Descartes' Ontological Argument

In the fifth Meditation, we meet the Ontological argument, in a form almost identical to the first put forward by Anselm in the Proslogion. In chapter two of the Proslogion, Anselm says that God is that which nothing greater can be thought, and since that which exists is better than that which does not exist, God must exist if he is the Supreme Being. Descartes arrives at it through his method of vivid and clear perception. He says that the idea of a supremely perfect God presents itself vividly and clearly to his mind. Something that is perceived as being vivid and clear is necessarily true in essence, but it is not necessarily extant outside of the mind. Descartes would point to perfect triangles and other geometric shapes as a good example of this. However, the idea of God necessitates existence, because existence is part of the very essence of a supremely perfect God. To have the idea of a non-existent God would then be an utter contradiction.