How Medieval Philosophers Attempted to Prove the Existence of a God

Historical analysis and comparison of theories from St. Anselm and Ibn-Sina

Ahad Sheriff
6 min readJan 13, 2020


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It takes greater faith to believe that an unseen God exists than it does to dismiss it. This is problematic to philosophers and people of religion since you cannot physically confirm that god is there. As a result, early-medieval philosophers such as St. Anselm and Ibn-Sina (Avicenna) attempted to prove the existence of god using vastly different methods rationally.

St. Anselm was an 11th-12th century monk and the Archbishop of Canterbury who was famous for his ontological proof, a philosophical argument for the existence of God.

The ontological argument is a fascinating argument for the presence of an all-knowing, perfect deity. While there are several different versions of this argument, in the end, it exists to show that it is self-contradictory to deny that there exists the highest possible being — god. Anselm described his case in the Proslogion as follows (Note: “something god-like” refers to “that-than-which-nothing-greater-can-be-conceived”):

“Even the fool is forced to agree that [something god-like] exists in his mind, since he understands this when he hears it, and whatever is understood is in the mind. And surely that [something god-like] cannot exist in the mind alone.”

In this passage, Anselm is claiming that it is a truth — by definition — that God is the highest being imaginable and that the notion of an all-powerful, all-knowing deity exists as an idea in our minds. Anselm continues his proof:

“Suppose it exists in the mind alone: then it can be conceived to exist in reality; which is greater…Therefore, if that [something god-like] exists in the understanding alone, the very being, [something god-like], is one that which a greater can be conceived. But obviously this is impossible. Hence, there is…