Descartes' Ontological Argument

Descartes' argument follows a similar format to Anselm's, and begins by defining God as 'The supremely perfect being' (or SPB). He goes on to analyse the concept of a SPB and states that this being would contain every supreme perfection, for example omnibenevolence, omnipotence, omniscience and so on. For Descartes, existence is also a perfection so it follows that the SPB must exist. Therefore, he concludes that in order for God to fulfill his role as the SPB he exists.

In addition, not only does Descartes argue that God exists, but that he exists necessarily. To strengthen this point he asks us to consider a triangle, because by definition it is impossible to conceive of a triangle with three interior angles that do not add up to the sum of two right angles. Further, it is impossible to conceive of a mountain without a valley because it is innate to the definition of a mountain. However this does not necessarily mean that a mountain or valley exist or not, but that the two are inseparable from each other as they are crucial to the essence of a mountain or valley.

Although it is impossible to conceive of a mountain without a valley that follows, that does not mean any necessarily exists.

This is the distinction that Descartes makes with God, that God must exist because existence is inseparable from his essence as a mountain is from a valley.

"therefore that he really exists: not that this is brought about by my thought, or that it imposes any necessity on things, but, on the contrary, the necessity which lies in the thing itself, that is, the necessity of the existence of God, determines me to think in this way: for it is not in my power to conceive a God without existence"
- Meditation V

Due to the definition of God as SPB, it is impossible to think of him without necessarily concluding that he must contain the perfection of existence.
Therefore, Descartes concludes a priori that God exists necessarily.