St. Anselm's Ontological Argument concludes that God, defined as "that than which nothing greater can be conceived," must exist in reality and not just in the mind. Anselm argues that if God exists only in the understanding, then a greater being could be conceived to exist in reality, which contradicts the definition of God. Therefore, God must exist in both the mind and in reality, affirming His existence. This argument emphasizes the necessity of God's existence based on the very concept of God itself.