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Why Descartes Can't Get Out of His Own Head but the Thomist Can

  • Writer: Nathan Liddell
    Nathan Liddell
  • Jun 28
  • 2 min read

According to Descartes, it is by means of the clear and distinct perception of an infinite, perfect being (God) that he can reason his way to the veridicality of his sense impressions. The existence and perfection of God work together in his reasoning to justify his belief in the external world. Since he is caused to experience things through his senses and since God is the primary cause of those experiences, those experiences must be true because a perfect being would not deceive him.


But, for God to have this grounding effect for knowledge, God must exist. Here Descartes reasons to the existence of God from thought alone (an ontological argument). In His fifth meditation, Descartes wrote the following:1


But if the mere fact that I can produce from my thought the idea of something entails that everything which I clearly and distinctly perceive to belong to that thing really does belong to it, is not this a possible basis for another argument to prove the existence of God? Certainly, the idea of God, or a supremely perfect being, is one that I find within me just as surely as the idea of any shape or number. And my understanding that it belongs to his nature that he always exists is no less clear and distinct than is the case when I prove of any shape or number that some property belongs to its nature (AT 7:65; CSM 2:45).

Descartes adds that he cannot conceive of God not existing. Together, these thoughts produce the following argument: 2


1. Whatever I clearly and distinctly perceive to be contained in the idea of something is true of that thing.
2. I clearly and distinctly perceive that necessary existence is contained in the idea of God.
3. Therefore, God exists.

Once he has established the existence of God via his ontological argument, he then adds the following premise:


4. Since God exists and is the primary cause of my sense experiences, my sense impressions are true.

Kant’s response, existence is not a predicate, is sufficient to show that this argument fails.3 It does not add anything to the content of the idea (or our understanding of the essence of a perfect being) to say that it exists. Rather, once we have formed an idea of the perfect being (that is we have established what its essence is, including necessary existence) then we must ask, does such a being actually exist (does it have actual existence)? (see Aquinas' De Ente et Essentia) We must discover the answer to this question via our sense impressions of the external world. Because God’s existence is an a posteriori fact, Descartets’ a priori argument simply cannot get out of the blocks. It cannot work. (Neither can any other ontological argument, for that matter.)


(For a Thomistic solution to Descartes' trap, see "Gilson's Thomistic Response to Skepticism" here.)



1. Andrew Pessin, "Descartes' Ontological Argument," Stanford Encyclopedia of Philosophy (Spring 2024 Edition), ed. Edward N. Zalta, URL = <https://plato.stanford.edu/entries/descartes-ontological/#SimpArgu>.


2. Ibid.


3. Ibid.

 
 
 

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