The Ontological ArgumentThe first argument we will look at is the so-called Ontological Argument. [a] The argument was first introduced by St. Anselm (1033-1109), Archbishop of Canterbury, in his book Proslogium. Anselm was one of the first Christian theologian who defended his faith on reasoning rather than by an appeal to Scripture or tradition. 
The Ontological ArgumentIn the Proslogium, Anselm began his argument by quoting a passage from Psalms (14:1) which says: "The fool says in his heart, 'There is no God.'" Now God, according to Anselm, is "that than which nothing greater can be conceived." So when an atheist, that is, the fool, says that "god does not exist", he means that "that than which nothing greater can be conceived" exists only in the mind but not in reality. But, Anselm added, for a thing to exists in reality is surely greater than that which exists only in the mind. Now obviously, "that than which nothing greater can be conceived" must exist in reality for that definition to hold.  As Anselm himself puts it:
According to Anselm's Ontological Argument, one cannot deny the existence of God without involving oneself in a contradiction. Anselm's God, by definition, exists!
The Ontological Argument gives one a feeling of unease; a feeling akin to seeing a magician pulling a rabbit out of a hat; there must be a trick somewhere! In fact, there are so many things wrong with Anselm's argument that almost every generation of philosophers has succeeded in poking holes in it!
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1. The Form of the Argument is SuspectThe first critique actually came from a contemporary of Anselm. It was a French monk, Gaunilon, who wrote a reply to Anselm entitled On Behalf of the Fool.  Gaunilon's critique was aimed mainly at the form of the Ontological Argument.
To this line of reasoning, he applied Anselm's logic on a "perfect" island. Suppose that beyond the boundary of man's known world (remember this was the eleventh century), there is an island. Suppose that this island is perfect and greater than all islands: an island than which none greater can be conceived. To use Anselm's logic, if this island exists only in the imagination it would be less great than that which exists in reality. Hence the "island than which none greater can be conceived" must exist in reality!  In a way, Gaunilon has shown that not only can the Anselmian magician pull a rabbit out of a hat, he can pull anything he wants, including the proverbial kitchen sink!
Used on an island, or any other thing, this form of argument is surely absurd. As Gaunilon wrote:
Anselm's reply was, in my opinion, weak and unconvincing. He asserted that an island is not something which can not be thought of as not existing.  In short, the Ontological Argument is applicable only to God!  This reply is unsatisfactory because it disallows any analogy by which the form of the argument can be tested.
Gaunilon's argument, in fact, exposes a basic flaw in the Ontological Argument. If the "most perfect island" is an object, according to Anselm, that can be imagined to be non-existent, then it follows that existence itself could not be an added perfection. Hence it makes no difference to the idea of perfection, whether the object (be it island or God) exists in reality or only in the mind. 
Existence is Not a PredicateIn the eighteenth century, the German philosopher, Immanuel Kant (1724-1804) further refined Gaunilon's objections to the Ontological Argument.
Existence, says Kant, is not a predicate. By that, he meant that saying that a being or creature or thing exists we are not adding anything to the properties that define it. [b] Take, as an example, the mythical centaur. We can define this by listing its properties; namely that it is a creature with a head, body and hands of a man attached to the body and legs of a horse. If we add to that statement that centaurs exists, we are not adding anything to the property of the creature. 
The concept is not changed whether one is merely thinking about it or one is thinking of it as existing. Hence, as applied to God, one's idea of a perfect being is not changed whether one is only thinking about it or thinking about it as existing. 
Now if supporters of the Ontological Argument assert that with God, existence is a part of the definition of the divine being such that it cannot be denied without self-contradiction, they face another difficulty. In this way they have made the statement "God exists" an analytic statement. Analytic statements are statements that simply restate the property that is already defined in the object; i.e. they tell us nothing new. All analytic statements are tautological. An example of an analytic statement would be "all husbands are male spouses". Nothing new is being conveyed by that statement for part of the definition of husband is "male spouse". Its the same with a triangle; for example "a triangle is a plan figure having three angles." Now Kant says:
What Kant is saying is that the truth of any analytic statement is purely conditional. It does not and cannot prove the existence of whatever regarding what it is talking about. So the assertion "God exists" simply means "If God exists (the condition), then he exists (part of the definition)". To sum up his argument (given in The Critique of Pure Reason ), Kant concluded:
ConclusionsOur conclusions on the Ontological Argument are:
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