Strength and Weaknesses of Ontological Argument

The Ontological Argument was, and still is, a hot-topic for debate among philosophers; many famous philosophers have published criticisms of the theory including Immanuel Kant and St. Thomas Aquinas. This obviously raises questions regarding whether or not this argument works.
While there is no clear-cut answer to these questions, I personally believe that the negatives of this argument outweigh the positives, thereby making it a weak argument. The first published criticism of Anselm’s Ontological Argument was from Gaunilo in his book In Behalf of the Fool (making reference to the fool in the book of psalms who didn’t believe in God).
While Gaunilo was a firm believer in God (and was in fact a monk), he disagreed strongly with Anselm’s method for proving his existence. His problem is with the strand of Anselm’s argument which is put forward in Chapter Two of Proslogion.

While Anselm claimed that the God, who is defined as perfect, must exist because an existent God is better than a non-existent God meaning that if he didn’t exist, he wouldn’t be perfect and therefore, wouldn’t be God, Gaunilo applied this logic to the example of a Perfect Island.
If the perfect Island didn’t exist in the real world, it would be a contradiction to call it the perfect Island. By this logic, the perfect Island must exist seeing as if it didn’t exist, it wouldn’t be perfect1. We obviously know that the Perfect Island does not in fact exist and, by Gaunilo’s reasoning, Anselm’s argument doesn’t work; if it doesn’t work with parallel arguments, it doesn’t work in the example of God. This criticism is very astute and, therefore, severely weakens the argument and its effectiveness.
However, Anselm directly responded to his contemporary Gaunilo’s criticism in an attempt to defend his argument and its ideals. Firstly, Anselm pointed out the fact the example of the Island (or any other examples for that matter) do not work because it, unlike God, is contingent and not at all necessary; its existence relies on the Earth and the Sea and it would have been entirely possible for it never to have existed. God, however, must exist and depends on the existence of nothing else.
Moreover, Anselm goes on to say that the perfect Island is impossible to define; will it become more perfect as it gets bigger? God, however, is specifically defined by Anselm allowing the Ontological Argument to be applicable to him.
This response deals with the criticism well and manages to re-strengthen the Ontological Argument to some degree. Another philosopher who disagreed with Anselm’s Ontological Argument was St. Thomas Aquinas. Again, he believed in God but disagreed with Anselm’s argument proving his existence.
Aquinas raises questions about God’s self-evident existence. He claims that things can be self-evident in two ways: in itself and both in itself and to us; even though something may exist self-evidently in itself, this self-evidence may not be known to us as humans and therefore, its existence would not be self-evident to us.
This is exactly what Aquinas proposes God to be. God is self-evident in himself because he is his own essence. However, seeing as this essence is unknown to us (as we do not know enough about him), the statement ‘God exists’ is not self-evident to us.
This, again, is another criticism which holds weight against the Ontological Argument, highlighting a glaring weakness in its logic. A third philosopher who published a criticism on the Ontological Argument (though in this case it was directed towards Descartes’ version of the argument which was published several centuries after Anselm’s in 1641) was Pierre Gassendi. In Descartes’ version of the Ontological Argument published in his Meditations, he claimed that God was entirely perfect and, as a result, must possess every possible perfection – including the perfection of existence.
Gassendi believed that this logic didn’t work because if a thing doesn’t exist, it is neither perfect nor imperfect; it merely doesn’t exist. Therefore, if God doesn’t exist, this logic could not possibly be applied to him in an attempt to prove his existence.
Descartes published a response to this criticism. In this response, he claimed that God could not be compared to anything else, proposing that ‘the relationship and essence is manifestly quite different in the case of God from what it is in the case of a triangle’; it is a part of God’s essence to exist.
While this does respond directly to Gassendi’s criticism, it does not particularly strengthen the argument seeing as Descartes gives no reason as to why God’s existence is a part of his essence; he merely states that it is true. Yet another philosopher who published a criticism of the Ontological Argument was Immanuel Kant.
Kant (who was an atheist) published a book called A Critique of Pure Reason in which he attempted to contradict both Descartes’ and Anselm’s versions of the Ontological Argument in two different ways.
In his first argument, Kant begins by hypothetically accepting that existing is indeed a defining predicate of God (which both Descartes and Anselm claim it is). He then goes on to argue that, even if this were true, there would be no contradiction in altogether rejecting the concept of God.
For example, you may understand that having a single horn on its head is a defining predicate of a unicorn. However, this does not mean that it would be contradictory to not believe in unicorns or magical horses with horns.
By this logic, you could also claim that you agree that if God did indeed exist, he would necessarily exist but that you do not believe in God or his necessity without contradicting yourself. In Kant’s second argument, he attacks the Ontological Argument at its base by claiming that ‘existing’ could not possibly be considered as a defining predicate as it does nothing to change the definition of the being in question; two people – one of whom believes in unicorns and one who doesn’t – would most likely share the same idea of what a unicorn is.
While these two people would disagree on whether or not unicorns were real, they would not disagree on what a unicorn was. Therefore, it is impossible to say that existing is a defining predicate of God because it adds nothing to the definition of God.
Overall, therefore, while the Ontological Argument is by all means a sound theory to some degree, the criticisms which highlight so many of the argument’s weaknesses show how the theory is weaker than in it is strong.