose to become a universal law. Kant then goes on to say that still for every action, in addition to its maxim, there is also an end, every action has an end. Mill and Aristotle also say this. Kant says if you have a categorical imperative there has to be an end that all rational beings see as a good end, this is mandatory. It can't be some kind of effect of our actions, because the kinds of things we produce in the phenomenal world only have value because we care about them. It has to be an end that all rational beings must care about; it can't be a utilitarian end, or one from consequences. If we value it as an end it has value, if we choose it as an end then there is a claim on others to see it as important as well, thus, this is a real mandatory end that humanity itself sees. Rational nature itself then has value.
The 2nd formula is "The Formula of Humanity" which states, I'm not just special because everyone thinks they are valuable. Can't treat other people as merely a means to an end. This gives one a claim to the help from other people. Slavery is an epitome of this formula as an example. It is wrong to treat people ONLY as a means to an end. (However, you are not using a grocery bagger as such because he gets paid). When you put the Universal Law Formula and Formula of Humanity together, you get another way of formulating the Categorical Imperative.
The 3rd Formula is "The Kingdom of Ends Formula." We ought to be thinking of ourselves as legislators for a kingdom of people who are ends to themselves and for Kant that is what we are doing when we are acting morally. We should only act on maxims that can be laws for a community (Kingdom) of rational beings. Thus, we are both subjects and sovereigns in this community, because we make our own laws and then we must obey them. This is the reason Kant thinks that the categorical imperative is binding on all of us because we impose it on ourselves and make the laws, not binding just because somebody might punish us if we disobey. We already accept the categorical imperative according to Kant without thinking about it. We end up with the ideas of autonomy and motivation. We end up with the idea that reason alone must be capable of motivating us to act a certain way which for Kant means we have autonomy (self rule), (motivated by reason as opposed to desires), which gives us free will. We can only be bound by moral laws if we have this kind of autonomy, if we are motivated by reason, if we have in a sense a free will. Kant thinks it goes in the other direction as well, if we have a free will then we are bound by the categorical imperative.
Thus, philosophers ask do we truly have free will? Also, to what extent are we moved by causation? Kant says laws govern causation. One type of law is Newton's laws of motion, scientific laws. Philosophers debate the question is human actions like these laws? Can we predict human actions? Do our desires cause us to act in certain ways; can our actions be predetermined? Some say yes. Aristotle calls this "efficient causation." Some call them "laws of natural necessity." Given the way the natural world works, things have to happen in a certain way and the world is governed by certain laws.
Kant says if we have a free will, then the laws that govern our choices are not going to be laws of natural necessity. If we have a free will, then our will or our practical reason will choose its own principles, its own laws to act on, and those will be the laws that will cause us to do certain things. If we have a free will, then our will chooses certain principles these must have form of a law for everyone; a universal law, this is the categorical imperative. Thus, for Kant, if we have free will then the categorical imperative is binding on us.
I recommend you read this work slowly and repeat key passages for better comprehension. Kant's work is a must for anyone interested in philosophy, and ethics.