Descartes' Ontological Argument The essential problem with Anselm's argument is that at the end of it all, the atheist can understand the definition and even have the concept in his or her mind, but still ask 'Is the concept realised? This was the objection of St. Thomas Aquinas. Name: St. Thomas Aquinas Occupation: Roman Catholic theologian Date: 1224/27-1274 Book: Summa Theologica (Summa Theologiae) Aquinas said that even if one understands the concept, it does not follow that what one understands exists actually rather than only mentally. Aquinas believed that Anselm's definition of God is not self-evident because we do not know the essence of God. As a result, Aquinas believed that we have to look at what is known to us, namely the effects of God. Thus Aquinas rejected the ontological argument in favour of arguments based on evidence of God in the world, e.g. the design and cosmological arguments. Question: What are we talking about when we say the 'essence' of God? Do you think Aquinas has a point? Descartes After Aquinas' dismissal the ontological argument remained out of fashion in philosophical circles for quite some time. It was revived by Rene Descartes. Name: Rene Descartes Occupation: French Philosopher & Mathematician Dates: 1596-1650 Book: Meditations on First Philosophy (commonly known as The Meditations) Descartes claimed not to have seen Anselm's argument.
Descartes describes God as a Supremely perfect being. The approach Descartes uses is as follows. Activity: Draw a triangle in pencil. Rub out one side. Is it still a triangle? Descartes contention is that one cannot separate having three sides from the concept of a triangle, just as one cannot have a mountain without having a valley at the bottom. When it comes to the question of God, Descartes finds that: "it is quite evident that existence can no more be separated from the essence of God than the fact that its three angles equal two right angles can be separated from the essence of a triangle" (Meditation 5) So Descartes is saying that existence is just as crucial to the concept of God as having three sides is to a triangle - it cannot be thought of without it. Descartes acknowledges that just because he thinks of a mountain as existing it does not mean that the mountain does exist somewhere. However, he says that in the case of God it is not his thought that makes God exist, but the necessity of God makes God exist.
Summary of the argument in Meditation 5 of Meditations on First Philosophy: Some things that exist in my mind have a determinate essence that is independent of my mind and cannot be separated from the thing. (e.g. triangles having three sides) Existence cannot be separated from the essence of God (any more than a valley can be separated from the essence of a mountain) [Therefore God exists] But So far I have not demonstrated that because I think of God existing this means that God exists (e.g. I can think of a winged horse, but horses do not have wings) I cannot think of a mountain without a valley. This does not mean that they exist, merely that a mountain and valley, whether they exist or not, are inseparable I cannot think of God except as existing, therefore existence is inseparable from God Therefore, God exists. To assert that God does not exist is then shown to be false by reductio ad absurdum since it means: The existing thing does not exist Descartes showing a bit of leg told you philosophy was sexy!
Objections from Descartes' Critics & Replies from Descartes Objection from Caterus (A priest and contemporary of Descartes) Objection: The argument is the same as Aquinas' statement of it. Therefore, it is open to the same objection Aquinas put forward, namely that existence is inseparable from the concept of God, but we cannot infer from this that the concept and its tied existence are actual. Descartes: Agrees with the above objection to Aquinas' formulation, but maintains that his version is not the same as Aquinas'. Descartes says that he clearly and distinctly perceives God's essence to include necessary existence, and what is clearly and distinctly perceived is true. Objection from a group of 17 th century philosophers & theologians Objection: Descartes says that after investigation of God's essence he clearly and distinctly perceives existence to be a part of it. However, how can anyone clearly and distinctly perceive what is infinite? Indeed, for many people the concept of 'God' involves contradictions. Therefore, Descartes cannot clearly and distinctly perceive God's essence. Descartes: Contradictions exist in the mind - it is when the mind cannot reconcile two ideas. The contradiction, therefore, is not in God, who is external to the mind it is in us, therefore it is no objection to God. Admittedly we cannot clearly and distinctly perceive God, but what we can clearly and distinctly perceive is existence as part of God's essence.
Gassendi's objection (an eminent contemporary of Descartes) Objection: The comparison of God's essence with a triangle's essence is invalid. Descartes does not compare essence with essence, but compares what he claims is the essence of God with a property of a triangle. Therefore, he should have compared, say, omnipotence (a property of God) with having three sides (a property of a triangle). If Descartes is to compare existence as God's essence, it should be with existence as a triangle's essence, which, of course, now no longer proves that God exists. Objection: Why is existence a perfection of God and not of a triangle? In fact, existence is not a perfection at all. Rather, it is the case that without it, there is no perfection. This must be so because if something does not exist, then it is not perfect, but neither is it imperfect (which it would be if existence was a perfection) - it is simply nothing. Descartes: A property is an attribute - anything that can be predicated of a thing. Therefore, existence is a property and it belongs to God's essence and only God's essence. Now summarise the arguments from Descartes and the objections to him: Arguments FOR (from Descartes) Arguments AGAINST (from objectors)