The ontological argument for the proof of God's existence is first attributed to whom?

The answer is Anselm of Canterbury because he first formulated and presented the ontological argument in his work, "Proslogion," in the 11th century. Anselm argued that God, as the greatest conceivable being, must exist in reality because existence is a necessary attribute of perfection. He posited that even the fool, who denies God's existence, can understand the concept of a being greater than which none can be conceived. This argument has since been explored and critiqued by various philosophers and theologians throughout history.