Agrippa's 5 tropes

  1. Dissent: For every thesis an anithesis
  2. Relation: Point of view matters.
  3. Infinite Regress: Justifications need justifications need justifications ... ad infinitum
  4. Axioms: Assume something to be true sans justification.
  5. Circularity: The conclusion appears in the premises.

3, 4, 5 (above), together, constitute the well-known Agrippa's trilemma.

One seemingly strong argument against skepticism has been to point out that it's a peritrope (self-refuting). However, for it to be so, it must be a sound argument and if that's the case the argument that skepticism is a peritrope is itself a peritrope and this further argument I just made is also one ... ad infinitum. Skepticism is unsound only if skepticism is sound.

We end in a rather mind-boggling contradiction

  1. For skeptics, skepticism is a good argument implies it's a peritrope.
  2. For dogmatists, skepticism is a peritrope implies it's a good argument.

Skepticism → Peritrope → Skepticism → ... ad infinitum/ad nauseum

How do we extricate ourselves from this quagmire?


9 Answers 9


The history of philosophy has many examples of people who think they have refuted scepticism - Descartes, Berkeley, Locke and Hume not to mention Russell, Ryle and Wittgenstein are all examples. And yet people keep returning to the issue.

One would be justified in thinking that the problem is to understand why this is so.

For example, in the case of Agrippa's argument, I see it as an effective objection to foundationalist views about knowledge, but not against knowledge as such. It is possible that other arguments do not have the radical conclusions claimed for them and they are actually aimed at different forms of scepticism. Some, as in the case of Agrippa's argument, may be right.

Another line of thought can be seen in Hume and Russell and Ayer. Scepticism is not per se an enemy to be overcome, but a friend and ally to be relied on. We need to chart a distinction between destructive scepticism and constructive scepticism. That would, at least, be a clearly different problem.

Stanley Cavell in his Claim of Reason and In Quest of the Ordinary argues that we need to understand, rather than refute, scepticism (he means radical or destructive scepticism). He sees it as an inescapable part of the human condition. His project goes beyond the boundaries of analytic philosophy, but I can't see that as a fatal objection.


You didn't explain why you think skepticism is self-refuting, but I've heard people say this would be because skepticism would demand skepticism of skepticism itself, so I'll address that.

Yes, you should be skeptical of skepticism, but that doesn't make it self-refuting.

This seems to conflate skepticism and what I might call universal non-belief.

Universal non-belief may indeed be self-refuting, because you'd need to not believe that non-belief is a good idea, at which point you wouldn't subscribe to non-belief (or something like that).

Skepticism is not that. Skepticism is closer to holding that truly proving anything (that's non-trivial, or without assumptions) is impossible, and you should therefore not have absolute certainty about anything.

Crucially, this does not mean you should believe nothing, but rather that you should not assume any belief to be foolproof, and therefore all your beliefs should be tentative and you should question your beliefs as far as possible.

Now, one could hold that truly proving anything is impossible, without being absolutely sure that truly proving anything is impossible and accepting that your belief in this impossibility may not be foolproof. To apply skepticism here, one could try to truly prove something, or find a way to theoretically do so. Failing that, you would stick to the tentative belief that truly proving anything is impossible. No self-refutation here.

  • 4
    @NikosM. "Nothing is [completely] certain" is not completely certain. It does, however, seem most likely, given our ever-changing/improving understanding of the universe, that different people appear to believe contradictory things, even with good understandings of logic, and that we just can't seem to prove anything if we start with no assumptions.
    – NotThatGuy
    Commented Feb 15, 2023 at 16:52
  • Is this somehow related to Popper's falsifiability in philosophy of science?
    – user64708
    Commented Feb 17, 2023 at 11:15
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    @irecorsan Falsifiability could be considered a part of skepticism (loosely speaking) and would factor into whether or not a claim should be accepted. One can question a claim regardless of whether or not it's falsifiable. All conclusions in science are treated as tentative, and questioning is encouraged, which could also be seen as skepticism.
    – NotThatGuy
    Commented Feb 17, 2023 at 12:31

Is the concept of knowledge, or our use of the word knowledge (and our use of other such terms in the same and other languages), such as to be broken down into other concepts/uses of other words? Or, on the contrary, John Cook Wilson reasoned that “we cannot make knowing itself a subject of inquiry in the sense of asking what knowing is”:

We cannot construct knowing—the act of apprehending—out of any elements. ... Knowledge is sui generis and therefore a ‘theory’ of it is impossible.

If the concept of knowledge is composite, then it was composed for a reason. Skepticism is thus useless; it would be easier to simply drop the concept of knowledge altogether than to apply the concept so as to defeat its own application. The alternative, here, would be like defining the concept of a color in terms of the impossibility of perceiving any color, or especially the one so defined.

Or even if the concept of knowledge is irreducible, it still is open to us to ask if the concept is important. Suppose Alice is talking to Elise and they get to a point in the conversation where Elise asks, "But how do you know that?" Alice replies, "Why, I don't know it at all! It's just true, though." At most, Alice will deny that anyone actively knows the opposite, either, in that she will deny that anyone knows such things at all (c.f. Wittgenstein's talk of hinge propositions). But so perhaps in a contextualist sense, the concept of knowledge is to be used only for things where it makes a difference whether the concept is applied; a highly esoteric statement like, "All is one in the All," does not sustain this difference (what would knowing this be, as opposed to not knowing it?).

So Agrippa's game, as set up, looks to be one where players can't win. They don't have the time to make an infinite series of moves; they're not allowed to make circles around the board (regardless of how nontrivial the circle is, say from one end of the board to another); and if they stop on one square of the board and say, "This is the first square," that too violates the rules of the game. But so what is the point of the game? And it does seem like just a game, now.

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    "Strange game. The only winning move is not to play." Philosophy = Armageddon?
    – Scott Rowe
    Commented Feb 15, 2023 at 2:28
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    @ScottRowe :-) - +1, just so you know that at least one person recognised the war games quote. I'm sure others also did. Also
    Commented Feb 15, 2023 at 9:25
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    @NotThatGuy I didn't deny that individual beliefs can be questioned, and I would even venture to say that some beliefs should be questioned (supposing that beliefs actually exist in the first place, though eliminative materialism calls that into question). I was addressing the trilemma as a reason for "invincible" abstract skepticism, pointing out that if the game of knowledge is rigged so no strategy is a winning one, then the game is indeed pointless. However, I do think that epistemic logic can be put to good use betimes so I'm not totally against knowledge-talk. Commented Feb 16, 2023 at 15:52
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    @candied_orange Skepticism's use as a tool to get closer to objective truth is "completely unsupported"?? A whole lot of scientific knowledge would disagree with you on that. Science and skepticism are intertwined, in that no conclusion should be taken as absolute truth and everything can be challenged. Had this not been the case, our scientific understanding would've probably been stuck with our incorrect theories from ancient times. Everyday life would also disagree with you on that, as appropriate skepticism leads a whole lot of people to what appears to be true on a daily basis.
    – NotThatGuy
    Commented Feb 16, 2023 at 18:25
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    @NotThatGuy that's not objective truth. Science doesn't do objective truth. Science does Occams Razor. That is, we prefer the most useful lies. An atom is defined as the smallest unit of matter. That's why we called it atom, after the first man. Then Marie Curie had to go and get sick. Now we're stuck with this stupidly inaccurate name for these packets of particles. Why? Because we gave a useful lie a name. We still use this useful lie. It's called Chemestry. Commented Feb 16, 2023 at 18:38

I don't think a positive answer is knowable. If the answer were "yes", then the invincible skeptic would doubt the answer, so could not know if skepticism were invincible.

So, to answer your final question, we can only extricate from the quagmire if skepticism is not invincible.

Since this metastatement about skepticism seems to be knowable, then that entails skepticism is not invincible.


As of the titular question: No, it is not.

Tim Button has formally shown in his book The Limits of Realism that all famous historical arguments for skepticism are incoherent/self-contradictory.

For what it's worth, he does show the same for all famous historical arguments for realism, hence the title of the book.

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    This answer could be improved by briefly detailing the arguments and Button's refutations.
    – NotThatGuy
    Commented Feb 17, 2023 at 11:04
  • @AgentSmith No, if I do make a meta-assessment of an argument like that it is logically inconsistent that is not self-refuting. If I would say that it is materially wrong that would be open to skeptical refutation. But that is not the point.
    – Philip Klöcking
    Commented Feb 17, 2023 at 13:18
  • @NotThatGuy True in principle but there is no way of briefly detailing arguments and their refutations if the discussion of each argument takes several pages in the book, not to speak of the refutations proper.
    – Philip Klöcking
    Commented Feb 17, 2023 at 13:20
  • @AgentSmith Again, not the point. When the thought that my thought may not be my own or any input might be implanted by a malicious demon are self-contradictory I can carry on from there or I basically should stop doing anything, including thinking. Good luck!
    – Philip Klöcking
    Commented Feb 17, 2023 at 13:38

As per my other answer, a skeptical argument is not different than an assertion of the presence or lack of presence of something.

For example:

A: I see a boat out there.

B: How do you know you see a boat out there?

What B is doing is asserting something, even if only indirectly. B asserts that A's sense (at that point) is incorrect and/or doubtful. But this is an assertion, and an assertion needs to be based on something, even if it is on a self-evident something.

So extracting the assertion out of a skeptical argument, makes it like any other argument and can be attacked or refuted all the same. One can simply ask the other to ground the implicit assertion the person makes on something, even on a self-evident something.

If one denies that an assertion needs to be based on something, then the skeptic loses the teeth to bite, since now the person cannot attack any statement made.

PS: That an assertion needs to be based on something holds for person A too. Possibly A can appeal to the self-evident fact of being a boat there, at least as a starting point.

Is skepticism invincible?

  1. A skeptical argument or challenge, as argued above, is like any other argument or statement and can be attacked or refuted all the same. Nothing invincible in that.
  2. Skepticism as a whole is nothing but the set of potential skeptical arguments or challenges. Nothing invincible in that either.

Note: you may be interested in this post about radical skepticism. Also you may be interested in the solution(s) to the problem of infinite regress in epistemology


Skeptism is self-refuting as noted by al-Ghazali which is why no major philosopher has been a sceptic. To do philosophy requires faith ... faith in truth as a category, faith also in our senses that they report the world faithfully.

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    Philosophy is rooted in skepticism. Plato's Academy morphed into, for obviously good reasons, a school of skepticism.
    – Hudjefa
    Commented Feb 15, 2023 at 0:03
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    @AgentSmith: No, thats not true. You're merely reading back the modern fetish with sceptism back in time. This is anachronistic. Commented Feb 15, 2023 at 0:12
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    "Faith in truth as a category"? That must be the strangest of the "y'all need faith too" argument I've heard. Some people seem desperate to find any justification whatsoever for their beliefs, even if all they can muster is a very doubtful "your justification is like mine". That rests entirely on the word "faith", while disregarding how and why faith-based beliefs are typically criticised (which wouldn't apply to whatever "faith in truth as a category" is supposed to mean).
    – NotThatGuy
    Commented Feb 15, 2023 at 11:50
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    This answer could be improved by summarising what al-Ghazali actually said. At the moment this answer is little more than "no", with no explanation offered for why this is the case, which makes for a very low quality answer.
    – NotThatGuy
    Commented Feb 15, 2023 at 12:49
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    @NotThatGuy: No, its not that strange. Post-modernity abandons truth as a category as its a modern form of scepticism. Al-Ghazali said we must have faith in our senses and our capacity for rational thought. Perhaps you are confused about my use of the word 'faith' here because it's more usual to use that term in religion. I used it deliberately to evoke the fact there is no further ground to this position. Faith, as a notion, is critically savaged in todays philosophy where sceptism is eulogised. And this, as I've described in the preceding, is just wrong. Commented Feb 15, 2023 at 13:17

While Mr. Klocking implies in his answer that fallibilism (IEP) easily and consensually among professional philosophers overcomes the problem of skepticism, contemporary skepticism (IEP) lives on, and that's okay. I'm going to outline a broader line of attack by a contemporary philosopher and go above and beyond a reference to a book. I'll refer to Robert Audi's Epstemology and it's chapter "Skepticism" as a basis for the response.

Robert Audi's approach to answering moderate skepticism is as follows:

To show a belief is tenable, first show in a second order claim that the belief in belief is possible with a general premise:

(1) An attentive belief to the effect that one is now in an occurrent mental state, such as thinking, is justified.

where "attentive belief is on based on careful attention to the matter in question, and where the justification is not absolute but simply strong enough to make it appropriate for a rational person to hold the belief".


(2) I have an attentive belief I am now in such a state, namely thinking.

Therefore, by deduction:

(3) My belief that I am thinking is justified.

He goes on to admit this reason is defeasible, such as applying a proposition about a history of recent troubles with hallucinations.

So, in short order, the notion that skepticism is indefeasbile, is not only open up to criticism through showing that reason itself, and hence the very conclusion is subject to defeasible conditions, but also by providing a positive argument that justification of belief is one of degree, not black and white, which would be an obvious false dilemma. Rationality entails accepting the fallibilistic nature of knowledge, thus dissolving the question of is there or isn't there knowledge.

So, one can accept both skepticism and credulity admitting they lead to fallibilism, by recognizing, as Audi puts it:

"These questions produce a tension. I want to believe [a certain proposition]... But I also want to avoid believing [the proposition] if it is not [true], for I have a deep-seated desire to avoid believing falsehoods... the former inclines us to believe readily... The latter ideal pushes us toward a kind of skepticism."

Thus, not only is the skeptical problem overcome by recognizing that radical skepticism itself is subject to skepticism, but positively it is the actual state of affairs of all people to find a balance between credulity and skepticism every day of their lives because rational people make attentive and reasonable presumptions pragmatically. To appeal to the classical logic of Agrippa is a good exercise, but accepting a non-naive realism is hardly a problem in contemporary discourse because of the acceptance of non-classical logics, a knowledge of the psychological, and a lot of progress made by epistemologists since the time of Pyrrho of Ellis.

  • True, However, as you can see, epistemology has to be adapted to skpeticism.
    – Hudjefa
    Commented Feb 17, 2023 at 2:07
  • 1
    @AgentSmith First, that would be an equivocation on invincible. Second, skepticism and credulity don't bend epistemology, they are epistemology. Skepticism broadly conceived isn't a competitive position, it's an aspect of reason.
    – J D
    Commented Feb 17, 2023 at 6:00

Agrippa's 5 tropes resoleved

  1. Dissent: Trust an other as thyself and you won't need the theses. An other is not an alien or evil, don't be mad in faith.
  2. Relation: Point of view doesn't matters for alitheya, be at skeptic's point.
  3. Infinite Regress: Not needed an justification if you are not an other-I.
  4. Axioms: Art the axioms needed by an action in one place and one time. Axioms are something here and now only, not there and when.
  5. Circularity: Don't use refers, break the vicious circle.

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