One of the most fascinating arguments for the existence of an all-perfect God is the ontological argument. Ontological arguments are arguments to prove the existence of God based on pure reason alone. They attempt to show that we can deduce God’s existence from, so to speak, the very definition of God. St. Anselm of Canterbury proposed the first and most well known ontological argument in 1078 in his Proslogion, but it was actually Immanuel Kant, an 18th century German philosopher, who first called the argument “ontological.”
In his argument, Anselm defines God as “that than which nothing greater can be conceived.” This can be interpreted as defining “God” as maximal perfection, or the greatest possible being. It doesn’t matter what one means when they use the term “God”. If the argument proves that such a being exists, then it does so regardless of what one wishes to call the being of maximal perfection. Anselm also argues that God exists as an idea in the mind. A being that exists, as an idea in the mind and in reality, is greater than a being that exists only as an idea in the mind. Thus, if God exists only as an idea in the mind, then we can imagine something that is greater than God (the greatest possible being that does exist in reality). According to Anselm’s argument, we cannot imagine something that is greater than God, for it is a contradiction to suppose that we can imagine a being greater than the greatest possible being that can be imagined. Therefore, concludes Anselm, God exists.
Gaunilo of Marmoutiers was an 11th century Benedictine monk who criticized Anselm’s ontological argument for the existence of God by using the same reasoning to prove the existence of the “perfect island”, or the greatest, most perfect island conceivable. His argument states that if the island that we are thinking of does not exist, then it cannot be the greatest conceivable island, because for it to be the great conceivable island it would have to