The Ontological Argument attempts to arrive at the reality of God from the idea of God. This argument was first articulated by Anselm (Proslogian, chapters 2-3), who is credited with formulating the core of the ultimate idea of deity. He presented his argument in two different forms. The first forms states that God is a being “something than which nothing greater (more perfect) can be conceived”; there is no reality beyond him to which he is inferior. Things existing in reality are greater than things existing in the mind only; therefore God must exist in reality, or he would not be the greatest possible being. In other words, since God exists in our minds, he must exist in reality as well, since it would be absurd to be able to think of something that is greater than that which nothing greater can be conceived.
The second form states that God is a necessary being, not a contingent being, since a necessary being is greater and truer than a contingent one. One must affirm what is necessary in a Necessary Being, and existence is logically necessary in a Necessary Being; therefore, a Necessary Being necessarily exists.
By way of analogy, Gaunilo of Marmoutier argued that Anselm must be wrong, because the argument could lead to a perfect anything, not just a perfect God. Gaunilo said it was absurd to believe that since he could conceive of an island than which nothing greater can be conceived, this island must exist. Anselm replied that since the island was contingent (its non-existence is conceivable), the analogy was false. Overall, though Gaunilo asked some interesting questions, he did not nullify Anselm’s argument.
Rene Descartes later offered a slightly adjusted argument: one must logically affirm what is essential to the nature of an object; existence is logically necessary in something that necessarily exists; therefore, one must affirm that a necessary existent does exist.
Descartes was not without his critics. Caterus claimed the argument only proved the conceptual existence of God. Kant rejected the entire statement, thus removing the obligation to believe in the subject of the argument. Hume went so far as to say that no argument for God is rationally inescapable. Since that time, the argument has been consistently raised that the concept of God only allows the possibility of God rather than proving his existence.
Kant also argued that the argument itself was structured incorrectly, since propositions asserting existence should always be synthetic rather than analytical; however, some philosophers have argued that Anselm’s Chapter Three argument (the second form) is immune from Kant’s critique.
More recently, panentheist Charles Hartshorne and Norman Malcolm revived Descarte’s second argument in this manner: ”A being who exists, but of whom it is conceivable that he might not have existed, would be less than God; for only a being whose existence is necessary rather than contingent can be that than which nothing greater is conceivable. But if such a necessary being does not exist, it must be a necessary rather than a contingent fact that he does not exist. Thus God’s existence is either logically necessary or logically impossible.”
The core criticism of all forms of this argument continues to be that while the concept of God can necessarily require his existence, that does not automatically mean that his existence in reality is assured. To address this problem, Alvin Plantinga has developed an ontological argument of his own that relies on a cosmological foundation to establish that the origination of the universe requires more than just a concept of God, and therefore God exists.
Because of the inability of this argument to lead to neither a rational proof of God nor a rational disproof, some Christian philosophers agree with Karl Barth that the Ontological Argument should be used more as an assurance to those who already believe in God than as an apologetic method for those who do not.
Plantinga’s recent writings have given increased strength to this argument. However, it remains the most difficult to understand, and many philosophers favor other arguments over this one.
 Paul Edwards, ed. “Ontological Argument for the Existence of God” The Encyclopedia of Philosophy, Volume 5. (New York: The MacMillan Company,1967), 539.
 Edwards “Ontological Argument for the Existence of God,” 538.
 Frame, John M. Apologetics to the Glory of God. (Philipsburg: P and R Publishing,1994), 115.
 “Ontological argument.” Basictheology.com. 2003. Internet. Available from www.basictheology.com/main/definitions.display.asp?did+55. Accessed April 23, 2003.
 Geisler, Norman. “Ontological Argument.” Baker Encyclopedia of Christian Apologetics. (Grand Rapids: Baker Books, 2002), 556.
 Geisler “Ontological Argument,” 557.
 Kung, Hans. “Does God Exist? Trans. by Edward Quinn. (New York: Vintage Books, 1975), 34.
 Edwards “Ontological Argument for the Existence of God,” 539.
 Geisler “Ontological Argument,” 557.
 Edwards “Ontological Argument for the Existence of God,” 540.
 Geisler “Ontological Argument,” 564.