The Ontological Argument from Proslogion by St. Anslem of Canterbury (1033-1109)

Whatever is understood exists in the understanding. And assuredly that than which nothing greater can be conceived cannot exist in the understanding alone. For suppose it exists in the understanding alone -- then it can be conceived to exist in reality, which is greater.   Therefore, if that than which nothing greater can be conceived exists in the understanding alone then the very being than which nothing greater can be conceived is one than which a greater can be conceived. But clearly this is impossible. Thus, there is no doubt that there exists a being than which nothing greater can be conceived, and it exists both in the understanding and in reality.