Although, like all the other arguments, it comes in different forms, the common denominator of the moral argument is that it sets out to prove the existence of God from the evidence that morality exists in the world. There are evidential reasons, it is argued, to believe that the universe is a moral one because the vast majority of people not only experience morality, but feel obliged to follow certain moral codes that are, broadly at least, common across time and cultures.

Aquinas's Fourth Way states that “there must be something which is to all beings the cause of their being, goodness and every other perfection: and this we call God” (Summa theologica). Aquinas bases his moral principles on Plato's theory of the Forms (see Chapter 1): the fact that we have the idea that things are less good, noble and so on implies that we have an idea of how such principles can be bettered and, ultimately, we can come to an awareness of the highest good, which, for Aquinas, is God.