Final answer:
According to Anselm, it is a contradiction to deny the existence of God because God is defined as a being than which nothing greater can be conceived. Anselm argues that it is greater for a being to exist in reality than just in the mind, so if God is the greatest conceivable being, then God must exist both in the mind and in reality.
Step-by-step explanation:
Anselm's argument is known as the ontological argument for the existence of God. According to Anselm, it is a contradiction to deny the existence of God because God is defined as a being than which nothing greater can be conceived. Anselm argues that it is greater for a being to exist in reality than just in the mind, so if God is the greatest conceivable being, then God must exist both in the mind and in reality. This argument is based on logic and reasoning, not empirical evidence or personal choice.