The three main writings of Anselm are The Monologion, The Proslogion, and Responsio.
The Monologion:
The Monologion was originally titled A Monologue on the Reason for Faith. In this work, Anselm began to introduce his theology which was rooted first in logical exposition, instead of religious belief, although to be sure, Anselm was a fervent Catholic and a member of the Buc monastery.
In the Monologue, Anselm argues that even according to basic logic, the existence of God should be easily discovered. He even goes so far as to say that "Anyone of even moderate intellegence should be able to easily reason the existence of God."
The core argument of the writing is that through the analysis of one's subjective understanding of moral goodness, one should find that some things are supremely good, or better, than others, which means that there must be something of superlative goodness—the Best. This best thing would need to be God according to the logic. The end of the book concerns God's relationship to moral goodness, arguing that God did not invent goodness, nor does he obey it, but rather, God is goodness personified, meaning that objective moral goodness is an extension of his nature.
The Proslogion:
After feeling that his first monologue was insufficient, Anselm extended his arguments in the Proslogion, or the 'Discourse,' which is written as a direct discourse with God, in which Anselm addresses the idea that one only needs to have the idea of God in one's head to prove the existence of it. He argues that actually, even an atheist should be able to arrive at the same conclusion, offering what has been called the "Ontological Argument for the Existence of God."
The ontological argument is this: There must be a best thing, a thing which exists and is better than all other things. The best conceivable extant thing is a loving God who would have created the world. This God would be personal, because it would be better to be personal than non-personal. The eventual conclusion of this line of reasoning is that such a God must exist, because it would be better to exist than not to exist. This type of logical system can also be found in Occum's famous theory, known as Occum's Razor.
Responsio:
When other abbots in the church read Anselm's works, there were many critics, most notably a man named Gaunilo who wrote a rebuttal called "For the Fool," which offers a parody argument that there must be an ideal island somewhere which meets a long list of arbitrary qualities. The nature of most rebuttals can be summed up in this: You can't move from human idea into reality. Just because someone thinks something must be some way, that doesn't mean it is that way.
In response, Anselm wrote the Responsio. His argument is that Gaunilo misunderstood the argument entirely. Both Gaunilo and Anselm paid each other the respect of not mentioning each other's names in their works. Anselm's rebuttal of the island argument is that the logic only works for the superlatives. What is better: An inanimate piece of land, or a being like God? God would be better according to Anselm, so the ontological argument does not produce the belief in an imaginary island.
