In philosophy, the uncreated or self-created cause to which every series of causes must ultimately be traced. Used by ancient Greek thinkers, the concept was adopted by the Christian tradition and became the basis of one version of the cosmological argument for the existence of God. According to this argument, every observed event is the result of a series of causes that must end in a first cause, which is God. The argument was given its classic formulation by St. Thomas Aquinas. It was rejected by many later thinkers, including David Hume and Immanuel Kant.