Ontology is a branch of metaphysics in relation to the nature of being. Christian philosopher Saint Anselm of Canterbury is most famous for his ontological argument for the existence of God.
He stated that the concept of God was “a being than which no greater can be conceived.” The word “conceived” applies to the mind, meaning that God is something which nothing greater can be thought of in our minds as an object of thought. However, the question is whether God exists in reality too.
Anselm’s reasoning is that if a being exists only in the mind but not in reality, then it means that a greater being is conceivable. Since God is a being than which no greater can be conceived, he therefore must exist in reality too.