The Five Tropes

In skepticism, the ancient greeks on 23/07/2011 at 2:24 am

I mentioned in a earlier comment that I think Sextus Empiricus’s Outlines of Phyrronism is the greatest book ever written. Of course, this is hyperbole on my part, but it gives a good indication of how strong the skeptical position has been for millennia.

Agrippa’s five tropes, published in Sextus Empiricus’s Outlines of Phyrronism, stand today as the greatest assault on all justificationist theories of knowledge. They are the following:

5-1 Diaphônia:
The mode that argues from disagreement. With respect to some matter that presents itself, there is undecided (anepikriton) conflict, both among the views of ordinary life and the views held by philosophers. Due to this, we are unable to choose or reject one thing, and must fall back on suspension.

5-2 Eis apeiron ekballonta:
Arguments that throw one into an infinite regress. That which is brought forward to make a given matter credible needs yet something else to make it credible, and so on ad infinitum. Since we thus have no starting point for our argument, suspension of judgment follows.

5-3 Pros ti:
Arguments from relativity. X only ever appears such-and-such in relation to the subject judging and to the things observed together with it. Suspension on how it really is follows.

5-4 Hypothesis:
Someone makes an assumption without providing argument. A dogmatist, if thrown back into an infinite regress of arguments, just assumes something as a starting-point, without providing an argument (anapodeiktôs). We suspend over mere hypotheses—they could be false, opposite hypotheses could be formulated, and so on.

5-5 Ton diallêlon:
Arguments that disclose a circularity. This mode is used when that which ought to confirm a given investigated matter requires confirmation (pistis—credibility) from that matter. We are unable to assume either in order to establish the other. We suspend judgment on both. (Vogt, Katja, “Ancient Skepticism“)

The three most popular objections–insofar that I have heard them said repeatedly, and without a sense of irony, in response to the skeptical challenge–have been the following. I now give you my worst of the worst:

1. Skepticism is self-defeating: How can a skeptic be sure that they do not know?

I first remember a professor raising this objection during an early undergraduate class. This immediately struck me as either a misrepresentation of the skeptic’s position or just lackluster teaching on the professor’s part. While this point could have been brought up only to dismiss it, fleshing out exactly what the skeptic’s position is, and the nature of knowledge, the professor did neither of the two, and instead spent the rest of the class time going over the ‘pros’ and ‘cons’ of this objection.

Does the skeptic deny the possibility of truth? Some may, of course, but they should not be taken seriously, in much the same way that some people claim that the Bible or revelation provides a properly basic belief should be not taken seriously. Does the skeptic deny belief? Many in fact dismiss belief as holding any important role in discovering the truth (the Phyrrhonian position of epoché is one example) by denying that the truth can ever be known.

Does the skeptic deny the supposed strength of justification? Ah! This, I saw at the time, was their real criticism of individuals that claimed to possess knowledge: once ‘knowledge’ is reformulated as ‘justified true belief’ and ‘true belief’ is put aside as possible, but ultimately unjustifiable, why should the skeptic claim to possess justification for denying justification? The skeptical argument is either true or false, not justified or unjustified. Assaulting skepticism by inquiries into the psychology of the skeptic is to miss the point.

It is far too easy to depict anyone you disagree with as lazy or a hypocrite–you need only attribute to them positions that make their position incoherent. And even if the skeptic should be lazy or a hypocrite, then what of it? Does that make the objection lose any of its strength?

2. Skeptics cannot consistently claim that appearances are not true: Practical action must occur, making a reductio of skepticism.

As covered previously, the skeptic suffers no problem of practical action. The skeptic does not claim that appearances are not true, only that we cannot reliably tell apart true from false appearances.

The problem is then only for the fideist, for they assume such a reliable procedure is possible: if the five tropes are true, then “the fideist ought to withhold assent on all subjects; and yet, the argument has no implications for what the fallibilist ought or ought not to believe, except for that the fallibilist ought to believe nothing if he were a fideist.”

If the skeptic wishes to take the elevator rather than the window when descending a skyscraper, then what matters for practical action is whatever action accomplishes her goals: if taking the window is the safe option and the elevator is a death trap (as a counterfactual: if Sarah the skeptic were lighter than a feather, then the window would be an excellent option while a frayed cable on the elevator would send Sarah plummeting to her death), then Sarah should choose the window. What matters is not justification of theories, but which theories are true. The skeptic need only prefer true theories. But can Sarah the skeptic ever claim to have justified the truth? Not without giving up her skepticism and becoming a fideist.

3. Moore’s ‘Here is a hand’ argument: More recently reformulated by BonJour in a more rigorous fashion, the ‘Here is a hand’ argument gives a IBE (Inference to the Best Explanation), so skepticism is not to be preferred.

Moore’s response never sat right with me, for it does little more than assume that the statement ‘Here is a hand’ is true, believed, and justified. Yes, it is psychologically compelling when one looks at ones hands to believe that one has hands, but even if such a claim were to be true and believed, is it in any way justified? Clearly, after considering the possibilities brought up by the skeptic, there are many other possible scenarios where I come to fervently believe something that is false, or I come to fervently believe something that is true, but for all the wrong reasons. According to the fideist account, they necessarily do not have knowledge in either of those two cases. On the other hand, Sarah the skeptic can take up any position that does not conflict with other assumptions she makes, for while she does not seek justification, Sarah seeks the truth, and incoherence indicates that at least one of the propositions the skeptic adopts is not in fact true.

On BonJour’s account, I am equally befuddled. I can take many things to be IBEs, but does that make them true? Realism, for instance, surely is an IBE, even though it is equally as corroborated as anti-realism. But does the nature of realism being an IBE necessarily imply that realism is true? Best explanations may, upon further reflection, turn out to be false. The possibility is inescapable. In other words, there is a horn facing the IBE: either they are only best explanations in light of our background assumptions, or they are best explanations not in light of our background assumptions. For the first, if our background assumptions are true, then an IBE will relate to truth in some important way, and this way may be in some sense justified, but then we are caught once again in the thicket, trying once more to explain how we know our background assumptions are true; for the second, if the IBE is an IBE by nature of some objective criteria outside our background assumptions, how can we come to know that any explanation is in fact an IBE? Thus, the justificationist must explain the differences between an IBE and an unjustified conjecture (get out of the fork) before proceeding.

Thus, the fideist, when responding to Agrippa’s five tropes, must take them seriously, and must not under any circumstances apply their own problems to the skeptic’s position (see 1, 2) or beg the question (see 3). Word-mangling or dodging the issue at hand may work well in the public arena, but it does little help in responding to the problem as raised by Sextus Empiricus. If anything, these responses are designed to provide succor to those that do not want to tackle the five tropes on level ground, and offer a defense-mechanism against any objection that may be raised. The boiling oil, the moat, the high and wide walls–they are all built to protect justificationist positions from addressing the five tropes by dismissing the skeptical position from the start.

And that is why they are the most common, but most intellectually facile, arguments against skepticism I have yet seen.


  1. It’s a fascinating line of argument made by justificationists.

    The purely logical case that knowledge (the justified belief kind) does not exist is as watertight as it gets. Most everyone even understands that deductive inferences assume that which they conclude in the premises, i.e. are question begging (supposing one is attempting justification). Anything else is invalid. Even inductive inferences only become stronger when they assume more of what they conclude in the premises, i.e. partially beg the question. This isn’t known to just a few sceptics or critical rationalists, but nobody seems to accept the implication: justification is logically impossible. This requires some bizarre double-think.

    There is another argument often trolled out. It is sometimes attributed to Ayn Rand, though I am sure she was not the first. It’s sometimes called “the fallacy of stealing the concept.”




    In other words, argument by reductio ad absurdum is invalid. Why? Because the arguer doesn’t believe in the premises. Yes, exactly, it makes that much sense. I think I may write up a post about this one at the Critical Rationalist Blog.

    • Lee,

      It turns out that I wrote on this same problem of ‘stealing the concept’ some time ago, but I moved it to my ‘drafts’ folder. I’ll publish it today.

  2. Anthony Flood wrote about this in his “Ayn Rand’s Use of Retortion”:


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