Kant's moral argument for belief in God in the Critique of PracticalReason may be summarized as follows. Kant holds that virtue andhappiness are not just combined but necessarily combined in the idea ofthe highest good, because only possessing virtue makes one worthy ofhappiness — a claim that Kant seems to regard as part of the content ofthe moral law (4:393; 5:110, 124). But we can represent virtue andhappiness as necessarily combined only by representing virtue as theefficient cause of happiness. This means that we must represent thehighest good not simply as a state of affairs in which everyone is bothhappy and virtuous, but rather as one in which everyone is happybecause they are virtuous (5:113–114, 124). However, it is beyond thepower of human beings, both individually and collectively, to guaranteethat happiness results from virtue, and we do not know any law ofnature that guarantees this either. Therefore, we must conclude thatthe highest good is impossible, unless we postulate “the existence of acause of nature, distinct from nature, which contains the ground ofthis connection, namely the exact correspondence of happiness withmorality” (5:125). This cause of nature would have to be God since itmust have both understanding and will. Kant probably does notconceive of God as the efficient cause of a happiness that is rewardedin a future life to those who are virtuous in this one. Rather, hisview is probably that we represent our endless progress towardholiness, beginning with this life and extending into infinity, as theefficient cause of our happiness, which likewise begins in this lifeand extends to a future one, in accordance with teleological laws thatGod authors and causes to harmonize with efficient causes in nature(A809–812/B837–840; 5:127–131, 447–450).
Kant holds that reason unavoidably produces not only consciousnessof the moral law but also the idea of a world in which there is bothcomplete virtue and complete happiness, which he calls the highestgood. Our duty to promote the highest good, on Kant's view, is the sumof all moral duties, and we can fulfill this duty only if we believethat the highest good is a possible state of affairs. Furthermore, wecan believe that the highest good is possible only if we also believein the immortality of the soul and the existence of God, according toKant. On this basis, he claims that it is morally necessary to believein the immortality of the soul and the existence of God, which he callspostulates of pure practical reason. This section briefly outlinesKant's view of the highest good and his argument for these practicalpostulates in the Critique of Practical Reason and other works.
Fourth, Kant concludes the Critique of the Power of Judgment with along appendix arguing that reflecting judgment supports morality byleading us to think about the final end of nature, which we can onlyunderstand in moral terms, and that conversely morality reinforces ateleological conception of nature. Once it is granted on theoreticalgrounds that we must understand certain parts of nature (organisms)teleologically, although only as a regulative principle of reflectingjudgment, Kant says we may go further and regard the whole of nature asa teleological system (5:380–381). But we can regard the whole ofnature as a teleological system only by employing the idea of God,again only regulatively, as its intelligent designer. This would be toattribute what Kant calls external purposiveness to nature — that is,to attribute purposes to God in creating nature (5:425). What, then, isGod's final end in creating nature? According to Kant, the final end ofnature must be human beings, but only as moral beings (5:435, 444–445).This is because only human beings use reason to set and pursue ends,using the rest of nature as means to their ends (5:426–427). Moreover,Kant claims that human happiness cannot be the final end of nature,because as we have seen he holds that happiness is not unconditionallyvaluable (5:430–431). Rather, human life has value not because of whatwe passively enjoy, but only because of what we actively do (5:434). Wecan be fully active and autonomous, however, only by acting morally,which implies that God created the world so that human beings couldexercise moral autonomy. Since we also need happiness, this too may beadmitted as a conditioned and consequent end, so that reflectingjudgment eventually leads us to the highest good (5:436). Butreflection on conditions of the possibility of the highest good leadsagain to Kant's moral argument for belief in God's existence (he nowomits immortality), which in turn reinforces the teleologicalperspective on nature with which reflecting judgment began.
One way to understand the problem Kant is articulating here is toconsider it once again in terms of the crisis of the Enlightenment. The crisis was thatmodern science threatened to undermine traditional moral and religiousbeliefs, and Kant's response is to argue that in fact these essentialinterests of humanity are consistent with one another when reason isgranted sovereignty and practical reason is given primacy overspeculative reason. But the transcendental idealist framework withinwhich Kant develops this response seems to purchase the consistency ofthese interests at the price of sacrificing a unified view of the worldand our place in it. If science applies only to appearances, whilemoral and religious beliefs refer to things in themselves or “thesupersensible,” then how can we integrate these into a singleconception of the world that enables us to transition from the onedomain to the other? Kant's solution is to introduce a third a prioricognitive faculty, which he calls the reflecting power of judgment,that gives us a teleological perspective on the world. Reflectingjudgment provides the concept of teleology or purposiveness thatbridges the chasm between nature and freedom, and thus unifies thetheoretical and practical parts of Kant's philosophy into a singlesystem (5:196–197).
If my maxim passes the universal law test, then it is morallypermissible for me to act on it, but I fully exercise my autonomy onlyif my fundamental reason for acting on this maxim is that it ismorally permissible or required that I do so. Imagine that I am movedby a feeling of sympathy to formulate the maxim to help someone inneed. In this case, my original reason for formulating this maxim isthat a certain feeling moved me. Such feelings are not entirely withinmy control and may not be present when someone actually needs myhelp. But this maxim passes Kant's test: it could be willed as auniversal law that everyone help others in need from motives ofsympathy. So it would not be wrong to act on this maxim when thefeeling of sympathy so moves me. But helping others in need would notfully exercise my autonomy unless my fundamental reason for doing sois not that I have some feeling or desire, but rather that it would beright or at least permissible to do so. Only when such a purely formalprinciple supplies the fundamental motive for my action do I actautonomously.
In other words, to assess the moral permissibility of my maxim, I askwhether everyone could act on it, or whether it could be willed as a universallaw. The issue is not whether it would be good if everyone acted on mymaxim, or whether I would like it, but only whether it would bepossible for my maxim to be willed as a universal law. This gets at the form, notthe matter or content, of the maxim. A maxim has morally permissibleform, for Kant, only if it could be willed as a universal law. If my maxim failsthis test, as this one does, then it is morally impermissible for me toact on it.
Third, insofar as I act only on material principles or hypotheticalimperatives, I do not act freely, but rather I act only to satisfy somedesire(s) that I have, and what I desire is not ultimately within mycontrol. To some limited extent we are capable of rationally shapingour desires, but insofar as we choose to act in order to satisfydesires we are choosing to let nature govern us rather than governingourselves (5:118). We are always free in the sense that we always havethe capacity to govern ourselves rationally instead of letting ourdesires set our ends for us. But we may (freely) fail to exercise thatcapacity. Moreover, since Kant holds that desires never cause us toact, but rather we always choose to act on a maxim even when that maximspecifies the satisfaction of a desire as the goal of our action, italso follows that we are always free in the sense that we freely chooseour maxims. Nevertheless, our actions are not free in the sense ofbeing autonomous if we choose to act only on material principles, because inthat case we do not give the law to ourselves, but instead we choose toallow nature in us (our desires) to determine the law for ouractions.
Many puzzles arise on this picture that Kant does not resolve. Forexample, if my understanding constructs all appearances in myexperience of nature, not only appearances of my own actions, then whyam I responsible only for my own actions but not for everything thathappens in the natural world? Moreover, if I am not alone in the worldbut there are many noumenal selves acting freely and incorporatingtheir free actions into the experience they construct, then how domultiple transcendentally free agents interact? How do you integrate myfree actions into the experience that your understanding constructs? In spite of theseunsolved puzzles, Kant holds that we can make sense of moral appraisaland responsibility only by thinking about human freedom in this way,because it is the only way to prevent natural necessity fromundermining both.
Kant's moral philosophy is also based on the idea of autonomy. He holdsthat there is a single fundamental principle of morality, on which allspecific moral duties are based. He calls this moral law (as it ismanifested to us) the categorical imperative (see ). The moral law is a product of reason, for Kant, whilethe basic laws of nature are products of our understanding. There areimportant differences between the senses in which we are autonomous inconstructing our experience and in morality. For example, Kant regardsunderstanding and reason as different cognitive faculties, although hesometimes uses “reason” in a wide sense to cover both. The categoriesand therefore the laws of nature are dependent on our specificallyhuman forms of intuition, while reason is not. The moral law does notdepend on any qualities that are peculiar to human nature but only onthe nature of reason as such, although its manifestation to us as acategorical imperative (as a law of duty) reflects the fact that thehuman will is not necessarily determined by pure reason but is alsoinfluenced by other incentives rooted in our needs and inclinations;and our specific duties deriving from the categorical imperative doreflect human nature and the contingencies of human life. Despite thesedifferences, however, Kant holds that we give the moral law toourselves, just as we also give the general laws of nature toourselves, though in a different sense. Moreover, we each necessarily give the samemoral law to ourselves, just as we each construct our experience inaccordance with the same categories. To summarize:
Third and finally, Kant's denial that things in themselves are spatialor temporal has struck many of his readers as incoherent. The role ofthings in themselves, on the two-object interpretation, is to affectour senses and thereby to provide the sensory data from which ourcognitive faculties construct appearances within the framework of our apriori intuitions of space and time and a priori concepts such ascausality. But if there is no space, time, change, or causation in therealm of things in themselves, then how can things in themselves affectus? Transcendental affection seems to involve a causal relation betweenthings in themselves and our sensibility. If this is simply the way weunavoidably think about transcendental affection, because we can givepositive content to this thought only by employing the concept of acause, while it is nevertheless strictly false that things inthemselves affect us causally, then it seems not only that we areignorant of how things in themselves really affect us. It seems,rather, to be incoherent that things in themselves could affect us atall if they are not in space or time.