The best known moral argument is that of Immanuel Kant. Kant’s argument is not based on the nature of morality, like the formal moral argument, or on morality’s perfectionism, like the perfectionist moral argument; rather, it is based on the rationality of moral behaviour.

Strictly speaking, Kant’s argument is not an argument for God’s existence, but rather for justice in the afterlife. For the sake of simplicity, though, here I have equated the two. The argument’s structure is this:

Kant’s Moral Argument

(1) Moral behaviour is rational.
(2) Morality behaviour is only rational if justice will be done.
(3) Justice will only be done if God exists.
(4) God exists.

Moral Behaviour is Rational

Moral behaviour, Kant is adamant, is rational behaviour; we have good reason to be moral. This is a fundamental principle of morality: if you ought to do something then you have a reason to do it. It makes no sense to say “I see that I ought to give money to charity, but I have no reason to.” If we ought to do something then that is a reason to do it. What is more, a moral reason is always a stronger reason for doing something than any other reason. If we have a moral reason to do a thing, and another reason not to do it, then rationally speaking we ought to do it. Moral behaviour is always rational.

Moral Behaviour is Only Rational if Justice Will be Done

Moral behaviour, though, would not be rational if there were no guarantee that it would be rewarded. If, as seems to be the case, sin often profits more than righteousness, then surely it is sinful rather than righteous behaviour that would be rational. Faced with a choice between doing that which is right and doing that which is wrong but which benefits us most, if there are no comebacks for immorality then it is rational for us to do that which is wrong but which benefits us most. It is only if there are comebacks for immorality, if justice will ultimately be done, that we have a reason to be moral.

Justice Will Only be Done if God Exists

Clearly justice is not administered in this life: some cheats prosper; some crime pays; bad things happen to good people. People do not always, or even often, get what they deserve. Life isn‘t fair.

If this life is all there is, then, justice is not done, and so moral behaviour is not rational; we have no reason to be good. We do, though, have a reason to be good. This life, then, cannot be all that there is. There must be something more.

The traditional Christian view of life after death has justice being meted out to sinners, and the righteous receiving the reward that they deserve. If this view, or some other like it, is correct, then we can explain the rationality of morality. If not, then, we cannot. The rationality of morality therefore establishes that something like the Christian view of the afterlife is correct.