The brilliant men of the Scholastic era divided into different schools of thought. One sect followed Realism, based on Plato. Plato believed that universals have an objective existence. These things are only shadows of the unseen reality in another world. Anselm took this to another level. He became archbishop of Canterbury in 1093. He is summed up in the statement, "I believe in order that I may know."
He is best known for developing Plato's ideas into the ontological argument for existence, and argument that sounds childish at first but one I have grown to love. I will quote from the Cairns textbook.
"Man has many goods that he enjoys in life. These goods are simply reflections of the one supreme Good through whom all exist. Because infinite regress is unthinkable, the cause of all must be the One whom we call God."
"Anselm wrote that everyone has an idea of a perfect supreme being in his mind. This idea must correspond to a reality that has an objective existence, for such a being lacking existence would not be perfect nor would it be that than which a greater cannot be conceived. Because no greater idea than that of God as the perfect Supreme Being can be conceived, God must exist in reality."
Basically, everybody in the world has a concept of a perfect supreme being. If they have this concept, then they must have that idea from a concrete evidence. Therefore, God exists. This also led to the idea that Christ's atonement was not a ransom to Satan, but a satisfaction of God's perfection which was violated by our sin. It satisfied God's wrath.
Anselm lived from 1033 to 1109. His ontological argument dominated the world until the time of Thomas Aquinas. I think of recent days, Ken Ham of Answers in Genesis has perfected the ontological argument in his apologetics about Noah's flood and dinosaurs/dragons.