St. Anselm, an 11th-century monk, developed a specific ontological argument for the existence of God. He began his argument by defining God as a “being than which none greater can be conceived,” or the greatest possible being. He then stated that this greatest possible being must exist in reality, since existing in reality is greater than existing only in the understanding. Anselm’s argument is ontological, since it argues from the idea of God to God’s existence.
Many years later, another monk named Gaunilo wrote an objection to Anselm’s argument. He attempted to prove that just because one can conceive of a “greatest possible something,” this does not mean it must exist in reality. Gaunilo gave an example of a hypothetical perfect island. One could imagine all sorts of amazing things on this island, but it would not necessarily exist. Gaunilo went on to explain that someone could say that this perfect island existed because existing in reality is greater than existing only in imagination. However, it would be absurd to believe that this argument proves that the island does exist in reality.
Anselm began his argument with a prayer, asking God for help in proving God’s existence. He quoted a portion of scripture in Psalms that says, “The fool has said in his heart, ‘There is no God.'” Anselm believed that, with God’s help, reason alone could prove God’s existence.
Anselm’s argument at first seems ridiculous; how could he possibly argue that God exists merely from the fact that one can imagine him to exist? On first glance, Gaunilo’s argument seems stronger. However, on closer inspection, we see that the two monks are framing their arguments about two different things. Anselm is arguing for the existence of a greatest being, while Gaunilo is arguing for the existence of a perfect island. By definition, an island has limits – it is surrounded by water. Of course it could not hold everything that would make it a “perfect island.” Such a thing could not exist. But, as Anselm argues, a being could possibly exist which was greater than all others. In this respect, Anselm’s argument seems to hold up.
The purpose of most arguments for God’s existence is to convince unbelievers to become believers. For this purpose, Anselm’s argument seems to be weaker than other types of arguments for God’s existence. A nonbeliever (and perhaps even some believers) would likely scoff at his argument and call it absurd. However, Anselm’s ontological argument for God’s existence does hold up under scrutiny and seems to be a pretty decent argument for God’s existence.