The Groundlaying has four parts: a Preface followed by three Sections. The Preface (pages iii-xvi in the 178x editions or pages 387-392 in the Academy edition) basically lays out Kant's assumptions, his starting points. He takes morality, if it exists, to be a priori, based, that is, on reason rather than experience, and to prescribe absolute and necessary duties for all rational beings with a will (i.e., beings who have the capacity to reason and to use that reason in making choices about what actions to take). In the First Section (1-24; 393-405), Kant begins to investigate the concept of a morally good will (i.e., one that acts from duty) and finds that such a will acts from maxims (i.e., principles of action) that can be willed as universal laws. Even before the end of this First Section, Kant has already found a version of the highest principle of morality, thus achieving one of the work's stated purposes. In the Second Section (25-96; 406-445), Kant pushes his investigation further to find that the morally good will must be operating according to a categorical imperative. In the course of this investigation of the will or of practical reason, he discusses several formulations of this categorical imperative. By the end of the Second Section, Kant has identified, but not yet established, the highest principle of morality: the principle of the autonomy of the will. In the Third Section (97-128; 446-463), Kant makes some progress toward establishing the principle of autonomy. He argues that if we presuppose that the will is free, as we must even though we cannot explain how this freedom is possible, then the principle of autonomy follows. The morality assumed in the Preface is thus defended.
Kant identifies a number parts of philosophy. One of these parts, the pure and a priori part of moral philosophy, is the metaphysics of morals (iii-v; 387-388). It is a necessary part for a variety of reasons, perhaps chief among them that moral judgments are universal and necessary and so are a priori, grounded in pure reason, and totally separate in their foundation from anything empirical or based in sense experience (v-xi; 388-390). Kant explains why he has written this particular work, which breaks new ground in its conception of a pure will, and states the purposes of the work: to find and establish the highest principle of morality (xi-xv; 390-392). At the end of the Preface, Kant mentions the methodology, at first analytic and then synthetic, of the work and how this methodology affects the layout of the book, sectioning it into three transitions or steps (xvi; 392).
Because what matters for morality is what rational beings will to do, Kant begins the First Section by saying some things about the good will: only a good will is good without qualification; it is good only through its willing; the idea of the absolute worth of a good will is strange but consistent with the purposes of nature (1-8; 393-396). To learn even more about the good will, Kant turns to the concept of duty, which contains the concept of a good will. He gives a series of examples to illustrate the difference between those actions that are in accordance with duty but that are done from inclination or habitual desire and those actions that are in accordance with duty and that are also done from duty. The unstated first proposition that emerges is that only actions done from duty have moral worth (8-13; 396-399). The second and third propositions follow on quickly: the moral worth of an action done from duty lies only in the maxim (i.e., the principle of action) by which the action is decided; duty is the necessity of an action from respect for the law (13-15; 399-400). With these three propositions in place, Kant then argues that for a will to be good it must be guided or determined by a principle of willing maxims as universal laws (15-17; 400-402). He follows this up with an example of false promising (18-20; 402-403). Kant wraps up his search for the highest moral principle by claiming that something very much like the principle he has found — the principle of willing maxims as universal laws — is actually used by ordinary humans in their moral thinking (20-22; 403-404). This claim of actual use then immediately raises the question of what role philosophy is to play if ordinary moral thinking has already got hold of the correct moral principle. Replying to the question, Kant finishes the first section by arguing that philosophy, as a critique of reason, is still needed in order to keep ordinary human reason from falling into confusion and error brought on by the natural dialectic or opposition between the competing claims of reason and inclination (22-24; 404-405).
Kant begins the Second Section with an attack on those who reduce morality to self-love or who think it a mere fantasy because no sure example of genuine action from duty can be found anywhere in our experience. He emphasizes that morality is not based on experience and in particular cannot be based on examples (25-30; 406-409). Kant complains that popular moral philosophy, precisely because it does not exclude experience, is unreliable (30-34; 409-411). After reminding the reader of what has been achieved so far, and in doing so also reminding the reader of the need to proceed using a non-empirical methodology, Kant says he will make the transition from popular moral philosophy, now recognized as inadequate and why, to a metaphysics of morals; he will make this transition by examining the faculty of practical reason to see how the concept of duty arises from it (34-36; 411-412). In practical reason, Kant finds the imperative, which is the formula of a command of reason, and the imperative tells a rational being with an imperfect will that it ought to do something. There are two kinds of imperative: hypothetical and categorical (36-39; 412-414). Hypothetical imperatives say that an action is good as a means to something else; categorical imperatives say that an action is good by itself without reference to anything else. Only the categorical imperative is an imperative of morality because only it commands objectively and unconditionally (39-44; 414-416). After finishing this classification of imperatives, Kant begins to wonder how these imperatives are possible, how the necessitation of the will, which these imperatives express, is to be thought. Kant argues that the hypothetical imperative is to be thought as analytic; the categorical imperative, however, presents more difficulties because we cannot use experience and examples to investigate it and because it is a synthetic a priori practical proposition (44-49; 416-419). At this point, Kant announces that, because of the difficulties it presents, he will put off until the Third Section further investigation of the possibility of a categorical imperative (49-51; 419-420).
Having temporarily set aside the question of possibility, Kant now wants just to see if the concept of a categorical imperative might provide the formula of a categorical imperative. He looks, and indeed he does find such a formula, the formula of universal law. He looks again, this time from the perspective of nature, and finds another formula of the imperative of duty: the formula of the universal law of nature (51-53; 420-421). Then, to illustrate these two formulations and the kinds of impossibility, either of thinking or of willing, that arise in trying to raise some maxims to the universality of laws of nature, Kant gives four examples of duties (53-57; 421-423). After giving an account of supposed exceptions to duties, Kant pauses to remind the reader that this investigation of practical reason must be conducted a priori and must reveal an a priori connection between the moral law and the will of a fully rational being because in such a being only reason determines the will (57-63; 423-427).
Preparing the way for a third formulation, Kant introduces the concepts of end, means, person and thing, absolute and relative worth. Using these concepts, Kant argues for the principle that persons are ends in themselves, have absolute worth, and thus must never be treated merely as things or as mere means (63-67; 427-429). Kant then applies the formula of this principle to the same four examples of duty (67-69; 429-430). After arguing that this principle is not borrowed from experience, Kant introduces the next formula, that of autonomy, as following from the previous two (69-72; 430-432).
This third formula of the principle, the formula of autonomy, is especially well-suited to be a categorical imperative because it reveals better than the other formulas that interest, being conditioned, cannot be the ground of moral law. Kant then introduces the concept of heteronomy to characterize those mistaken principles, used in all previous efforts to find the principle of morality, that attempt to make anything other than the will itself as the source of moral law and obligation (72-74; 432-433).
Working from the concept of a lawgiving rational being, Kant next develops the empire of ends formula of the categorical imperative. This formula uses the concept of an empire of ends, a systematic union of different rational beings through common laws. In this empire, reason refers every maxim of the will as universally lawgiving to every other will, and reason does this from the idea of the dignity of a rational being that obeys only laws that it gives itself (74-77; 433-434). Kant concludes that only morality and humanity have the inner, unconditional, and incomparable worth that is dignity, and that autonomy is the ground of this dignity (77-79; 434-435). After pointing out that all maxims have a form, matter, and complete determination and that it is possible to exploit these features in order to bring the idea of reason that lies behind the imperatives closer to intuition and feeling (79-81; 435-436), Kant begins a long review of what has been accomplished so far (81-86; 436-440).
In the first subsection of the work, Kant explicitly identifies the principle of the autonomy of the will as the highest principle of morality and also identifies it as a synthetic a priori proposition. Mere analysis of the concepts of morality shows, however, that the principle of morals is the principle of autonomy (87-88; 440-441).
Having found, but not yet established, the highest principle of morality, Kant spends some time considering previous failures to find the highest principle. All these failures are marked by the concept of heteronomy, a concept which applies to a will that searches outside of itself for the law that is to determine it (88-89; 441). He then takes a closer look at several heteronomous principles, two of which are empirical (happiness and moral sense) and two of which are rational (ontological concept of perfection and theological concept of perfection). Kant lists many defects of these principles, but finds the ontological concept of perfection the least objectionable (90-93; 441-443). The basic flaw in all these principles of heteronomy is that they, by depending ultimately on the contingent natural constitution of human beings, break the necessity that must be a feature of moral imperatives (93-95; 443-444). The section ends with Kant bringing together the absolutely good will and autonomy, the former a topic with which he began and the latter a topic at which he has lately arrived, and reviewing what has been shown so far and previewing what still lies ahead (95-96; 444-445).
Kant begins the Third Section with a negative explanation of the freedom of the will in terms of lawful causality and independence from external causes. This explanation gives rise to a richer, more fruitful, positive concept of freedom which leads to autonomy, to the categorical imperative, and to morality. A free will is thus identified with a will under moral law. So, if a free will is presupposed, morality, along with its principle, follows (97-99; 446-447).
Kant next argues that a will that can act only under the idea of freedom is, for practical purposes, actually free. So the presupposition of freedom is warranted and morality is established (99-101; 447-448).
Kant initiates a sustained reflection on his just-completed argument that established the principle of autonomy on the presupposition of freedom of the will. He asks why one should subject oneself to this principle. The answer has something to do with interest, but not an impelling interest since that would not give a categorical imperative. The interest we do sometimes take in virtuous personal characteristics, though, only reveals a prior commitment on our part to morality; it does not show on what the bindingness of the moral law rests. So, based on these reflections, Kant thinks there might be an inescapable circle in the argument (101-104; 448-450). In this circle, freedom is established through moral law and then moral law is established through freedom. After a brief mention of how this circle comes about through the reciprocal concepts of freedom and of lawgiving, Kant suggests a way to escape the circle: there are two standpoints, one in which we conceive ourselves through freedom as a priori efficient causes, the other in which we represent ourselves according to our actions as effects (105-110; 450-453).
Continuing to use these two standpoints, Kant goes on to show how a categorical imperative is possible as a synthetic a priori proposition and then to show by example that the deduction is even confirmed by the practical use of common human reason (110-113; 453-455).
Kant then comes across yet another problem: there is an apparent contradiction when we ascribe both freedom and natural necessity to ourselves. But he solves this contradiction by again appealing to the two standpoints; as members of an intelligible world, we are free and, as members of the world of sense, we are determined by natural necessity (114-118; 455-457). Kant has made extensive use of this two-world theory and of the two standpoints in order to defend his position on freedom, and he now begins to argue that his defense has not taken reason beyond its limits. First, the defense has not tried to give an explanation of how freedom is possible; such an explanation is impossible because freedom is not a possible object of experience (118-121; 457-459). Second, and for the same reason relating to possible experience, his defense of freedom has not tried to make understandable how there can be interest in moral law through reason's causality (121-3; 459-461). Nearing the end of the work, Kant gives a summary of results (124-6; 461-462). Though not everything is explainable, Kant has still found that there is room enough within the boundary of moral inquiry to find and establish the highest principle of morality, the principle of autonomy, and now points out that staying within that boundary actually has benefits for morality (126-7; 462-463).
In the Concluding Remark, Kant argues that his deduction of the highest principle of morality cannot be faulted for leaving some things unexplained, for the explanations are impossible within the boundary of reason in general (127-8; 463).