Immanuel Kant, Groundwork for the Metaphysics of Morals
The purpose of the Groundwork is to establish the supreme principle of morality (4: 392). Morality rests on its pure part, in which a rational being gives laws to itself a priori (i.e. independently of any evidence from observation) (4:389). The Groundwork proceeds from common rational moral cognition via philosophical moral cognition and the metaphysics of morals to the critique of pure practical reason (4:392). It does not return to the metaphysics of morals (the rational part of ethics prior to its application to humans-4:388).
Transition from common rational moral cognition to philosophical moral cognition
The only thinkable thing that is absolutely good is a good will (4:393). Its worth is independent of what it effects or accomplishes (4:394). A good will is the purpose for which nature has given human beings reason; happiness is not the purpose of reason, for instincts would secure happiness more effectively (4:395-396).
1) An action has moral worth only when its maxim rests on duty (e.g. to be beneficent where one can) and not on inclination (e.g. sympathy for another's plight) (4:397-399).
2) An action from duty has its moral worth not in its aim but in its maxim (the subjective principle of the will) (4:400).
3) Hence duty is the necessity of an action from respect for the law.
Explanation of how (3) follows: One cannot have respect for the object aimed at by the action, because it is an effect and not the activity of the will. Nor can one have respect for inclination, from whose influence duty is supposed to abstract. (Respect is the consciousness of the subjection of my will to a law without any mediating influence.) So the moral worth of an action cannot stem from the effect expected to be obtained by it or from any principle that needs to get its motive from this expected effect. So only the representation of the law in itself constitutes the moral good. Since we have abstracted from impulses, this law's only content can be lawfulness itself: Always act so that I could also will my maxim to be a universal law (4:401-402).
Example: Would I be content if my maxim to get out of an embarrassment through an untruthful promise were to be valid as the following universal law: anyone may make an untruthful promise who is in an otherwise unavoidable embarrassment. No, for with such a law there would be no promises, so that my maxim would destroy itself (4:402-403).
Transition: Although common human reason does not think of this principle abstractly, it implicitly recognizes it in the moral judgments that it makes, indeed more securely than the more subtle reason of the philosopher. Nevertheless, because our wishes and inclinations can tempt us to reason our way out of the strict laws of duty, common human reason must take a step to practical philosophy, to receive information about the source and correct determination of its principle (4:404-405).
What does Kant mean by a maxim? He defines it as "the subjective principle of the volition" in accordance with which an action is done (4:400). His examples include: (1) to get myself out of embarrassment through an untruthful promise (4:403, cf. 4:422); (2) from self-love to shorten my life when by longer term it threatens more ill than it promises agreeableness (4:421); (3) to indulge in gratification rather than trouble myself with expanding and improving my fortunate natural dispositions (4:423); (4) not to contribute to the welfare of, or to assist, anyone else struggling with great hardships (4:423) These maxims are possible general policies for the conduct of one's life. They combine a type of circumstance (e.g. one's life threatening more ill than it promises agreeableness), a kind of action to be performed in that circumstance (e.g. shorten my life), and a motivating inclination (e.g. self-love). They all fail Kant's test of universalizability. Maxims that might pass the test are: (5) always to take an afternoon walk for the sake of my health; (6) to be faithful to the spouse to whom I have promised fidelity. Passing the test makes it morally permissible to act on the maxim, unless the action also implements another maxim that fails the test. Passing the test does not make it morally obligatory to act on the maxim; it might or might not be morally obligatory to act on a maxim that passes the test.
Note: For my reflection, I chose to address the question what exactly Kant means by his key term "maxim", which he nowhere explains clearly. There are lots of other questions you could address in a reflection on the preface and first section of the Groundwork. Here are some examples:
1. How convincingly does Kant establish his initial thesis that the only absolutely good thing is a good will?
2. Does Kant really mean that an action that conforms to my duty has no moral worth if I have an inclination to do it, but has moral worth if I do it solely out of duty, with no inclination to do it? Doesn't he have things backwards? (On this issue, see Marcia Baron's "Acting from duty" in our textbook (pp. 92-110).)
3. Why does Kant think that morality rests on reason's making laws for itself, without any reference to experience?
4. How else could one ground morality than by appealing to a self-legislating reason? What are the comparative pros and cons of the alternatives? (Kant himself gives a table of all the logically possible alternatives in his Critique of Practical Reason (5:40), and argues that each of them is unsatisfactory.)