Descartes' Ontological Argument

Descartes' Ontological Argument builds upon that put forth by Anselm, retaining the general structure and conclusion, but changing the definition of God and, existence as a perfection rather than as merely "greater than reality". He uses this argument to demonstrate that the existence of God cannot be disputed or that there is no reason to doubt his existence, rather than the more common arguments to "prove" God's existence.

Descartes begins his argument by defining God as the "supremely perfect being". This means that God contains all perfections, as Descartes believed that perfection meant "not lacking in anyway". This means that existence is a part of perfection as to not exist would be lacking, and therefore God possesses that quality. He concludes that God must exist if he is to fulfill the definition of the supremely perfect being, as he is only truly perfect if he exists. Anything less than existence is not perfection.

A more in-depth look at Descartes' reasoning provides a stronger argument. He believed that there are things that cannot be doubted, such as mathematics. Mathematics, once demonstrated, cannot be disputed as it just is. He uses the more specific example of a triangle to illustrate his point more fully. A triangle has three sides and its internal angles are the sum of 180 degrees. This is always true of a triangle, even if no one has ever had the idea of a triangle of experienced one. It is these qualities that make a triangle, a triangle. In the same way the nature of the triangle is immutable (i.e. it cannot change), the nature of God is also immutable. Descartes argued that existence was a predicate of God.

He uses another example of mountains and valleys. He argues that God cannot be thought of without also thinking of him existing, in the same way you cannot think of a mountain without a valley - it is an innate part of its definition.

In this way, Descartes is not "arguing God into existence", he is illustrating that existence is a quality of God (like the qualities of a triangle just are), whether you acknowledge it or not. He explains the problem of some people not believing in God through the argument that some knowledge is innate. Through this he means that everyone is imprinted with certain knowledge, that they have the potential to know.

Immanuel Kant argued against Descartes by saying that existence is not a predicate of God. Using the semantics of language he shows the flaw in Descartes' argument of believing that possessing existence equates to existing.