St. Anslem's Ontological Argument asserts that God is the perfect ground for all conception and all reality, so-much-so that the denial of His existence is logically impossible. Therefore, God exists.
Historically, St. Anselm’s argument was held to be untenable. Gaunilo, monk of Marmoutier, responded with his famous In Behalf of the Fool, in which he protests that the conception of a thing does not necessitate its reality. Moreover, Gaunilo charges that people refer to those things they already know (men are known by the characteristics that men have), but the conception of a supreme essence that is greater than all is not something that any person can refer to on their own. Therefore, “one might more appropriately say that it cannot be understood not to exist and cannot be understood even to be able not to exist.”
Knowing, for Gaunilo, implies a certainty that understanding does not: one can understand the existence of another person in a different country, but that in no way means that one knows that other person actually exists. To that end, Gaunilo does not deny that St. Anselm’s argument carries a certain force with it, but asserts that it must be “more cogently argued.”
To wit, the main challenge to the Ontological Argument can be summed up by the simple phrase “it assumes that all ideas have their parallel in reality.” To say this, however, places reverse emphasis on the direction of the Ontological Argument, and ultimately ends in circularity. The Ontological Argument moves from a direction of cause to effect, conception to concretion, but the popular contention that reality contains the parallels of the ideas moves from effect to cause, concretion to conception, and thereby slips the limits of St. Anselm’s a priori intentions.
What St. Anselm was describing was that there is no parallel in reality to a supreme God, for that would make the physical parallel God and not the Christian deity. Therefore, God is wholly beyond parallel in both humanity’s conception of the supreme deity, and in the observable world around us. However, for God to be perfect, He had to really exist, free from parallels, in order to be God. God is His own cause, and our understanding of His supremacy beyond reality and human conception is the effect of His truly being real.
Moreover, if all ideas are paralleled in reality, then reality can be said to be the reflection of an idea. Who conceived the idea then? Certainly not contingent beings who need the reality they exist in to survive! Presumably then, God, who is beyond parallel in conception and reality must have conceived the idea of reality. So unless one is willing to admit to St. Anselm’s a priori assertion, one ends up with an ineffective argument that reads something like this: ideas have their parallel in reality, ergo reality is composed of ideas. Nothing is ventured and nothing is gained from such speedy trips around Pi (π).
© Christopher J. Freeman
 Hopkins, Jasper and Herbert Richardson, ed. & trans. Anselm ofCanterbury V.I, 120
 Ibid., 120
 Dr. William Mundt, “Fundamental Arguments for God’s Existence” Concordia Lutheran Theological Seminary, November 30th, 2004.