Descartes' Ontological Argument

Descartes writes his argument from a sceptical stance, and it is similar to Anselm’s in that it begins with a definition of God, in this case as ‘the supremely perfect being’ (SPB). He goes on to describe the SPB as having all the perfections. He continues to say that existence is perfection, so God must therefore exist in order to fulfil his definition as the SPB. Descartes ends his argument by saying that God exists necessarily, or in other words, he possesses the perfect quality of existence.

Descartes uses the example of a triangle to illustrate his point. A triangle will always have three sides and its angles will always equal 180 degrees. Whether or not we have ever experienced a triangle, these facts will always be true of it. In the same way that a triangle and three sides are interlinked, God and existence also are linked as the nature of these two is immutable.

By describing existence as a predicate of the SPB, Descartes encounters the issue of the differentiation between possessing the quality of existence and actually existing in reality.