This week I've been given to study from my highschool teacher Descartes' Demon argument but I have several doubts I fully understand it ,but let me put this in clear order :

1) I understand that this argument is about whether we can have any knowledge at all about external world which includes existence itself .

2) The argument says that there exists an evil demon who is capable of deceiving us in the same way we suppose God to be able.

3) The argument is solved by the fact that if I am deceived it follows that I exist otherwise I can't be deceived.

Now my question concerns the third point:

How do I know that this evil demon is not feeding me the thought that "if I am deceived it follows that I exist " ?

My evil demon is so smart that he knows that giving me this thought I would think that I exist because I would have been able to think that.

But again how do I know that the demon is not again fooling me by feeding me the above thought ? I would think that I outsmarted him but he would be the smarter because he knows this and that's why he does this.

This can keep going for ever so it seems to me that I can never be sure of what I am thinking .

So the question breaks down to this :how do I know I am not still playing devil's game when I say that I must exist in order for this demon to deceive me ?

  • What you're referring to is a kind of radical skepticism. Asking proof of something doesn't always make sense. Sometimes you hit rock bottom, the point at which you believe certain things without a need for proof, or act without any kind of justification. Let's say you came up with an argument supporting you in this case - you could then ask for justification for that (like "how do you know that argument isn't a false one implanted by the devil"). Explanations run out eventually.
    – Franz
    Commented Jul 11, 2017 at 21:36
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    But if this evil demon feeds you that thought wouldn't you have to exist to be fed it? The argument is disjunctive: if I am deceived, even about the very nature of deception, I have to exist, and if I am not deceived I have to exist anyway.
    – Conifold
    Commented Jul 11, 2017 at 23:33
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    @Conifold, you of all people, should know better :-( . See my comment to the accepted answer. Commented Jul 13, 2017 at 16:57
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    @AlexanderSKing I thought about pointing out the flaws of cogito, but it seemed out of place here since OP was having trouble with how it is even supposed to work. Charity before criticism :)
    – Conifold
    Commented Jul 14, 2017 at 18:12
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    @Conifold this is a philosophy forum: criticism is charity. Commented Jul 14, 2017 at 18:21

4 Answers 4


Many would are argue that you are right, the demon is still successful in his deception.

DesCartes claims in the cogito that he has proven the existence of an "I", since for there to be deception, there has to be thinking, and for there to be thinking there has to be an "I" that does the thinking. Hence "I think, therefore I am".

Several philosophers, notably many empiricists like Locke and Hume, pointed to a major flaw in DesCartes argument, which is that all that DesCartes proves is that "thinking occurs", not that "I am thinking". Later on, Russell in his essay "Analysis of Mind", makes the analogy with rain: when we say "it rains", there's no need for a "rainer" to do the raining. Similarly, when thinking occurs, there's no need for there to be a thinker doing the thinking, the thoughts just occur.

From David Hume's perspective: When we try to observe an "I" that is doing the thinking, we can't find anything. All we observe is the thoughts and emotions themselves - what we think of as the "I" is just the collection of thoughts and impressions that we have. This is known as the Bundle Theory of Self. See this post for a more detailed explanation.

To recast the argument in Cartesian demonic terms: The demon is fooling you (and this "you" is just an illusion) by making you think that an "I" is necessary to do the thinking, when in fact thoughts can occur perfectly well on their own.

One can try to save DesCartes argument by replacing thinking with observation. Thinking might not require a thinker, but observation by definition requires an observer. If we prove that observation is occurring, then there must be an observer: "I observe therefore I am".

Thinking is not so much of a challenge, after all computers think all the time, but conscious, first-person perspective is. This is the basis for philosophical Zombie arguments.

The bundle theorist can still refute this version of the argument, but they have to do a lot more work do to so. How do we explain first-person perspective and consciousness, if all there is, is thoughts, no selves?

Here, the bundle theorist has to resort to Higher-Order and Self-Representational Theories of Consciousness: Consciousness and first-person perspective are just thoughts about other thoughts or thoughts that are about themselves. See this post for details.

  • thanks for the answer,definetly what I was looking for. ;)
    – Jean Leroi
    Commented Jul 14, 2017 at 11:42
  • Oh, well done. This was a delightfully well-formed question and equally well-formed answer. It's nice to have this common concern dealt with so cleanly.
    – Ryder
    Commented Jul 15, 2017 at 20:34
  • I'm curious: why do you use the form "DesCartes"? I've not seen it before, and I could see it being confusing for new students or casual SE readers.
    – Ryder
    Commented Jul 15, 2017 at 20:36
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    I don't buy the "It rains" argument. In that sentence the word "it" is an expletive, while in "I think", the word "I" is necessary. If one instead said the equivalent "The cloud is raining", the argument would no longer make sense. Commented Sep 30, 2019 at 12:56
  • You don't need to go that far to get rid of the self, if you pay attention to the physical data. Dennett would disagree that observing requires an observer on simpler grounds. We know the nervous system to be a parallel architecture. If you need an 'I" to be observing, then are there as many of them as there are sensory pathways? You can be 'observing' in lots of ways at once. The only thing that unifies them is conscious recall.
    – user9166
    Commented Sep 30, 2019 at 23:13

How do I know that this evil demon is not feeding me the thought that "if I am deceived it follows that I exist " ?

Interesting thought that the evil demon could be feeding us bad logic. But feeding us that particular piece of logic seems like a paradox - very similar to the liar paradox (e.g. "I am lying right now").

Descartes' point is regardless of how the demon is affecting our mental capacity, feeding us good logic or bad logic, there must still be a recipient of the logic (namely us), and for Descartes there is no getting around that. There must be something for the demon to "aim" at - a subject/object relation. Otherwise it's just the demon casting the good/bad logic to the void, breaking down the thought experiment.

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    According to many empiricists, Descartes logic as you explained it is faulty. All he proved is that "thinking happens" - but there is no reason for there to be a thinker who does the thinking, the thoughts themselves do the thinking - just as when it rains, there doesn't have to be a "rainer" doing the raining, or when the wind blows, there doesn't have to be a "winder". Lookup Hume, Locke's, William James, and other's responses to Descartes cogito. Commented Jul 13, 2017 at 16:56

How do I know that this evil demon is not feeding me the thought that "if I am deceived it follows that I exist " ?

But if you understand it the origin doesn't matter, ad hominem (genetic fallacy) in the informal logical sense. The origin or genesis of the sentence, or proposition, may make us suspicious, may make us bring greater scrutiny to it, but it can not rule it out simpliciter as something that may speak the truth.

The truth (on the most-widely accepted view, importantly, this is Descartes' view of truth) hangs on whether the thing said matches what is the case. That the thing is integral with the assertion in the sentence. For example, this is a sentence. You are now reading a sentence. Such must be true, whether or not the sentence got here due to the pernicious will of a treacherous being.


Do you mean that you doubt what the sentence says is true? One could say that one takes it to be true, by devil's trick, but it is not true. That, I believe, is somehow unanswerable. Though, Nietzsche said that it makes no sense to assert that one doesn't exist. One says nothing at all. Since, "not existing", is put forward propositionally as a kind of existing.

One might also possibly say, it seems to me, that one finds the whole issue unintelligible, that existence doesn't mean anything. Or, that the proposition can't say what it means to say.


Another way to solve this conundrum that I believe is more complete, but less satisfying, is:

"I think, therefore thoughts exist."

So if the demon feeds you bad logic, he is feeding you thoughts, and that doesn't change the fact that thoughts exist.

Basically we are avoiding the problem that comes with trying to define "I", which is implicit in your question.

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