{ 1 } - the mere concept of a supremely perfect being requires that this being exist.
{ 2 } - the order and design that we discover in the world shows that there must be a supreme mind who formed the world.
{ 3 } - the mere existence of the world requires an explanation -- and this explanation can only be God.

The "third way" of St. Thomas Aquinas argues that the existence of contingent beings (beings whose non-existence is possible) requires that there be a necessary being (a being whose non-existence is impossible). This necessary being is God.

Another form of the argument says that there must be a first cause -- a being that brought other things into existence, while nothing brought it into existence.