A monk tried to prove God exists just from the definition of 'God'

In 1078, Anselm of Canterbury argued that God, by definition, is 'that than which nothing greater can be conceived'. He reasoned that a being that exists only in the mind is less great than one that also exists in reality, so the greatest conceivable being must exist. Philosophers have argued about whether the logic actually works for nearly a thousand years, including a famous rebuttal from a monk named Gaunilo within Anselm's own lifetime.

— Anselm of Canterbury, Ontological argument — Proslogion, 1078