Trying to prove that God exists is a difficult argument and many people have tried many different ways. The Ontological argument is one argument; at the centre of the argument is the concept of existence. The Ontological argument has been argued from a group of philosophers for the existence of God. "Ontological" means talking about being and so that being is the existence of God. The ontological argument differs from other arguments in favour of God because of the fact that it is an a priori deductive argument.
There are two main arguments for the ontological argument which seeks to prove the existence of God a priori. The first argument is from Anselm; he was the Archbishop of Canterbury and therefore started his argument from a theistic point of view. Anselm believed that no belief in God was absurd and he used a reducio ad absurdum argument, which tries to show that God not existing could not be believed because in not believing you are adopting a nonsensical argument. His starting point was his definition of God, ‘God is the thought than which nothing greater can be conceived’. Firstly Anselm attacks the idea that there is no God, even the thought that there is no God requires the concept of God. The greatest possible being has to exist in reality as well as in the mind to be the greatest possible being therefore we can conceive of the greatest possible being because it also has to exist in reality. A criticism that was raised about the greatest possible being is that we as humans all have different ideas of what it could be but Anselm responded by saying that when we reach the idea of such a high level being such as God then the idea of what it is becomes very similar. Therefore God exists according to Anselm. Also God is considered to be a necessary being, which will always exist, does not rely on other beings and cannot not exist, therefore God must exist if he is to be called a necessary being.