Kant begins by distinguishing between two kinds of principles by which people decide how to behave. The first are maxims, which are “subjective” rules applied to specific circumstances. The second category is that of laws, which are universal and objective rules determined through reason. People use their own reason constantly to distinguish between laws and maxims in their daily lives. Kant gives the example of someone who believes they should seek revenge on anyone who insults them. Even such a person understands that “this is no practical law but only his maxim” (17). Each individual’s actions are shaped by that person’s free will as it reasons between maxims and laws. People are also motivated by imperatives, which Kant defines as rules “indicated by an ‘ought’” (18). However, many imperatives can be subjective because they are conditional and are motivated by a specific goal or object. For example, the imperative that someone “must work and save in his youth in order not to want in his old age” (18) is conditioned on whether that person actually wants to grow old or anticipates some other form of support later in life. Kant labels imperatives that are universal and not at all conditional, such as that one should not knowingly make false promises, as “categorical imperatives” (18-19).

Practical laws cannot be based on a principle that seeks pleasure or displeasure in a specific object. While it is true that rational beings seek pleasure and happiness, valuing happiness above all else is just the subjective “principle of self-love” (20). Kant completely rejects the traditional philosophical idea that there are “higher” and “lower” forms of desire based on what the object of desire is. Instead, Kant asserts that “it does not matter at all where the representation of this pleasing object comes from but only how much it pleases” (20). Pleasure or loss of pleasure is always conditional, even though it is universal that every rational being does seek happiness. This is because, while the desire for pleasure and the desire to avoid displeasure are both universal, what causes pleasure or displeasure cannot be universal.

Kant’s next theorem emphasizes that the object or representation of an individual’s desire is what determines whether or not it is a practical law. If the object of the desire is “an empirical condition” (24), meaning something shaped by one’s senses, then it does not qualify, because it is subjective. Kant gives the example of someone holding the maxim “to increase” their “wealth by every safe means” (25). That person finds themselves in possession of a bank deposit left by someone who has died and left no papers recording it. Kant asks whether this could be the basis of a universal, practical law. Kant concludes it cannot, because if this maxim were applied universally, no one would ever trust a deposit from any bank. In addition, Kant’s motive would be the desire for money, which is not universal and thus cannot be the basis for a practical law.

Kant adds that both maxims and practical laws do contain a “lawgiving form” (26) that demonstrate the existence of a free will and its ability to influence actions—its causality. Further, individuals can act in accordance with both the moral law and their maxims. Lastly, an individual’s free will has to find a basis within a law but not within the object to which the law applies. For Kant, this is proof that freedom is linked with moral law and practical reason, since free will can and does operate under moral guidelines that are independent of individual desires. Still, Kant admits that the question remains of how one can determine what the moral law is. He argues that people “can become aware of pure practical laws just as we are aware of pure theoretical principles” (27) by following rational thought and not relying solely on the subjective and empirical. As proof, Kant describes two hypothetical individuals: someone who does not fulfill their desire when threatened with punishment and death, and someone who refuses to lie under oath against someone even when threatened with execution.

Pure reason is thus synonymous with practical reason, and Kant argues that this synonymy is enough to prove the existence of a moral, universal law. All beings capable of freedom of will and the causality resulting from the exercise of that free will are bound “by needs and sensible motives” (29) as well as by obligations and duties. However, this does not mean that moral law is something obvious and unambiguous. Humans can only aspire toward what Kant describes as the “holiness of will” (29), the striving of free will toward the fulfillment of moral law. Using reason to try to live according to the moral law is an “unending progress,” which is how Kant defines “virtue.” Absolute certainty about the moral law, on the other hand, is dangerous.

Kant argues that free will is not entirely constrained and shaped by outside forces. Instead, the will has autonomy (freedom of action). This autonomy is the basis of all moral laws and duties, since without autonomy, no morality can exist. Such a will can be motivated by more than just empirical circumstances and goals, so one’s own happiness cannot be the basis for a practical law. Instead, it is the concept of obligation that makes an individual extend their desire for happiness into desiring the happiness of others as well. To demonstrate this, Kant points out how absurd it would be if someone claimed that lying for their own benefit “fulfilled a true human duty” (32) or if someone strongly recommended that you work for a person who is openly selfish and exploits others.

In fact, for Kant, not even a selfless desire for universal happiness can count as a practical moral law. This is because happiness is conditional, so it can “give general rules but never universal rules” (32). Not only do the sources of individual happiness vary, but every individual has different resources and faces different obstacles in achieving happiness. Because of this, while achieving happiness is both rare and dependent on outside factors, Kant argues that the fulfillment of one’s moral obligations is always possible. Kant gives the example of someone who won a game through cheating. They might have won more money, which would ostensibly increase their happiness, but they may also hate themselves for cheating, because they violated a moral law.

Kant rejects the idea that punishments and rewards are necessary for directing rational beings toward happiness, seeing such an argument as contradicting the principle of free will. Kant also argues against the idea that every individual is guided by a “special moral sense” (35) rather than by reason. To prove the point, Kant draws on the past work of philosophers and theologians to devise a chart listing the “determining grounds,” or bases, of morality. The chart has six axes: external and internal, subjective and objective, and internal and external. Kant concludes that these arguments are either dependent on the empirical and subjective or do not fit with the principle of freedom. Instead, Kant believes that only “the formal practical principle of pure reason” can explain practical imperatives and “the principle of morality” (37).

Kant reiterates that “pure reason can be practical” (37), by which he means that it can shape an individual’s will outside of any empirical and external factors. The moral law is independent of empirical evidence and is “of a supersensible nature” (38). Kant goes on to argue that moral law allows individuals to interact in the “archetypal world,” a realm of pure reason, and act in the ”ectypal world” (38), the world in which ideas and reason have impact on objects in the form of actions. From this, Kant asks how pure reason can come up with ideas a priori or completely theoretically and how it can be a “determining ground” (39) of the will. Kant argues that speculative reason is still based on “possible experience” (39). As for the idea of pure reason as an “immediate determining ground of the will” (39), Kant argues that this is only possible through the idea of freedom. In Kant’s view, practical laws “are possible only in relation to the freedom of the will” and “freedom is necessary because those laws are necessary” (40). This means that moral laws must exist because free will and causality belong to all rational beings. In other words, without freedom, there would be no need for morality. Kant argues that it would not even be possible to conceive of a moral law without the concept of individual free will.

Next, Kant addresses the arguments of David Hume, “who can be said to have really begun all the assaults on the rights of pure reason” (44). Hume questioned the very concept of cause and effect, believing that the connection between a cause and its effect can only be deduced—and never directly known—through empirical evidence. Kant claims that Hume’s arguments will lead to skepticism over mathematics, and he thus derides Hume’s treatment of mathematics as “analytic”—understanding concepts by their relations with each other. Instead, Kant suggests that mathematics should be viewed as “synthetic” (45), an approach that would encompass both the concepts and their relationships as well as how those concepts are understood and measured. Viewed this way, mathematics itself is evidence against Hume’s radical skepticism. Further, Kant argues that if Hume’s views on causality were universally true, he would not be able to even rationally deduce that effects (actions carried out by a will in the ectypal world) emerge from a cause (freedom of will).

Stemming from this assertion, Kant concludes that we can have certainty even about noumenal objects beyond our immediate experience just by extrapolating from the fact that we are rational agents with an “unconditioned” (48) ability to interact with the objects around us. In other words, we know with absolute certainty that we are free to act and interact in ways that are not determined strictly by external forces. From this, we can safely assume that we can affect even what is outside our immediate experience.