St Thomas Aquinas (1225-1274)

Aquinas tried to prove that God exists in five ways:

    1. the argument from motion: things move; nothing (natural) moves that is not moved by something else; so there must be a first or prime mover, i.e. God

    2. the cosmological argument: each thing that exists has a cause; there must be a cause of nature (or everything); this cause must be supernatural (not part of nature); it is God.

    3. the argument from necessity: each thing is either contingent or necessary; all contingent things at some time do not exist; if everything were contingent then at some time nothing would have existed; in that case nothing would exist now, since nothing comes from nothing; but things do exist now; so not everything is contingent, a necessary being (i.e. God) must exist.

    4. the argument from gradation: there is a great chain of being from the lowest to the highest; there must be some ultimate standard by which all things are measured; so there must be a perfect being, i.e. God.

    5. the teleological argument: all things in nature have a purpose; so they must be designed; so there must be a designer, i.e. God.

Like Plato, Aristotle, and the Stoics, Aquinas also believes in a natural moral law that we can discover using reason.  Anything against nature or society or reason is wrong, he argued.