Saint Anselm of Aosta, Bec, and Canterbury, perhaps during a moment of
enlightenment or starvation-induced hallucination, succeeded in formulating an
argument for God's existence which has been debated for almost a thousand years.
It shows no sign of going away soon. It is an argument based solely on reason,
distinguishing it from other arguments for the existence of God such as
cosmological or teleological arguments. These latter arguments respectively
depend on the world's causes or design, and thus may weaken as new scientific
advances are made (such as Darwin's theory of evolution). We can be sure that
no such fate will happen to Anselm's Ontological Argument (the name, by the way,
coined by Kant).
In form, Anselm's arguments are much like the arguments we see in
philosophy today. In Cur Deus Homo we read Anselm's conversation with a skeptic.
This sort of question-and-answer form of argumentation (dialectic) is very much
like the writings of Plato. The skeptic, Boso, question's Anselm's faith with
an array of questions non-believers still ask today. Anselm answers in a step-
by-step manner, asking for confirmation along the way, until he arrives at a
conclusion with which Boso is forced to agree. This is just like Socrates'
procedure with, say, Crito.
Later philosophers have both accepted and denied the validity of
Anselm's famous ontological argument for the existence of God, presented in both
the Proslogium and Monologium. Anselm did not first approach the argument
with an open mind, then examine its components with a critical eye to see which
side was best. Anselm had made up his mind about the issue long before he began
to use dialectic to attempt to dissect it. "Indeed, the extreme ardor which
impels him to search everywhere for arguments favorable to the dogma, is a
confession his part that the dogma needs support, that it is debatable, that it
lacks self-evidence, the criterion of truth." (Weber, V)
In chapters 2-4 of his Proslogium, Anselm summarizes the argument. A
fool is one who denies the existence of God. But even that fool understands the
definition of God, "a being than which nothing greater can be conceived." But
the fool says that this definition exists only in his mind, and not in reality.
But, Anselm observes, a being which exists in both reality and in the
understanding would be greater than one that merely exists only in the
understanding. So the definition of God, one that points to "a being than which
nothing greater can be conceived", points toward a being which exists both in
reality and in the understanding. It would be impossible to hold the conception
of God in this manner, and yet deny that He exists in reality.
The argument was criticized by one of Anselm's contemporaries, a monk
named Gaunilo, who said, that by Anselm's reasoning, one could imagine a certain
island, more perfect than any other island. If this island can exist in the
mind, then according to Anselm, it would necessarily exist in reality, for a
'perfect' island would have this quality. But this is obviously false; we
cannot make things exist merely by imagining them.
Anselm replied, upholding his argument (in many, many words) by saying
that they are comparing apples and oranges. An island is something that can be
thought of not to exist, whereas the non-existence of "that than which a greater
cannot be conceived is inconceivable." (Reply, ch.. 3) Only for God is it
inconceivable not to exist; mere islands or other things do not fit this quality.
Copleston sums it up succinctly (for Anselm doesn't): "it would be absurd to
speak of a merely possible necessary being (it is a contradiction in terms),
whereas there is no contradiction in speaking of merely possible beautiful
islands.
St. Thomas Aquinas rejects the argument, saying that the human mind
cannot possibly conceive of the idea of God by reason alone (a-priori), as
Anselm might. The argument does not make sense by itself, and must first
provide an idea of the existence of God with an analysis of God's effects (a-
posteriori), to which Thomas turns. I think there is evidence in Anselm's
writings that he would disagree, saying that the idea of God is an innate one
given to us by God, and needs no other revelation to bring it about.

"Hence, this being, through its greater likeness, assists the
investigating mind in the approach to supreme Truth; and through its more
excellent created essence, teaches the more correctly what opinion the mind
itself ought to form regarding the Creator." (Monologium, ch. 66)

Although St. Thomas was obviously a believer, he was not swayed by the
idea of reason alone being sufficient to prove God's existence. His objection
of the human mind's capability to ascertain God is echoed by other philosophers
such as Kierkegaard (who was also a Christian): "The paradoxical passion of the
Reason thus comes repeatedly into collision with the Unknown...and cannot
advance beyond this point. [Of God:] How do I know? I cannot know it, for in
order to know it, I would have to know the God, and the nature of the difference
between God and man; and this I cannot know, because the Reason has reduced it
to likeness with that from which it was unlike." (Kierkegaard, 57)
Anselm disagrees, and explains why illumination of God ...