Anselm of Canterbury
Notes
Anselm (c. 1033–1109), Benedictine monk, Archbishop of Canterbury from 1093, and the originator of the Ontological Argument in the Proslogion (1078): God is that than which nothing greater can be conceived, and since it is greater to exist in reality than in the understanding alone, God must exist. His contemporary Gaunilo of Marmoutiers replied on behalf of "the fool" with the perfect island, and the argument has been doing the same two steps ever since.
Wikipedia: https://en.wikipedia.org/wiki/Anselm_of_Canterbury