Ontology is a branch of metaphysics that relates to the nature of being. Christian philosopher Saint Anselm of Canterbury is most known for his ontological argument for the existence of God. His argument consists of six premises. Firstly, Anselm describes God as “a being than which no greater can be conceived.” By this, he means that nothing can surpass God because he is infinitely great. However, the word “conceived” applies to the mind, meaning that God exists as an object within the mind, which is the second premise. The third premise is that a being which exists in both mind and reality is greater than a being that only exists in the mind. Therefore, if God exists only in the mind, then it means that it is possible for us to conceive a being that is greater than God. However, this brings us back to the first premise which states that God is “a being than which no greater can be conceived.” Since we cannot imagine a being that is greater than God, it means that he must exist in both mind and reality.
On the other hand, St Thomas Aquinas is known for his Summa Theologica, which are five arguments which supposedly prove the existence of God. Rather than concepts similar to Anselm’s ontological argument, Aquinas’ proofs rely on our senses. The first argument is motion where Aquinas believes that anything that moves must be moved by another thing. For example, A is moved by B, B is moved by C, C is moved by D, and so on. However, there would be no motion in the first place if not for the First Mover that caused A’s movement. The First Mover itself is unmoved and we call it God. Aquinas calls this the First Mover for the first unmoved mover, which is what we call God. The second argument is efficient cause. Everything has its own efficient cause. For example, A was created by B, B was created by C and so on. Similarly to the first argument, there must be a First Maker that was not created by something else and we call this First Maker God. The third argument is contingency of being. Aquinas believed that the existence of everything in the world is contingent. Nothing would exist unless there is a being whose existence is necessary and accounts for everything else. This necessary being is called God. The fourth argument is from degrees of perfection. We have certain standards and we make a lot of judgement about different things. However, we would not have any standards unless there is an all-perfect being that we can compare things to. Aquinas labels this all-perfect being as God. The last argument is design. Everything seems to have a design or purpose. For example, a creature’s wings are designed to enable the creature to fly. So if there is a design, there must also be a designer and this would be God.