Analyzing Descartes’ ontological argument for the existence of God

Rachel asked:

Explain in detail any one of Descartes’ three arguments for god’s existence in the meditation. Is the argument valid? is it sound? Analyze and explain in detail.

Answer by Tony Fahey

Hi Rachel, as far as I know, and of course I may be wrong, there are only two arguments for God’s existence in Descartes’ Meditations. The first appears the third mediation, and the other in the fifth. However, since you are only interested in one let us look at the argument of the fifth meditation which is known as the ontological argument. This is an a priori argument for the existence of God. That is that evidence is not based on empirical evidence, but on analysis of the concept of God.

According to Descartes each of us possesses the idea of a perfect entity. Inherent in that idea is the fact that a perfect entity must exist – because, as Anselm had said, a perfect entity can only be perfect if it has existence. Neither could we conceive of a perfect entity if there was no such thing. We are imperfect, said Descartes, so the idea of perfection cannot come from us. Descartes reasons that the idea of a perfect being must have been placed in him by a really existing perfect being – God. That God exists was therefore as self-evident to Descartes as that a thinking being must exist.

Descartes believed that the idea of God was innate; it was something we are born with. The more self-evident a thing is to one’s reason, the more certain it is that it exists. From this he concluded that he was a thinking being and that there exists a perfect entity, God. With this as his departure point, Descartes lays out his theory of dualism. With regard to all the ideas we have concerning outer reality, there is possibility that we are deceived. We think we have a body but we may be dreaming; we cannot be certain that we have a body. Descartes believed his body and the non-conscious natural world was non-essential, that is, contingent. It is important to realise that Descartes is not saying that the material world does not exist, but that its existence is radically unlike that of the mind. His body is not part of his essence, therefore, if his body ceased to exist, his mind would not cease to be all that it is. In other words, Descartes would continue to be Descartes even if he had no body.

There are two kinds of reality – two substances – says Descartes. One is thought, or mind, the other is extension, or matter. The mind is purely conscious and occupies no room in space and therefore cannot be subdivided into smaller and smaller parts. Matter, on the other hand has no consciousness. Descartes maintained that both mind and matter originate from God, because only God exists independently of anything else. Although both substances come from God, they are independent of each other. Thought is independent of matter and conversely, the material processes are independent of thought.

One of the main problems with Descartes’ ontological arguments is one that was identified by the monk Guanilo when the same argument was advanced by his contemporary Anselm circa 1093. According to Gaunilo, the ontological argument would imply that anything, no matter how fictitious or chimerical, which was thought in the mind, would have to exist in reality. To prove his point he uses the example of an ‘island more blessed than any other, a perfect island… greater than which nothing greater can be conceived’. Given Anselm’s ‘proof’, he argues, if one can conceive of such an island in this way, then it follows that such an island must exist in reality as well as in the mind – this, of course, is absurd.


Answer by Geoffrey Klempner

There is a way of seeing the ontological argument as being valid but not sound. A being like God, being perfect in every way, cannot contingently exist. That seems reasonable. A perfect island can never be THAT perfect. We can imagine some universe where there is a perfect island and another universe where there isn’t a perfect island. But IF we can conceive of God as God, that is to say, having the attributes of perfection attributed to Him by Anselm and Descartes, then God, IF he exists in any possible world must exist in all possible worlds.

That’s a valid argument for the existence of God, from the assumption that the conception of God is not incoherent or self-contradictory. But that’s an assumption which a critic of the ontological argument would question.


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