Explain Anselms ontological argument for the existence of God.

Anselm's ontological argument holds that God must necessarily exist, by definition. This is an a priori argument, meaning that its conclusion is reached through purely deductive reasoning, not relying on any outside observations. For example, if all bachelors are unmarried men there are no married bachelors. For Anselm God, can be defined as the greatest possible being. He also holds that a being would be less great if it didn't exist. Therefore, because God can't be anything less than the greatest possible being, he must exist. However, Anselm's apprentice countered his claim by arguing that the same thing can be argued about the greatest possible anything, using the greatest island as an example. The greatest possible island doesn't exist. Therefore the argument doesnt prove the existence of the island. Anselm therefore can't claim that this argument for God proves God's existence. Anselm countered that God is a necessary being; unlike the island, God doesn't rely on outside factors such as a surounding sea to exist, therefore the argument applies to God but not the island.