From what I understand, Descartes wrote refutations to criticisms made of his Meditations. I'm just interested in what he would say to Wittgenstein, who I think has the most compelling critique of radical doubt.

Wittgenstein wrote in On Certainty, "If you tried to doubt everything, you wouldn't get as far as doubting anything. The game of doubting itself presupposes certainty."

According to Descartes, nothing is indubitable - we have to doubt anything and everything until we can clearly and distinctly perceive it.

But is everything really subject to doubt? Let's follow this to it's logical conclusion. Were we to doubt everything, we would have to doubt:

  1. The meaning of our own words. Descartes would have to doubt the very meaning of each word in the sentence, "I think; therefore I am." He would have to doubt what the word doubt even means.

  2. He would have to doubt the "laws of logic" so to speak. He would have to doubt every rule of inference that he uses to make his arguments. He would have to doubt the laws of thought themselves (excluded middle, law of identity, non-contradiction).

So to me, it would seem that subjecting everything to doubt is simply nonsensical. You would not get as far as doubting anything indeed.

  • I'm not sure "I think; therefore I am." is something that has to be "thought" in words to be known. Commented Aug 11, 2014 at 4:51
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    For me, cogito ergo sum is a finger pointing at an evident truth. you may argue about the logic of the finger but the truth remains.
    – nir
    Commented Aug 11, 2014 at 11:34
  • I think you're misinterpreting Descartes here. Descartes most definitely believes some things are indubitable... "clear and distinct ideas". Descartes abandons the sort of project you describe in an aside in Meditation I.
    – virmaior
    Commented Aug 13, 2014 at 8:15
  • vimalor, do you have a quote? en.wikipedia.org/wiki/Cartesian_doubt "René Descartes, the originator of Cartesian doubt, put all beliefs, ideas, thoughts, and matter in doubt" poorly referenced true.
    – user6917
    Commented Aug 13, 2014 at 8:32
  • @user3293056 oddly wikipedia is not your best source on this sort of thing. Descartes does start with that project, but he abandons it while still in Meditation I. It's more boogeyman useful to others than the actual project he embarks on in Meditations II-VI.
    – virmaior
    Commented Aug 13, 2014 at 8:34

3 Answers 3


Descartes: The cogito works most reliably for an actually existing sufficiently rational subject. A madman could try radically doubt everything at the same time. In that case it can happen that the cogito fails to work, and the madman is unable to convince himself of his own existence. A rational subject only doubts a manageably small number of assumptions at a single instant in time. I explicitly mentioned the difference between rational doubt and madness, because my evil demon thought experiment comes quite close to actual madness.

Wittgenstein: Why do you say "works most reliably"? Do you want to imply that the cogito could fail for me, even so I'm sufficiently rational? Well, let's try it: "I doubt that your proposed doubt is actually possible, therefore I actually exist." Hmm..., somehow I'm still not really convinced that I exist.

Descartes: Strange, it worked for me. I doubted, therefore I existed. But you are right, I seem to be unable to convince me now that I actually existed. It probably doesn't work for past or future existence, just for present existence. Well, let's try it: "I doubt I actually existed, therefore I actually exist." Strange, it really doesn't work. Maybe I don't actually exist? Yes, of course, I died a long time ago, so maybe I no longer exist. Or maybe I still exist, but I'm not the subject that does the thinking and doubting here.

Wittgenstein: OK, you seem to admit that the cogito doesn't work unconditionally in all cases. But what about doubting the "laws of logic"? Won't this prevent you from drawing any conclusion at all?

Descartes: Of course I have to doubt the "laws of logic", and I explicitly did this. This is one reason for the evil demon thought experiment. The "laws of logic" are an idealization of a paper and pencil reasoning process anyway, and hence don't model rational human reasoning in time, monologue and dialogue accurately. I don't need modus ponens for my conclusion, all I need is to convince myself beyond doubt of my own existence.

Wittgenstein: But why should it be important for anybody but you, if you can convince yourself of your own existence beyond doubt?

Descartes: Please don't change the subject, just because I managed to defend cogito against your objections. Anyway, thanks for this interesting exchange.

  • I would not be so fast in thinking that some ONE can actually define madness. Cogito Ergo Sum is not an algorithm it is a LETTER. From Columbus that he reached a new continent. If someone can not reach clarity of his own existence -- it is his problem.
    – Asphir Dom
    Commented Sep 15, 2014 at 16:31
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    @AsphirDom Yes, it is a LETTER, but one with a description of how to find the new continent. Now this description might not be helpful for everybody, and there might be some people that will never be able to reach the new continent, not even with the best and most suitable description. (Regarding madness, Descartes had an enlightenment, but feared it might be only madness. It might be hard to distinguish between enlightenment and madness, precisely because some ONE can't actually define madness.) Commented Sep 16, 2014 at 10:36

My take on this is that Descarte is looking for an axiomatic basis to base his epistemology on. So his 'radical doubt' was simply a tool to help him where to look. He is not intending that radical doubt is actually feasible per se; its only possible in an imaginative or intellectual capacity.


There is no response. The two authors work from completely different assumptions about the flexibility of words. Wittgenstein would say that something akin to Cartesian certainty is nonsense. Even the "evil demon" thought experiment creates a context where another mind must exist, specifically the evil demon.

Wittgenstein looks at the requirements of language to have meaning. Could the word "doubt" have meaning outside of a language? It's not even certain that question makes sense. What does it mean to say you doubt baseball? It is a category mistake. Doubting other minds is similarly so. If language is something shared, then any language, including the word "doubt," requires someone to share with. Wittgenstein's use theory of meaning would cast similar aspersions on the Cartesian meditation, because Descartes makes the philosopher's mistake of trying to remove a word from context. No one uses "doubt" the way Descartes does.

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