With the idea of the highest good in place, Kant offers the following simple argument for
theism. Morality obligates each of us to seek the good, and so it obligates us to seek the
highest good. But morality cannot obligate us to seek the impossible. Hence, the highest
good must be attainable. It is not attainable without a cause adequate to the effect, which
is to say, unless there is a God with the power to proportion happiness to virtue. God's
existence is therefore a necessary condition for the possibility of the highest good, and so
it is a necessary condition for our obligation to be moral (1997, pt. 1, bk. 2, ch. 2, sec. 5).
The intuition behind Kant's argument is profound even though his description of the
highest good is idiosyncratic. What may seem particularly bothersome about the
argument is that Kant himself creates a problem for value theory and then argues that
there must be a God to solve the problem. The ancient and medieval philosophers, among
others, did not see the tension in the concept of the highest good in such stark terms to
begin with, so the need to bring God to the rescue was not as glaring. Nonetheless, some
of them did think that the highest good must be reachable and that it is not reachable
without the existence of an afterlife. A comparison of Kant's notion of the highest good
with that of Aquinas is illuminating. Aquinas accepted the Aristotelian position that all
humans desire happiness by nature, that happiness is our natural end. But if we
investigate what would truly fulfill the human longing for happiness, we see that it is
something unattainable without God and the possibility of the enjoyment of seeing God.
Aquinas's view of the ultimate human end is an extension and deepening of Aristotle's
view in book 10 of the NE that happiness is contemplation of the highest things (2000,
bk. 10, ch. 7). As Aquinas describes it, to seek happiness is to seek the satiation of the
will; to be happy is to have nothing left to will (1992, I, ii. q. 5, art. 8). The will is satiated
in the possession of reality, which, for human beings, is accomplished through an act of
the intellect, an intellectual vision. The human desire for happiness is not satisfied with
anything less than a total vision of reality. This vision is contained in the Beatific Vision,
a vision of God in whom all things are seen.^5
Aquinas does not construct his explanation of human happiness in the form of an
argument for theism, since it appears in a part of the Summa Theologiae that presupposes
his famous arguments for God's existence at the beginning of the work. But a
transcendental argument for the existence of God is implicit in Aquinas's account of the
nature of happiness. The natural end for humans requires union with an eternal being who
satisfies our natural craving for happiness. Without such a being the end of human living
is unattainable. Either there is a God or human beings aim at the impossible by nature.
So, whereas Kant argues that morality puts an impossible demand on us if there is no
God, the Thomistic argument understands nature as structured in such a way that it aims
at the impossible if there is no God. The former argues that in the absence of God there is
something wrong with morality, whereas the latter argues that in the absence of God
there is something wrong with nature.
Aquinas, like the Greeks, assumed that nature is orderly and teleological in structure.
There would be no point to the existence of natural desires unless they are capable of
fulfillment (1992, I, q. 75, art. 6, corpus), and therefore the conditions for their fulfillment
reveal important metaphysical truths. In contrast, modern thinkers are generally wary of
drawing any conclusions from human needs and desires. If we come to believe that our
natural human desires cannot be satisfied in this life, the typical response is to conclude
that we should change the desires. This modern option displays a remarkable degree of
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