An ontological argument is an argument for the conclusion that God exists based on premises derived from rationale; an argument founded on reasoned, a priori premises. The first ontological argument was proposed by St. Anselm of Canterbury in the 11th. Century A.D. Anselm claims to derive the existence of God from the concept of "something than which nothing greater can be conceived." (Anselm 78) He reasoned that, if such a being fails to exist, then a greater being can be conceived. But, nothing can be greater than the greatest thing imaginable, which means that a being which no greater can be conceived (i.e. God) must exist.
A formal statement of Anselm's argument would look like this:
1.) God is "something than which nothing greater can be conceived"
2.) God exists in the understanding (your mind), where you conceive of Him as the most perfect being possible.
3.) It is possible to conceive God existing in both the understanding and in reality.
4.) If God exists just in the understanding we could also conceive of him as really existing; but then this would be greater.
5.) Things that exist in reality are greater than those that exist only in the mind. God cannot, then, exist just in the understanding since we would then be able to conceive of something greater. It is impossible to conceive of a being greater than the greatest conceivable being.
6.) God must exist in both the understanding and in reality.
Premise (1) of this argument is Anselm's conception of God. Premise (2) is a simple logical truth; anything can be conceived of in the understanding. Premises (3) and (4) combine the ideas of God and his existence in reality. Anselm continues in premise (5) that anything conceivable would certainly be better in reality. So, by...