Explore key ideas about the existence of God in the Ontological Argument. (2017 Edexcel exam question)

The Ontological Argument (OA) is an a priori argument, unlike the Design and Cosmological Argument. It attempts to prove God’s existence from exploring the meaning of the word ‘God’ through logical reasoning.
The classical OA comes from two scholars. Saint Anselm defined God as “that than which nothing greater can be conceived”. God, like dragons, exist in the mind. But, God must exist in reality too, since he is that than which nothing greater can be conceived, it is more perfect than not for him to exist - so he must exist. Rene Descartes developed Anselm’s argument, defining God as “a supremely perfect being". From this definition, Descartes believes that we can conclude God exists because existence is a predicate of a perfect being. Descartes says trying to imagine God without this predicate of existence is illogical; like how imagining a triangle without three sides.
The modern OA was developed by Alvin Plantinga, who brought in the notion of 'possible worlds', used to suggest that possibilities in this universe are infinite. Plantinga argument is that: firstly, there is a possible world in which there exists a being with ‘maximal greatness’, and secondly, a being only has maximal greatness if it exists in every possible world. Plantinga’s conclusion is, therefore: there is a possible world in which there is a being that is maximally great; if it is maximally great, it is existent in our world; there is a God in our world.