The ontological argument is an argument for God's existence and can be stated in this way: “God is the greatest being imaginable. One of the aspects of perfection or greatness is existence. Thus, God exists.” Or put another way—“The fact that God can be conceived means that he must exist.”
This argument for God’s existence was developed by the twelfth century theologian and philosopher, Anselm. It is based on Anselm’s declaration that God is “that which nothing greater can be conceived.”
The ontological argument is unlike the cosmological and teleological arguments in that it does not argue from evidence in the natural world. Thus, it is not a “cause and effect” argument. The famous French philosopher, Rene Descartes, believed in the validity of the ontological argument.