Kant has repeated two points about the relationship between God and morality on several occasions.
We can know what is right and wrong independently of knowing anything about God.
The moral rules apply to God as well as to human beings; they apply to all rational beings.
Finally, as you might imagine, Kant thinks that the fear of eternal punishment would not provide a moral motivation. The desire to have a pleasant afterlife is self-interested and so actions motivated by that desires would not count as actions from the law but, at best, as actions in accordance with the law. Basically, that would be no better than the shopkeeper who is honest because that is what is good for business.
So you would think that God plays no role in Kant’s ethics. But you would be wrong. Kant thinks that belief in God and the afterlife are presuppositions of moral thought. Instead of going from belief in God to conclusions about ethics, however, Kant thinks the intellectual path goes from ethics to belief in God. That is unusual, to say the least. Let’s see how he does it.
I should say at the outset that it is probably best to read only three sections: I, V, and VI. The first sets out the problem, the fifth explains how belief in God addresses the problem, and the sixth presents a mercifully compact summary. If you want to do the whole thing, start with those three first.
This sets out the problem Kant is going to address. An antinomy is a conflict between two equally reasonable statements. Kant is going to start by laying out such a conflict here. He intends to show that it is not a genuine conflict in subsequent sections.
The problem concerns what Kant calls the highest good. The highest good “which is practical for us” involves a necessary connection between virtue and happiness. I assume he means that the only way the moral law is going to be practical for us is if it contributes to our happiness. He is assuming that if this is not so, we will not follow the moral law.
So either the desire for happiness leads us to behave morally or moral behavior makes us happy. But the desire for happiness cannot be a moral motive; that is a point he insists on. And virtue does not, in fact, always lead to happiness.1 Those are the two reasonable statements. They do not conflict with one another, but, taken together, they seem to show that virtue and happiness are necessarily separate things, at least for us.
Kant worries that if we cannot show that virtue and happiness come together, then “the highest good is impossible in accordance with practical rules” and “the moral law … must be fantastic and directed to empty imaginary ends and must therefore itself be false” (5:114).
Just to be clear, Kant is going to say that this is not the case. He thinks the highest good is possible and the moral law is not false. This is just his way of arguing that the problem he intends to solve is important.
One solution to the problem lies in Kant’s theory of free will. That, to refresh your memories, held that we can take two standpoints on ourselves. On the one hand, we can see ourselves as members of what he calls the sensible realm; that means that we are subject to the relations of cause and effect in the natural world that we observe with the five senses. On the other hand, we can see ourselves as belonging to what he calls the intelligible realm; there, we are purely rational and free from the causes of the natural world. Here we get a cool vocabulary term: when you are in the intelligible, rational realm, you are a noumenon.
The problem is that morally virtuous behavior does not necessarily make us happy in this world, the sensible world.
Kant goes on a digression criticizing two theories of ethics in ancient philosophy, Epicureanism and Stoicism, for their attempts to show that virtue leads to happiness in this world (5:115-117).
He proposes instead of happiness a feeling of contentment that arises, he thinks, when we act freely. By “freely,” he means free from all natural causes, including our own desires (5:118).
Because Kant believes that acting freely is the same thing as acting morally, he thinks he has solved the problem. Morally virtuous behavior can make us content because it is free from natural causation. While that is not exactly the same thing as being happy, it is pretty close. (5:119)
Kant contrasts what he calls speculative reason, which concerns what we can know about objects, with what he calls practical reason, which concerns what we will do.
He maintains that if practical reason leads to a conclusion that speculative reason does not reach, practical reason should take priority and we should accept its conclusions.
His reasoning is a bit terse. He seems to be saying that one or the other has to win because otherwise reason could contradict itself (5:121, at the end). But that does not mean that the practical side has to win; it just means that one branch of reason has to win. To put it another way, you could equally well avoid conflicts by saying speculative reason always wins. So I am not confident that I understand his reasoning here.
In this section, Kant approaches the problem from a different direction. The problem is that there is a gap between the intelligible realm and the sensible realm. In the intelligible realm, we are rational, free, and moral. In the sensible realm, our behavior is driven by natural causes, not free, and, at best, in mere conformity with the moral law. To be in what Kant calls “complete conformity” with the moral law, we would have to be solely in the intelligible realm.
But complete conformity is beyond our abilities. That is reserved for God, who is perfect. Complete conformity with the law is “holiness,” and that is beyond what any being in the sensible realm could achieve.
This gap between the way we are in the sensible realm and the way we should, ideally, be shows that we must postulate, that is, assume, that we have immortal souls. Kant’s reasoning goes like this (5:122).
Complete conformity of the will with the law is “practically necessary.”
The closest beings in the sensible realm can come to complete conformity is to make endless progress towards it.
Endless progress is possible only if we have immortal souls.
Therefore, the existence of immortal souls is a “postulate” of practical reason.
I gather that this means you should believe in an immortal soul if you are practically rational. And you should believe it even if there is no more direct proof that immortal souls exist.
In sum, the reason why it makes sense to hope for eternal life after death is not that it might be blissful. Rather, it is that if there were such an eternal life, it would be possible to approach moral perfection (5:123).
It is not obvious why “we can be perfect only if we are immortal souls” leads to the conclusion “therefore, we are immortal souls” rather than the conclusion “but we are mortal and so we cannot be perfect.”
Perhaps Kant’s point is that, for practical purposes, we have to act as though we are immortal souls that could achieve perfection. That is the thought that has to guide our behavior. We cannot prove it theoretically, but a sincere commitment to morality requires us to act as if we knew it was true.
The problem was that there is a gap between being morally good and being happy. This is not a problem for purely rational creatures like God. But it is a problem for creatures who inhabit the sensible realm, like us.
The solution is to assume that God exists and that he closes the gap. Here is the argument (5:124-125). I am going to break it up and add numbers to make the structure more clear.
(1) there is not the least ground in the moral law for a necessary connection between the morality and the proportionate happiness of a being belonging to the world …
(2) Nevertheless, in the practical task of pure reason, that is, in the necessary pursuit of the highest good, such a connection is postulated as necessary: we ought to strive to promote the highest good (which must therefore be possible).
(3) Accordingly, the existence of a cause of all nature, distinct from nature, which contains the ground of this connection, namely of the exact correspondence of happiness with morality, is also postulated.
(4) However, this supreme cause is to contain the ground of the correspondence of nature not merely with a law of the will of rational beings but with the representation of this law, so far as they make it the supreme determining ground of the will, and consequently not merely with morals in their form but also with their morality as their determining ground, that is, with their moral disposition.
(5) Therefore, the highest good in the world is possible only insofar as a supreme cause of nature having a causality in keeping with the moral disposition is assumed.
(6) Now, a being capable of actions in accordance with the representation of laws is an intelligence (a rational being), and the causality of such a being in accordance with this representation of laws is his will. Therefore the supreme cause of nature, insofar as it must be presupposed for the highest good, is a being that is the cause of nature by understanding and will (hence its author), that is, God.
(7) Consequently, the postulate of the possibility of the highest derived good (the best world) is likewise the postulate of the reality of a highest original good, namely of the existence of God.
(8) Now, it was a duty for us to promote the highest good; hence there is in us not merely the warrant but also the necessity, as a need connected with duty, to presuppose the possibility of this highest good, which, since it is possible only under the condition of the existence of God, connects the presupposition of the existence of God inseparably with duty; that is, it is morally necessary to assume the existence of God.
After making this argument, Kant reminds us that he denies that God is the ground of all obligation; reason does that (5:126).
Then he criticizes the Epicureans, for basing their ethical system entirely on happiness, and the Stoics, for basing their ethical system entirely on virtue (5:126-127).
Christianity, by contrast, has it right. It demands perfection even in this world and holds out the prospects of happiness in the next world. It treats moral virtue not as a means to being happy but as a condition for being worthy of being happy (5:130).
There are three postulates of practical reason.
The last paragraph is important. It says that these are postulates for practical purposes. They do not enable us to “cognize,” that is, understand, the nature of our souls, the intelligible realm, or God.