The Ontological argument is an a priori argument that seeks to prove the existence of God on the grounds of reason. The two classical formulations of the argument were put forward by 11th century Bishop Anselm and the 17th century French mathematician and philosopher Rene Descartes. Both arguments justify the claim that God exists on the very fact that by very definition it is a logical contradiction to say anything else.
It could be argued that the Ontological argument is a successful argument in a belief in God on the grounds of its a priori logic. Anselm famously stated that God is ‘that than which nothing greater can be conceived.’ Anselm is applying to the logic that since God by very definition and notion accepted by all to be great, and thus cannot and is not reliant on anything else. Anselm logically applies the philosophy of Boethius and argues that God cannot not exist. Anselm put forwards the fact we all understand the concept of God, and therefore ‘the fool is the one who denies God’s existence.’ This belief can be seen and viewed as being logical as well have concepts that we understand before needing further experience. For example we know that by very definition 1+1 is 2, we do not need any further proof. Anselm justifies a belief in God on the same grounds, that we all understand God, and based on the fact we understand God, God thus must exist.
However there are problems with this logic which most certainly weaken using an ontological argument to justify a belief in God. The Christian philosopher Thomas Aquinas argued that a priori arguments to justify a belief in God fail on the grounds that our own knowledge and language cannot comprehend the greatness of God, furthermore the 18th century philosopher Immanuel Kant argued that a priori arguments completely fail since we cannot verify God existing using a priori. These two points are valid and to be taken with credit. For anyone seeking further clarification in a belief in God mere statements may not be credible enough, and further experience and explanation are needed. Therefore the points made by Kant and Aquinas can be seen as weakening a justification in arguments of ontology for believing in God.
Though the a priori logic of the ontological argument does weaken a justification in the belief in God it does however use philosophical reasoning which does give it credibility and conviction. Anselm, as well as Norman Malcolm, apply the rational of necessity to the concept of God, furthermore Rene Descartes argues that God is perfect by definition and cannot lack anything. In both regards arguing from the position of necessity, that God is not dependent on anything else, is logical. If we are to accept that God is perfect and great than we therefore have to accept the premise that God is necessary and cannot be dependent on anything else. Furthermore, on this great God must exist, since it would be impossible not to if He were necessary.
The major issue with this claim however is that by simply asserting that something exists does not make it exist in reality. If that logic were to be applied to anything than any illogical concept could arise, or be brought into existence. Bertrand Russell famously argued that the existence of Santa could be true if I were to accept the principle that men exist, I therefore would have to accept the fact that Santa to also exists. Additionally Immanuel Kant, the fiercest critic of the ontological argument, stated that simply arguing existence is a feature, or predicate of God does not therefore make something exist. We once again return to the problem of the a priori nature of the argument, experience is needed to make sense of such a fundamental claim that ‘God exists.’ I cannot simply state that God exists, therefore have proven God’s existence.
Though the argument from necessity and existence do question the ontological argument it could be argued that the fundamental premise of the argument that Anselm upheld, to doubt God existing is illogical does make sense since there is still possibility in such a concept, as much so as life after death. We assert there is life after death without knowing factual proof. The modern philosopher Alvin Plantinga holds a similar view. By applying modal logic he states that it is possible for God to exist. If God were to exist in said world X, he too would have to exist in this world since good is both maximally great and perfect. Therefore God is necessary. Applying probability and the fact that a concept of God is greatly accepted is valuable, and can be seen to give justification in believing in God.
In conclusion the very a priori nature of the ontological argument does not justify a belief in God since belief in God would already be accepted by the said person reading the argument. To the theist the argument is logical, however to the atheist the argument offers as much justification in God existing as does believing in Santa or the Easter Bunny. Brian Davies famously argues the logical fallacy of the argument and its circular nature. It begins with the idea that God exists, and finishes with the point God exists. Without accepting the idea of the argument you therefore by default will not accept the conclusion. It is therefore true to say that the ontological argument does not offer justification in a belief in God.