Cartesian Skepticism

The main features of Cartesianism are:

(1) the use of methodical doubt as a tool for testing beliefs and reaching certainty.

– A Companion to Epistemology, p 57

It seems odd to me that keiths, who denies the possibility of certainty, is a champion of Cartesian skepticism.

A Cartesian skeptic will argue that no empirical proposition about anything other than one’s own mind and its contents is sufficiently warranted because there are always legitimate grounds for doubting it.

… A Cartesian requires certainty.

– A Companion to Epistemology, p 457

keiths is not a Cartesian Skeptic.

Cartesian scepticism, more impressed with Descartes’ argument for scepticism than his own reply, holds that we do not have any knowledge of any empirical proposition about anything beyond the contents of our own minds. The reason, roughly put, is that there is a legitimate doubt about all such propositions because there is no way to justifiably deny that our senses are being stimulated by some cause (an evil spirit, for example) which is radically different from the objects which we normally think affect our senses.

A Companion to Epistemology, p 457

keiths is not a Cartesian Skeptic.

Is it even possible to be Cartesian Skeptic?

169 thoughts on “Cartesian Skepticism

  1. keiths:

    KN concedes that there are possible worlds in which he is being fooled. What he fails to establish, and in fact cannot establish, is that he is not in one of those possible worlds.


    That’s true.

    You denied it earlier:

    The fact that there’s an implicit certeris paribus clause of infinitely disjunctive logical possibilities only entails that I would not be entitled to my knowledge claim in all possible worlds. It does not mean that I am not entitled to claim that in the actual world I am directly perceiving physical things.

    It’s good that you see your error now.

    It just doesn’t mean what you think it does.

    It doesn’t mean that we should adopt agnosticism about the external world (external to what?).

    External to our minds, or at least to the part of our minds doing the (potentially non-veridical) perceiving.

    That would be the case if knowledge required certainty. But we all agree that it doesn’t.

    It doesn’t require certainty, but it does require likelihood. I explained this to walto using a hypothetical dialogue:

    Exchange #1:

    Xavier: Do you believe X?

    Yolanda: I not only believe X, I know it.

    Xavier: How likely is it that X is true?

    Yolanda: Extremely unlikely.

    That exchange is nonsensical because the knowledge claim clashes with the likelihood assessment. It remains nonsensical if you change the last line to this:

    Yolanda: I have no idea whatsoever.


    The fact that I cannot know whether or not I’m a brain in a vat, plugged into the Matrix, or being tricked by a malign genie doesn’t tell me anything interesting or important with how I — or anyone — should improve our epistemic practices, or how we should inquire better (or worse).

    It tells you something profoundly and mind-blowingly important. It answers one of the fundamental questions of epistemology — whether we can know things about the external world — in the negative.

    It doesn’t tell us what might be of interest — say, how we can distinguish between science and ideology, how to explain what is wrong with confirmation bias, or the harms involved in epistemic injustice or epistemologies of ignorance.

    Cosmology doesn’t tell us those things, either, but still we pursue it. Why should it be any different with regard to fundamental epistemological questions?

    I don’t see how keiths’s Cartesian skepticism can accommodate the distinction between ‘normal’ persons who experience a shared world along with other cognitive agents and cognitive agents who have become unmoored from objective reality due to sensory deprivation and social deprivation.

    It’s easy. Recall the distinction I drew between two types of non-veridical perception:

    You need to distinguish between two types of non-veridical perception:

    1) Non-veridical perception due to shortcomings or malfunctions in the perceptual apparatus; and

    2) Non-veridical perception due to the non-veridicality of the sensory information arriving at the perceptual apparatus.

    An optimally functioning perceptual apparatus can still be fooled if the sensory information it’s operating upon is non-veridical.


    If epistemology is upended by skepticism, then so much the worse for that kind of epistemology.

    So much the better for that kind of epistemology, because it is an honest epistemology. Surely you can see the irony of pursuing an epistemology in which you lie to yourself about what you can and cannot know.

    Epistemology is still necessary for critically engaging the world in which we live, move, and have our being.

    Right, and an honest epistemology, in which we substitute “know*” for “know” when speaking of the external world — can do that quite nicely.

  2. keiths is almost certainly correct.

    And keiths certainly acts as if his senses are veridical.

  3. KN,

    Adding to the list of mistakes that you’ll want to avoid repeating as you reformulate your position on Cartesian skepticism:

    6. You committed the “argument from consequences” fallacy, claiming that Bad Things Would Happen if we accepted Cartesian skepticism. That’s false (see below) but also irrelevant to the validity of the skeptical position. Things are as they are, whether you consider that to be good news or bad.

    7. You warn of Dire Consequences, but you never seem to be able to identify any that hold up to scrutiny. Most recently you wrote:

    It looks like a retreat from everything that actually matters in human life for the sake of mere logical possibility.

    I replied:

    I don’t see how it’s a retreat from anything that matters in human life. What specifically are you thinking of?

    Your argument from consequences is already a fallacy, and now it appears that the consequences don’t even exist. The bogeyman isn’t real, KN.

    8. You keep forgetting that Cartesian skepticism is an epistemological position, which means that it deals with knowledge. Shifting the topic to justification, and then arguing for a watered-down version of same, won’t work.

    The Sentinel Islander scenario exposes the problem. Your logic, when employed by the islander, leads to a false claim of knowledge. You can argue all day for a vitiated version of justification, but that won’t magically transform the islander’s false claim into a true one.

  4. KN,

    A few more for the list:

    9. You made this claim:

    I’ve given reasons for why the [simulation] scenario lacks justification, and that’s all I need in order to say that, as far as I can tell, I know I’m not in a simulation.

    As I noted at the time, that’s bad reasoning. A lack of evidence for belief X does not automatically constitute evidence for belief not-X. I offered the following counterexample:

    Let X be the claim that there are an odd number of dollars in Dinesh D’Souza’s checking account right now (ignoring any cents). I lack justification for believing X, but that hardly entitles me to claim that I know that Dinesh D’Souza doesn’t have an odd number of dollars in his account. I simply don’t know either way.

    10. You described skepticism as a “dilemma” that we needed to be “saved” from, but were never able to justify either characterization.

    We don’t need to be saved from skepticism. It works just fine as an epistemological position.

    11. At one point you agreed with a statement of Cartesian skepticism:

    Now the position looks like, “we can know things about the real world, unless {A, B, C, D . . . }, where none of the items in that set can be ruled out on a priori grounds”

    I can agree with that, of course. I just don’t see the point of insisting on it.

    [emphasis added]

    One day later, you were writing:

    I think that Cartesian skepticism rests upon a grave and profound error…

    You see why I keep urging you to slow down and think things through?

  5. Perhaps there should be an award for…

    Persistence in the face of indifference?

  6. Ah! Keiths is here.

    As new resident philosopher, perhaps you could consider a conundrum.

    A member posts a comment: “tomorrow I will do X”. But tomorrow passes and she does not do X. Does that make the member dishonest and render the statement a lie?

  7. No. I’d say she’s wrong but not dishonest, assuming that she believed it when she wrote it.

    Are you referring to this?


    You have a rather extreme “mote and beam” problem.

    I’ll address it later today in Noyau and provide a link to it from this thread.

    If so, you’re stupider than I thought. This won’t turn out any better for you than it did the last time, and you’re foolish to ask for more humiliation.

  8. keiths: No. I’d say she’s wrong but not dishonest, assuming that she believed it when she wrote it.

    Well, sure, the statement is not a lie until tomorrow passes and there is no follow up of either *does X* or “I changed my mind”. Even if someone forgot they promised to “do X tomorrow” they have the option of saying “sorry, I forgot. I’ll do it now” when prompted.

    Perhaps one should just “do X tomorrow” rather than make promises.

  9. Alan Fox: How do you know this?

    I infer it from his confidant statements about what others have written and what others believe.

    He has no business claiming that KN has contradicted himself if he cannot trust what he is seeing in the words that he thinks KN wrote. Why is he accusing KN unless he trusts his senses?

    Why is he trusting his senses if they are not worthy of trust? Why are they worthy of trust if they are not truthful?

    His claimed skepticism is nothing of the sort.

    He either trusts his senses or he lives a life of utter and complete faith in that which he cannot possibly know. How ironic is that?

  10. Alan,

    Your masochism is not my top priority right now. You’re just going to have to wait.

    Now go play with your cousin Mung.

  11. keiths:

    Your masochism is not my top priority right now.You’re just going to have to wait.

    Now go play with your cousin Mung.

    Please take all the time you need. Or change your mind. Just be honest with yourself. I’m glad you have more important things to do. You should indeed get out more. Smell the roses.

  12. Cartesian skepticism

    Any of a class of skeptical views against empirical knowledge based on the claim that claims to empirical knowledge are defeated by the possibility that we might be deceived insofar as we might be, for example, dreaming, hallucinating, deceived by demons, or brains in vats.

    Dictionary of Philosophy of Mind

    Compare to what keiths describes as Cartesian skepticism.

  13. Further, in answer to irrelevant objections by keiths:

    The gist of Cartesian-style skeptical arguments is that some empirical proposition (e.g. that there are trees) cannot be known because we might be deceived (e.g. we might be brains in vats hallucinating that there are trees). Related forms of these arguments attack our justification for believing some empirical proposition on grounds of possible deception. These ‘justification’ versions undermine claims to knowledge insofar as justification is a necessary condition on knowledge. The arguments I examine below are all of the ‘knowledge’ variety, but they can easily be transformed into arguments of the ‘justification’ variety by simply replacing all occurrences of ‘knowledge’ with ‘justification’.

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