Whilst researching philosophical skepticism, I found this answer to the question here which states the following:

[Jon Erison] Extreme skepticism is in fact self-defeating. According the the Wikipedia article linked in the question, "philosophical skepticism is an approach that denies the possibility of knowledge". Therefore philosophical skepticism can not claim even that philosophical skepticism is correct. Therefore a critic of philosophical skepticism can not be compelled to accept any of its claims.

Would my reasoning below be correct, in assessing the validity of the claim "Extreme skepticism is self-defeating"?

Assuming the philosophical skeptic claimant makes the claim:

Knowledge is not possible

(and thus the claimant "denies the possibility of knowledge").

The Claimant (a) believes in this statement; and (b) believes it is true. What is not certain at the moment, is whether the claim is justified, which would determine if the claimant "knew" the claim.

To illustrate why the claimant may not consider it justified there is an example provide by SEOP illustrating the non-consensual nature of belief:

for example, when Kai reads that astronomers no longer classify Pluto as a planet, he acquires a new belief (in this case, the belief that astronomers no longer classify Pluto as a planet).

Then If the claimant takes this further (Let P = " that astronomers no longer classify Pluto as a planet") :

Kai believes that the author(s) of the book state P.

Kai believes that he has read that the author(s) of the book state P.

Kai believes that he has perceived that he has read that the author(s) of the book state P

Kai believes that he has perceived that he has perceived ...

And from hereon, it's just perceptions of perceptions all the way down.

(Just as a side note some definitions, such as that provided by dictionary.com, define perceive as:

to become aware of, know, or identify by means of the senses:

so just to clarify I'm only referring to "aware of" not "know" otherwise that would completely defeat the purpose of this question )

From this,(Assuming Kai stopped the process) Kai may either have "faith" ( faith, as in believing a perceived unjustified proposition) in a proposition or claim that:

Any belief is unjustified

(There's also the possibility that he doesn't claim anything afterward, in which case he wouldn't be deemed a skeptic?.)

So returning to the original claim, the claimant now justifies the claim

Knowledge is not possible


Any belief is unjustified

which then makes the former claim a True Justified Belief, and thus knowledge?

Would the fact that the claim used to justify "Knowledge is not possible" is unjustified, make "Knowledge is not possible" unjustified?

  • 1
    It is more accurate to say that careless radical skepticism, which hastens to assert its denials, is self-defeating. But even radical philosophical skeptics aren't careless, they only suspend judgment on all claims, including their own. Self-defeat is more of a pitfall for philosophical novices.
    – Conifold
    Commented Oct 10, 2020 at 6:36
  • @Conifold So it is conceivable that a radical philosophical skeptic could state: "Knowledge is not possible" and their position still deemed logically consistent? Would they have to remain silent forever after in order to remain consistent? And What sort of statements would qualify a careless radical skeptic as self-defeating?
    – TomDot Com
    Commented Oct 10, 2020 at 14:00
  • Why stay silent? Language provides plenty of means to avoid claiming anything said. What Pyrrhonians say is "the only justified attitude with respect to any claim is suspension of judgment", this claim included. And make arguments both for and against any claim made by others, where they are free to assume anything others assume, or its opposite.
    – Conifold
    Commented Oct 10, 2020 at 14:12
  • @Conifold What definition of Judgment does a Pyrrhonic Skeptic employ? From your previous comment, I'd assume that "Blue Cows eat grass" would not qualify as a judgment but a proposition, so what would they quailfy as a judgment? From this article, It seems that any proposition with a copula 'is' (equality signifier) would be a judgment, so would suspending judgment involve never stating any relation between two things, or any proposition with a copula in it? (Except possibley "i read Grass is green" or propositions like it)
    – TomDot Com
    Commented Oct 11, 2020 at 1:19
  • - where the relation has been reported as being perceived, rather than stating it blankly)
    – TomDot Com
    Commented Oct 11, 2020 at 1:51

4 Answers 4


One may argue that the error consists in taking the assumption of the possibility of knowledge that knowledge is not possible. The skeptic may argue that that which implies a contradiction is simply false (basic propositional logic), so the assumption should be false: there is no possibility of knowledge that knowledge is not possible. This reaffirms a specific instance of skepticism.

One may argue that the contradiction should trace back to the claim of skepticism, but this needn't be the case.

Edit: this answer is inspired by Jonathan Dancy's approach in An introduction to contemporary epistemology.


I would not say that skepticism is self-defeating - after all, while the skeptic does not buy any knowledge-claim, he does not himself need to endorse that knowledge is indeed impossible. By the same token, of course, there is no cure against skepticism. But why care about the skeptic at all? He is but the fool licensed to mock and revile, not to offer conclusions himself.

  • Along with the self evident truth in the assertion, 'philosophical skepticism is self-defeating', IMHO you might add that, for human beings, 'pessimism is not an option',
    – user37981
    Commented Oct 9, 2020 at 20:03
  • It may be self-defeating to claim that 'knowledge is impossible', but the skeptic need not do so; indeed, no serious skeptic would.
    – Turtur
    Commented Oct 9, 2020 at 20:19

No, Wikipedia's definition of philosophical scepticism as "an approach that denies the possibility of knowledge" is obviously self-contradictory and therefore probably false, unless you want to believe that all Sceptic philosophers are terminally illogical, which can easily be doubted.

To understand anything, you have to start from ordinary dictionary definitions, to try and understand what the words involved may mean.

Scepticism 3. the doctrines or opinions of philosophical Skeptics.

So, what is a Sceptic?

Sceptic (Philosophy) a member of one of the ancient Greek schools of philosophy, esp that of Pyrrho, who believed that real knowledge of things is impossible

The notion of "thing" at the time of the Ancient Greeks probably meant things in the so-called "real world", i.e., essentially the physical world, as opposed to things in the mental world.

Descartes is one of the major historical figures of extreme scepticism, but his scepticism concerned only our knowledge of the physical world. He famously made the distinction between his own body, that he could reasonably doubt that it existed, and his own mind, that he could not logically deny that it existed.

So, it all depends on what people mean by "extreme". The most extreme scepticism is the denial that knowledge is possible, but then this implies that you cannot know that knowledge is impossible, so either you have to suspect that some philosophers are illogical, or perhaps that they don't really mean the most extreme of scepticism.

Socrates is probably the only philosopher to have been reported as saying that the only thing he knew was that he knew nothing, and rational people understand that this should not be interpreted literally.

Descartes' scepticism only says that we can doubt that we know the material world. That is not exactly the most extreme of scepticism. Rather, it is very reasonable, and indeed I don't know myself of any good counter-argument to it.

Beliefs, of course, are not knowledge. However, we can justify our beliefs. For example, I can justify my belief that there is a tree in my garden by the empirical fact that I see a tree in what looks like my garden. This is all I need to justify my beliefs and there is no other sort of justification available to us concerning things like trees and subatomic particles.

In judicial courts, the judge often has to rely on a so-called "eyewitness". Even whenever material evidence is available, the judge still has to rely on his or her own visual perception. I don't know of any other way.

A justified belief, even a justified belief which happens to be true, is still a belief, notwithstanding the truly unbelievable theory that knowledge is justified true belief. I may have a true belief which I don't know is true, and hence is not knowledge, yet the theory says it is knowledge.

There is also no infinite regress as far as perception is concerned. You perceived or you don't. When you perceive, you know that you perceive, even when the perception is misleading. I may be wrong about the fact that there is a tree in my garden, not about the fact that I perceive a tree in my garden.

The theory that knowledge is justified true belief is terminally circular. To justify any belief about the physical world, you would need to assume that some more fundamental beliefs about the material world are true. This is what science does.

Scientific theories about the material world are justified by assuming that observations of the material world are true, which would require more justifications of the observations, and you thereby go into infinite regress. Alternatively, you just accept that science is a belief, not knowledge, but a really well justified belief, and it certainly is.

Further, it seems that most scientists have stopped believing that science is true, to fall back instead on the more secure idea that science is good enough to predict some future events we are interested in.

  • Why is it illogical to belief that "Knowledge is not possible", and not claim that they do not "know" said proposition? Wouldn't this be internally consistent, because they have only stated belief in the proposition, not knowledge?
    – TomDot Com
    Commented Oct 11, 2020 at 1:40
  • In your penultimate paragraph you recognise that "justifications of the observations ... go into infinite regress" and also claim that we can "justify our beliefs" ealier. Does this mean that it is valid to justify a belief, when said justifications are eventually unjustified in this "infinite regress"? So essentially, Can a justified Justification be formed from justifying it with an unjustified justification?
    – TomDot Com
    Commented Oct 11, 2020 at 1:47
  • @TomDotCom 1. I don't understand your first comment. 2. In the standard notion of justification, we justify a belief by explaining why we believe what we believe. This is the justification of beliefs in terms of other beliefs. In the JTB theory of knowledge, either we would have to justify knowledge from beliefs, which is obviously faulty, or we have to justify knowledge of p in terms of our knowledge of some p2, and then p2 in terms of some p3 etc. ad infinitum, which is also faulty. There might be a way out but I am not aware that there is any. Commented Oct 11, 2020 at 10:50
  • (1) Sorry that I wasn't clear enough, Suppose someone claimed that "Knowledge is not possible". They necessarily believe this proposition, but it is not known whether they know it or not. Seeing as in (2) it might not be possible to actually justify anything, then currently it can't be said that they know said proposition. Why would this make them illogical? If they claimed to know that "Knowledge is not possible" I'd agree that they are illogical, but as far as a I'm aware they are not claiming this.
    – TomDot Com
    Commented Oct 11, 2020 at 11:43
  • @TomDotCom I didn't say anyone claimed knowledge is impossible and I didn't claim that someone who would would be illogical. Commented Oct 11, 2020 at 13:08

The subset of skeptics who pretend to suspend all judgments is not self-contradictory or self-defeating, but let us say that the situation is one of a logic game in which this kind of skeptic plays a question each round. So, you claim to know something, and then they ask, "How do you know that?" You offer a reason, they ask how you know that, and so on.

As long as you offer statements/judgments/etc., the game will go on and in that sense the skeptic wins (or at least never loses). However, you can quickly break the cycle by playing a question yourself: ask the skeptic, "What do you mean by, 'How do you know'?" Now this skeptic has to play an assertion. Voila, either they claim a meaning for their use of the word "know," and then you win---or they claim not to mean anything by that word, and they then forfeit the game.

  • So in the case that you win (Skeptic claims definition of word "know") is this because the skeptic has judged the word "know"? That is for example they state: "Know is ...", which contains a copula, and thus qualifies as a judgment?
    – TomDot Com
    Commented Oct 11, 2020 at 1:28
  • 1
    Also, if in this hypothetical where you ask the Skeptic "How do you know?", what if they responded with "Could you provide me a definition of 'How do you know' to use?", rather than claim one themself. Could they escape then, making a judgment, as they could now respond (assuming you did give them a definition) "Using that definition of 'How do you know', I thus do not know"?
    – TomDot Com
    Commented Oct 11, 2020 at 1:35

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