Kant did not initially intend to write the Critique of Practical Reason, let alone three Critiques. It was primarily the reactions to the Critique of Pure Reason and the Groundwork of the Metaphysics of Morals that encouraged Kant to develop his moral philosophy in the second Critique. In this brief introduction I outline some of the major events that took place in Kant’s development between the first and second Critiques. I illustrate that the story of the Critique of Practical Reason’s origin reveals that it is especially suited to being accompanied by certain background source materials that help illuminate its aims and contents.

On what kind of human action and omission is treated here. We find it grounded in experience that both some thoughts of the soul and some movements of the body stem from the will of the soul. Others, on the other hand, are not subject to the will (§325 Met.). For example: it is based on my [2] will that I now direct my thoughts to considering the good deeds of God, which he has shown me on previous occasions, but not that I see the person who encounters me or hear the shrieks of someone making noise (§219, 786 Met.), nor that I think of those things that occur to me in such circumstances (§238 Met.). No less does it stand under my will whether I now want to stand or sit, but not whether I digest the meal I have eaten or not (§519 Met.). What ranks among the actions of human beings. Since what stems from our will has its ground in the will (§29 Met.) and thus in us (§197 Met.), and similarly the movements of the body that are subject to the will have their ground in the state [Zustand]1 of the body (§878, 882 Met.), both the thoughts of the soul as well as the movements of the body that stem from the will rank among our actions (§104 Met.). Which are free. And since the will has a freedom to choose among possible things that which pleases us the most (§510 Met.), so are such human actions free, and therefore receive the name of free actions. To be precise, the movements of the body, by means of which the desires of the soul are fulfilled, are free with respect to the soul (§884, 885 Met.). On the other hand, since we encounter no freedom independently of the will (§492, 519 Met.), so is there also [3] no freedom in human action, whether it consists in thoughts of the soul or movements of the body, if it is not subjected to the will. Which are necessary. The action in such cases is necessary and therefore receives the name of a necessary action. Here we are only concerned with the free actions of human beings and in no way with those that are necessary.

August Wilhelm Rehberg (1757–1836) was a civil servant in Hanover, but he also made several important contributions to the philosophical debates of his time. This chapter contains the first English translation of Rehberg’s review of the second Critique, which was highly influential and read by figures such as Reinhold and possibly Fichte as well. In the review, Rehberg doubts that pure reason can be practical. One of the most important statements of the review is Rehberg’s claim that the feeling of respect must be something sensible and, as such, must contain an element of pleasure, despite what Kant says. Kant was aware of the review and is thought to have responded to it in later works such as the third Critique.

That it is neither unnecessary nor superfluous to elucidate Herr Prof. Kant’s Critique of Pure Reason, by means of which the content of this important but difficult to comprehend work is disclosed to those readers who lack the leisure time and patience, even if they are not entirely lacking in ability, to delve into the system of the deep-thinking philosopher, and disclose it in such a way that such readers are now able to understand its content with little effort and are able to busy their philosophical reflection with it, this, it seems to me, has been admitted by all experts and half-experts who have to some degree expressed themselves about it by complaining about the obscurity of the work. Herr second court chaplain Schulze has therefore done an agreeable and important service to philosophy and her devotees with these Elucidations, in that he has provided us with this clear commentary, approved by Herr Prof.

Johann Georg Heinrich Feder (1740–1821) was a well-respected and well-known professor of philosophy at the University of Göttingen, especially during the 1770s and early 1780s. A turning point took place in Feder’s life and career, however, when he edited the infamous Göttingen review of the first Critique, which was originally written by Christian Garve and to which Kant responds in the Prolegomena. This chapter contains a complete translation of Feder’s review of the second Critique, which therefore captures the opinion of one of Kant’s most well-known and infamous critics. Feder discusses a number of topics in the review, including: whether pure reason can be practical without the assistance of feeling and inclination, the nature of good and evil and their relationship to pleasure and displeasure, and the idea that respect for the moral law is respect for ourselves as legislators.

Christian August Crusius (1715–1775) was one of the most important German philosophers in the middle of the eighteenth century. His series of four German textbooks offered a systematic and sophisticated alternative to Wolffianism. Kant was at the beginning of his academic career when Crusius’ philosophical works were first published, so it is not surprising that Kant would come to be influenced by Crusius’ philosophy. This chapter contains a translation of selections from books 1 and 2 of Crusius’ Guide to Living Rationally (1744), capturing his theory of the will and desire, his theory of freedom, his voluntarist theory of ethics, his theory of the end of human life, and his moral proof of the immortality of the soul. The selections will help readers better understand Kant’s reference to Crusius’ moral philosophy as one based on the “will of God” (5:40), among many other things.

Philosophers need no courtesies when they make their remarks publicly known to each other, even where they diverge. Truth—their only goal—vindicates them. Whoever seeks and assesses truth with sincerity, in whatever corner of the earth they may live, is a friend of the philosopher.

Gottlob August Tittel (1739–1816) was a professor at a Gymnasium in Karlsruhe and is best known as an early critic of Kant’s theoretical and practical philosophy. This chapter contains selections from his On Herr Kant’s Reform of Moral Science (1786), the first book-length commentary on the Groundwork. In this text, Tittel makes several important objections to the Groundwork, many of which have since become classic, such as: that the categorical imperative is empty, that the motive of duty for duty’s sake is impotent in human nature, and that Kant is a covert consequentialist. Tittel’s most famous objection, however, is that the categorical imperative is not a new principle of morality, but merely a new formula of a principle philosophers have known for a long time. Kant was aware of the Reform, and he responds to Tittel in a footnote in the Preface to the second Critique (see 5:8.28–37n).

In the Groundwork of the Metaphysics of Morals, the author analytically explicates the following from the common, confused concepts of morality: that it is absolutely necessary to arrive at one ultimate practical principle that can be known a priori, and thus make reason alone the source of the human being’s moral capacity if such a being does not want to entirely renounce all morality and surrender itself to only those sensations that are part of its animal nature; that this practical law of reason concerning human moral conduct, when conceived purely, leads to the idea of metaphysical freedom, and thus discloses the human being to itself in a higher form entirely independent of the sensible world and its laws, as well as in a very particular dignity, as the member of a distinct, intelligible world.

Christian Garve (1742–1798) was a well-respected writer and translator in late eighteenth-century Germany. One of his most influential translations was that of Cicero’s On Duties, to which he appended three volumes of commentary, and which has been suspected to have influenced Kant while he was writing the Groundwork. This chapter contains the first English translation of an extended footnote from Garve’s essay ‘On Patience’, in which he engages with Kant’s moral philosophy and to which Kant responds in the first part of the ‘Theory and Practice’ essay. The focus of the footnote is whether happiness or virtue is the final end of creation, and whether it is possible for human beings to strive to be worthy of happiness without also striving for happiness itself.

Intending to someday publish a Metaphysics of Morals, which is a pure moral philosophy completely cleansed of everything that might be empirical, Herr Professor Kant issues this Groundwork in advance, wherein he presents the subtleties that the Metaphysics of Morals unavoidably contains, in order to not mix them up with more accessible doctrines in the future, and wherein he wishes to seek out and establish the most supreme principle of morality, which he considers a business that is separate from every other moral investigation in its purpose.

This text, as with all the other works by the Königsberg philosopher, unmistakably bears the mark of an original, deep-thinking mind and contains many characteristic ideas and excellent observations that are often mentioned only in passing and which cannot easily be condensed in a short summary. As undeniable as this is, we regret that Herr Kant cannot be absolved of the charge that, in the present text as in other previous ones, he often substitutes a dialectical illusion for a proof or (to use his own expression) a cloud for Juno. [see 4:426.17–18] We consider it to be that much more of a duty to illustrate this with some examples, given Herr Kant looks down upon the most famous philosophers from his dialectical peak, just as a giant looks down upon the race of pygmies, and does not seem to be very far [106] from mistaking his subjective reason for objective reason or for an ideal of human reason.

Pistorius’ review of the Groundwork was without a doubt the most important early review, at least with respect to its influence on the second Critique. Pistorius raises several important objections in the review, many of which are now regarded as classic responses to Kant’s moral theory in the literature. These include: the empty formalism objection, the claim that Kant is a covert consequentialist, that only hypothetical imperatives can bind human beings, and that there is a distinction to be made between happiness through instinct and happiness through reason. Kant was aware of the review and responds to Pistorius explicitly in the second Critique, such as in the second chapter of the Analytic, where he replies to a “certain reviewer,” i.e., Pistorius, who claimed against the Groundwork that “the concept of the good was not established before the moral principle.” (5:8.27–9.2)