Pyrrhonian Problematic, The

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Knowledge and Justification

If a belief is to count as knowledge, then it must be true. But truth is not enough: lucky guesses and, more generally, beliefs that are only accidentally related to the facts they purport to describe do not amount to knowledge. What else, besides truth, is needed for a belief to count as knowledge, then? There is no agreement regarding how to fully answer that question, but there is a line of thought regarding how to begin such an answer that is widely shared: for a belief to amount to knowledge it has to be justified or supported by reasons, or rationally grounded, or warranted, or have some sort of positive epistemic status. (These, and other, words are sometimes used as synonyms, whereas sometimes they are intended to mark important epistemological distinctions. I use them interchangeably.) The justification in question here is usually qualified as epistemic, to distinguish it from the kind of justification that, for example, an assassin's mother might have in believing that her son is innocent despite mounting evidence against him.

It is possible to adopt many different attitudes with respect to any proposition p (say, the proposition that Paris is the capital of France). For instance, it is possible to believe that Paris is the capital of France, to be happy that Paris is the capital of France, to hope that Paris is the capital of France, and so on. Some of these attitudes can be called doxastic attitudes. What distinguishes a doxastic attitude from other attitudes we can adopt toward a proposition is that one can be justified or unjustified (in the epistemic sense) in adopting a doxastic attitude.

There are three basic doxastic attitudes: belief, disbelief, and suspension of judgment. (It might be that there are other attitudes that we might be justified or unjustified, in the epistemic sense, in adopting, but belief, disbelief, and suspension of judgment are basic in the sense that any other doxastic attitude will be such only because it entails one of these three basic attitudes.) To disbelieve that Paris is the capital of France is to believe that it is not true that Paris is the capital of France (and so, depending on how you count, you might think that there are only two basic doxastic attitudes: belief and suspension of judgment). To suspend judgment with respect to the proposition that Paris is the capital of France is to be in a mental state that is opposed both to believing and disbelieving the proposition. Suspension of judgment must therefore be carefully distinguished from having no attitude whatsoever with respect to a certain proposition. There is a difference between never having considered the question whether there is an even number of stars in the Milky Way and, having considered it, suspending judgment with respect to the question.

Academic and Pyrrhonian Skepticism

If the connection between knowledge and justification presented earlier is correct, then we can know a proposition only if we are justified in believing it. Skepticism with respect to a range of propositions is the claim that the only justified attitude with respect to the propositions in that range is to suspend judgment. We are all skeptics, in this sense, with respect to some range of propositions. For instance, it seems obvious that the only correct attitude with respect to the proposition that there is an even number of stars in the Milky Way, once we have considered it, is to suspend judgment. This is ordinary skepticism. But most of us are nonskeptics with respect to many propositions. For instance, it seems obvious that the only justified attitude with respect to the proposition that Paris is the capital of France is to believe it, whereas the only justified attitude with respect to the proposition that Tony Blair is the president of the United States is to disbelieve it. Philosophical skepticism extends well beyond ordinary skepticism, claiming that we should suspend judgment with respect to propositions that we ordinarily think we are justified in believing.

It is customary to distinguish between two different kinds of philosophical skepticism, which can be called, following an ancient tradition, Academic skepticism and Pyrrhonian skepticism. Academic skepticism referred originally to a phase in the history of Plato's Academy that stretched approximately from the third to the early first century BCE. The main figures of Academic skepticism were Arcesilaus (mid-third century BCE), Carneades (mid-second century BCE), and Clitomachus (d. 110/109 BCE). The main sources for Pyrrhonian skepticism are the writings of Sextus Empiricus in the late second century CE.

Academic and Pyrrhonian skepticism differ in the scope of propositions that, according to them, we should suspend judgment about. Let's call those propositions that do not contain any epistemic concepts ordinary propositions and let's call those propositions to the effect that someone knows an ordinary proposition can be called epistemic propositions.

Academic skeptics think that the only justified attitude with respect to most (perhaps all) ordinary propositions is suspension of judgment. However, Academic skeptics do not suspend judgment with respect to epistemic propositions: On the contrary, they think that the only justified attitude with respect to them is to disbelieve themthat is, they think that we are justified in believing that we do not know almost anything of what we take ourselves to know. (When contemporary authors discuss skepticism, chances are they are referring to this aspect of Academic skepticism: to the claim that we do not know certain propositions that we ordinarily take ourselves to know. However, the tradition is to classify as a skeptic with respect to a certain proposition only someone who thinks we should suspend judgment with respect to that proposition, not someone who thinks that we should dissent from it.)

Pyrrhonian skeptics, meanwhile, extend their skepticism to epistemic propositions as well. Both Academic and Pyrrhonian skeptics leave it open whether Paris is the capital of France or not: maybe it is, maybe it is not, but we are not justified in believing that it is or believing that it is not. According to Pyrrhonian skeptics it is also an open question whether we know that Paris is the capital of France: maybe we do, maybe we do not, but we are not justified in believing that we do or that we do not. Academic skeptics, on the contrary, do not leave this question open: they think we are justified in believing that we do not know that Paris is the capital of France.

The modes of Agrippa

From now on, the focus will be on Pyrrhonian skepticism exclusively The Pyrrhonians had a number of ways, or modes, to induce suspension of judgment. The importance of Pyrrhonian skepticism to contemporary epistemology derives primarily from these modes, and in particular from a subset of them referred to collectively as the modes of Agrippa. There are five modes associated with Agrippa, but three of them are the most important: the mode of hypothesis (or unsupported assertion), the mode of circularity (reciprocal), and the mode of regression to infinity.

The three modes of Agrippa function together in the following way. Whenever the dogmatist (Sextus refers to those who are not skeptics as dogmatists) asserts his or her belief in a proposition p1, the Pyrrhonian will challenge that assertion, asking the dogmatist to justify p1, to give reasons for thinking that it is true. The dogmatist will then either decline to answer the challenge or adduce another proposition p2 in support of p1. If the dogmatist refuses to answer the challenge, the Pyrrhonian will be satisfied that the only justified attitude to take with respect to p1 is to suspend judgment, because no reason for it has been given (thus appealing to the mode of hypothesis). If the dogmatist adduces another proposition p2 in support of p1, then either p2 will be identical to p1 or it will be a different proposition. If p2 is the same proposition as p1, then the Pyrrhonian will also suspend judgment with respect to p1, because no proposition can support itself (thus appealing to the mode of circularity). If, however, p2 is different from p1, then the Pyrrhonian will ask the dogmatist to justify his or her assertion of p2. And now the dogmatist offers no reason in support of p2, offers p2 itself or p1 as a reason, or adduces yet another proposition p3, different from both p1 and p2. If the dogmatist offers no reason for p2, then the Pyrrhonian will invoke the mode of hypothesis again and suspend judgment in accordance with it; if either p2 itself or p1 is offered as a reason to believe in p1, then the Pyrrhonian will invoke the mode of circularity and suspend judgment in accordance with it (because not only can no proposition be a reason for believing in itself but also no genuine chain of reasons can loop); and, finally, if the dogmatist offers yet another proposition p3, different from both p1 and p2, as a reason to believe p2, then the same three possibilities that arose with respect to p2 will arise with respect to p3.

The dogmatist will not be able to continue offering different propositions in response to the Pyrrhonian challenge forevereventually, either no reason will be offered, or a proposition that has already made an appearance will be mentioned again. The Pyrrhonian refers to this impossibility of actually offering a different proposition each time a reason is needed as the mode of infinite regression. The three Pyrrhonian modes, then, work in tandem to induce suspension of judgment with respect to any proposition whatsoever.

Agrippa's Trilemma

The Pyrrhonian use of the three modes of Agrippa to induce suspension of judgment can be presented in the form of an argument, called Agrippa's trilemma. It is at least somewhat misleading to present the Pyrrhonian position in terms of an argument, because in presenting an argument one is usually committed to the truth of its premises and conclusion, whereas Pyrrhonian skeptics would suspend judgment with respect to them. Nevertheless, presenting the Pyrrhonian problematic in the form of an argument does not do much violence to this skeptical position, because what is important is not whether the Pyrrhonian skeptics themselves accept the premises or the validity of the argument, but whether their audience does. Problems still remain regarding the coherence of anyone (be they Pyrrhonian skeptics or not) who accepts the soundness of an argument whose conclusion is that we are not justified in believing anything. It is doubtful, though, whether anyone accepts Agrippa's trilemma: "Dogmatists" certainly do not, and neither do Pyrrhonian skeptics. It is not a coincidence that Wittgenstein's dictum about throwing the ladder after using it to climb echoes Sextus's less-pleasing image of the laxative that purges itself together with the "humours" of the body it is designed to expel. Still, even if we do not think that the argument is sound, we stand to learn something interesting about the structure of an epistemological theorybecause each of the premises of the apparently valid argument looks plausible at first sight.

Before presenting a reconstruction of Agrippa's trilemma some definitions need to be introduced. Say that a belief is inferentially justified if and only if it is justified (at least in part) in virtue of its relations to other beliefs. A justified basic belief, by contrast, is a belief that is justified but not in virtue of its relations to other beliefs. An inferential chain is a set of beliefs such that every member of the set is allegedly related to at least one other member by the relation is justified by. Agrippa's trilemma, then, can be presented thus:

(1) If a belief is justified, then it is either a basic justified belief or an inferentially justified belief.

(2) There are no basic justified beliefs.


(3) If a belief is justified, then it is justified in virtue of belonging to an inferential chain.

(4) All inferential chains are such that either (a) they contain an infinite number of beliefs; (b) they contain circles; or (c) they contain beliefs that are not justified.

(5) No belief is justified in virtue of belonging to an infinite inferential chain.

(6) No belief is justified in virtue of belonging to a circular inferential chain.

(7) No belief is justified in virtue of belonging to an inferential chain that contains unjustified beliefs.


(8) There are no justified beliefs.

Premise (1) is beyond reproach, given our previous definitions. Premise (2) is justified by the mode of hypothesis. Step (3) of the argument follows from (1) and (2). Premise (4) is also beyond reproachthe only remaining possible structure for an inferential chain to have is to contain basic justified beliefs, but there are none of those according to premise (2). Premise (5) is justified by appeal to the mode of infinite regression, and (6) is justified by appeal to the mode of circularity. Premise (7) might seem to be a truism, but some authors have argued that denying it is the only plausible way out of Pyrrhonian skepticism.

It is interesting to note that Agrippa's trilemma is perfectly general; in particular, it applies to philosophical positions as well as to ordinary propositions. In fact, when Agrippa's trilemma is applied to epistemological theories themselves, the result is called "the problem of the criterion."

Many contemporary epistemological positions can be stated as a reaction to Agrippa's trilemma. In fact, all of premises (2), (5), (6), and (7) have been rejected by different philosophers at one time or another. In the remainder of this entry, we examine each of these responses.

Rejecting Premise (2): Foundationalism

Foundationalists claim that there are basic justified beliefsbeliefs that are justified but not in virtue of their relations to other beliefs. In fact, according to foundationalists all justified beliefs are either basic beliefs or are justified in virtue of being inferentially related to a justified belief (or to some justified beliefs). This is where foundationalism gets its name: The edifice of justified beliefs has its foundation in basic beliefs.

But how do foundationalists respond to the mode of hypothesis? If a belief is not justified by another belief, then is it not just a blind assertion? If basic beliefs are justified but not by other beliefs, then how are they justified? What else besides beliefs is there that can justify beliefs?

To this last question, many foundationalists reply: experience (if they are talking about empirical knowledge, of course; a priori knowledge raises interesting problems of its own, and it is also subject to Agrippa's trilemma). To a rough first approximation that glosses over many important philosophical issues, experiences are mental states that, like beliefs, aim to represent the world as it is, and, like beliefs, can fail in achieving that aimthat is, experiences can misrepresent. Nevertheless, experiences are not to be identified with beliefs, for it is possible to have an experience as of, for example, facing two lines that differ in length without having the belief that one is facing two lines that differ in lengtha combination of mental states that anyone familiar with the Müller-Lyer illusion will recognize.

There are three important questions that any foundationalist has to answer. First, what kinds of beliefs do experiences justify? Second, how must inferentially acquired beliefs be related to basic beliefs for them to be justified? Third, in virtue of what do experiences justify beliefs?

traditional and moderate foundationalism

With respect to the first question, we can distinguish between traditional foundationalism and moderate foundationalism. Traditional foundationalists think that basic beliefs are beliefs about experiences, whereas moderate foundationalists think that experience can justify beliefs about the external world. Take, for example, the experience that you typically have when looking at a tomato under good perceptual conditionsan experience that, remember, can be had even if no tomato is actually there. A moderate foundationalist would say that that experience justifies you in believing that there is a tomato in front of you. The traditional foundationalist, however, would say that the experience justifies you only in believing that you have an experience as of a tomato in front of you. You may well be justified in believing that there is a tomato in front of you, but only inferentially.

A traditional argument in favor of traditional foundationalism relies on the fact that whereas you can be mistaken regarding whether there is a tomato in front of you when you have an experience as of facing a tomato, you cannot, in the same situation, be mistaken regarding whether you are undergoing such an experience. From the point of view of traditional foundationalism, this fact indicates that the moderate foundationalist is taking an unnecessary epistemic riskthe risk of having a foundation composed of false beliefs.

The moderate foundationalist can reply that the traditional foundationalist must undertake a similar risk. For, while it is true that if one is undergoing a certain experience then one cannot be mistaken in thinking that one is undergoing that experience, one can still be mistaken about one's experiencesfor instance, perhaps one can believe that one is in pain even if the experience that one is undergoing is actually one of feeling acutely uncomfortable. And if it were just as difficult to distinguish between the true and the false in the realm of beliefs about our own experiences as it is in the realm of beliefs about the external world, then we could be wrong about which of our own beliefs are basically justified and which are not. If this kind of metafallibilism is accepted, then why not accept the further kind according to which basic justified beliefs can be false? Of course, the resolution of this dispute depends on whether, as the moderate believes, we can be mistaken about our own experiences.

deductivist and nondeductivist foundationalism

What about our second question: How must basic beliefs be related to inferentially justified beliefs? Here, too, there are two different kinds of foundationalism: deductivism and nondeductivism. According to the deductivist the only way in which a (possibly one-membered) set of basic justified beliefs can justify another belief is by logically entailing that other belief. In other words, there has to be a valid argument whose premises are all basic justified beliefs and whose conclusion is the inferentially justified belief in question. Given that the argument is valid, the truth of the premises guarantees the truth of the conclusionit is impossible for all the premises to be true while the conclusion is false. Nondeductivism allows relations other than logical entailment as possible justificatory relations. For instance, many foundationalists will claim that good inductive inferences from basic justified beliefs provide their conclusions with justificationeven though inductive arguments are not valid, that is, even though it is possible for all the premises of a good inductive argument to be true while its conclusion is false. Although these are independent distinctions, traditional foundationalists tend to be deductivists, whereas moderate foundationalists tend to be nondeductivists. Notice that for a traditional, deductivist foundationalist, there cannot be false justified beliefs. Many contemporary epistemologists would shy away from this strong form of infallibilism and take that consequence to be an argument against the conjunction of traditional foundationalism and deductivism.

primitivist, internalist, and externalist foundationalism

The question that is most interesting from the point of view of the Pyrrhonian problematic is our third one: What is it about the relation between an experience and a belief that, according to the foundationalist, allows the former to justify the latter? (Analogous questions apply to nonfoundationalist positions too, and the discussion to follow is not restricted to the specific case of foundationalism.) There are three different proposals about how to answer this question that are the most prominent. The principles that assert that a subject is justified in having a certain belief given that he or she is undergoing a certain experience can be called epistemic principles. Our third question can then be stated as follows: What makes epistemic principles true?

The first proposal, which we shall call primitivism, claims that the question cannot have an intelligible answer. There is no more basic fact in virtue of which epistemic principles obtain. They describe bedrock facts, not to be explained in terms of anything else, but are instead to be used to explain other facts. Epistemological theorizing, according to the primitivist, ends with the discovery of the correct epistemic principles.

The other two positions are nonprimitivist. Internalist nonprimitivism holds that epistemic principles are true in virtue of facts about ourselvesfor instance, one prominent internalist view is that which epistemic principles are true for a given subject is determined by which epistemic principles that subject would accept under deep reflection. Externalist nonprimitivism holds that epistemic principles are true in virtue of facts that are not about ourselvesfor instance, one prominent externalist view is that certain experiences provide justification for certain beliefs because the obtaining of those experiences is reliably connected to the truth of those beliefs (reliabilism), or because i.e., it could not easily happen that those experiences obtain without those beliefs being true (an appeal to "sensitivity" or "safety" conditionals).

Both externalists and internalists think that primitivists are overlooking real facts, whereas primitivists think that there are fewer things in heaven and earth than are dreamt of in nonprimitivist philosophy. Within the nonprimitivist camp externalists think that internalists have too subjective a conception of epistemologyto some extent, thinking it so, or being disposed to think it so under conditions of deep reflection, makes it so for at least some traditional internalists. Internalists, for their part, are likely to think that externalists are no longer engaged in the same project that both skeptics and internalist epistemologists are engaged in, the project of determining "from the inside" whether one's beliefs are justified or amount to knowledge, because the obtaining of a relation between a subject's belief and the external world is something that the subject is in no position to ascertain "from the inside."

Rejecting Premise (5): Infinitism

Infinitism, the claim that infinite evidential chains can provide justification to their members, is the answer to Agrippa's trilemma that has received the least attention in the literature. This is due, at least in part, to the fact that infinitism has to deal with what might seem like formidable obstacles. For instance, it seems that no one actually has an infinite number of beliefs. To this objection, the infinitist is likely to reply that actually occurring beliefs are not needed, only implicit beliefs that are available to the subject to continue constructing his or her inferential chain if called on to do so (by others or by him- or herself). The plausibility of this reply depends on whether good sense can be made of the notion of implicit belief and the notion of an implicit belief's being available for a subject.

Even leaving that problem aside, the infinitist, like the coherentist, maintains that justification can arise merely in virtue of relations among beliefs. Infinitists will then have to respond to many of the same objections that are leveled against coherentismin particular, they would have to respond to the isolation objection mentioned in the next section.

Rejecting Premise (6): Coherentism

Coherentists reject two related features of the picture of evidential reasons that underlies Agrippa's trilemma. The first feature is the idea that justification is an asymmetrical relation: if a belief p1 justifies a different belief p2, then p2 does not justify p1. The second feature is the idea that the unit of justification is the individual belief. Putting these two rejections together, the coherentist believes that justification is a symmetrical and holistic matter. It is not individual beliefs that are justified in the primary sense of the word, but only complete systems of beliefsindividual beliefs are justified, when they are, in virtue of belonging to a justified system of beliefs. The central coherentist notion of justification is best taken to be a comparative one: A system of beliefs B1 is better justified than a system of beliefs B2 if and only if B1 has a greater degree of internal coherence than B2. One crucial question that coherentists have to answer, of course, is what it takes for one system of beliefs to have a greater degree of coherence than another. Many coherentists have thought that explanatory relations will be crucial in elucidating the notion of coherence: The more explanatorily integrated a system is, the more coherence it displays.

The main objection that coherentists have to answer is called the isolation objection. The objection centers on the fact that, according to the coherentist, the justification of a system of beliefs is entirely a matter of relations among the beliefs constituting the system. But this runs against the strong intuition that experience has an important role to play in the justification of beliefs. To illustrate the problem, suppose that you and I both have a highly coherent set of beliefsyour system, it is safe to assume, contains the belief that you are reading, whereas mine does not, and it contains instead the belief that I am swimming (because, let us suppose, I am swimming right now). Suppose now that we switch systems of beliefssomehow, you come to have my set of beliefs and I come to have yours. Given that coherence is entirely a matter of relations among beliefs, your system will be as coherent in my mind as it was in yours, and vice versa. And yet, our beliefs are now completely unjustifiedthere you are, reading, believing that you are swimming, and here I am, swimming, believing that I am reading. In other words, certain transformations that preserve coherence in a system of beliefs do not seem to preserve justification.

In reply, coherentists argue that it is possible to give experience a role without sacrificing the idea that justification is entirely a matter of relations among beliefsone idea is to require that any minimally acceptable system of beliefs contain beliefs about the experiences that the subject is undergoing. It is fair to say that there is no agreement regarding whether this move can solve the problem.

Rejecting Premise (7): Positism

One position that can be traced back to some ideas in Ludwig Josef Johann Wittgenstein's On Certainty (published posthumously in 1969)and, perhaps, also to José Ortega y Gasset's Ideas y Creencias (1940)is that evidential chains have to terminate in beliefs that are not properly said to be either justified or unjustified. This position, which we shall call positism (not to be confused with positivism), shares many features with foundationalism: for instance, both positists and foundationalists agree that inferential chains have to be finite and noncircular. But, whereas the foundationalist thinks that the starting points of inferential chains are beliefs that are justified by something other than beliefs, the positist thinks that the starting points of inferential chains are beliefs that are not justified by anythingthey are posits that we have to believe without justification. Despite this difference between the positist and the foundationalist, the positions are structurally similar enough that analogues of the questions posed to the foundationalist can be asked of the positist.

First, then, which beliefs are such that they are not justified and yet are the starting points of every inferential chainin other words, how do we identify which are the posits? One answer that can be gleaned from Wittgenstein's On Certainty, which we will call relativistic positism, is that this is a matter that is relative both to time and society, because what the posits are is determined by some function of the actual positing practices of the members of one's society at a certain time. Thus, according to Wittgenstein the proposition that no one has been to the moon was a posit for a certain long period of timeit was a proposition that no one felt the need to justify, and that was presupposed in many justificatory practices. For obvious reasons, though, that proposition can no longer appropriately function as a posit. Other epistemologists, nonrelativistic positists, think that which beliefs are properly posited depends on some objective truth about which beliefs have to be presupposed to engage in the practice of justifying beliefs at all. One prime candidate for playing this role is the first-person belief that I am not being deceived by an evil demon into thinking that I am a normally embodied and situated human being.

The second question, regarding how posits must be related to inferred beliefs to justify them, can receive answers that are completely analogous to the foundationalists'. The third question, applied to positism, is the question why certain beliefs are properly posited. Relativistic positists answer that this is so because of a certain societal fact: because they are taken to be so by an appropriate subsector of a certain society at a certain time. Nonrelativistic positists answer that a certain belief is properly taken as a posit just in case every justificatory act that we engage in presupposes that the belief in question is true.

One objection that positists of both sorts have to face is that they are transforming a doxastic necessity into an epistemic virtuethat is, they are concluding that certain beliefs can properly serve as the starting points of inferential chains because that is how in fact they are treated (relativistic positism) or because otherwise it would not be possible to engage in inferential practices at all (nonrelativistic positism). The Pyrrhonian skeptic, of course, will reply that the mere fact that most members of a society accept a certain belief without justification, or even the fact that if we do not do so then we cannot justify anything else, does not mean that it should be accepted without justification.


Perhaps one of the most interesting developments in relation to the Pyrrhonian problematic is that more and more epistemologists are arguing that the proper way to reply to Agrippa's trilemma is to combine some of the positions that, for ease of exposition, we have presented as mutually exclusive (this development is explicit in contemporary authors such as Sosa, but, some will argue, it is already present in Descartes). Thus, for example, many contemporary epistemologists put forward theories that contain elements of both internalism and externalism, or foundationalism and coherentism. It is a testament to the endurance of the Pyrrhonian problematic that philosophers continue in this way to grapple with it.

See also Agrippa; Ancient Skepticism; Arcesilaus; Carneades; Classical Foundationalism; Coherentism; Descartes, René; Greek Academy; Internalism versus Externalism; Ortega y Gasset, José; Plato; Pyrrho; Sextus Empiricus; Skepticism, History of; Wittgenstein, Ludwig Josef Johann.


Barnes, Jonathan. The Toils of Scepticism. New York: Cambridge University Press, 1990.

BonJour, Lawrence. The Structure of Empirical Knowledge. Cambridge, MA: Harvard University Press, 1985.

Burnyeat, Myles. "Can the Skeptic Live His Skepticism?" In Doubt and Dogmatism: Studies in Hellenistic Epistemology, edited by Malcolm Schofield, Myles Burnyeat, and Jonathan Barnes. Oxford, U.K.: Clarendon Press, 1980.

DePaul, Michael R., ed. Resurrecting Old-Fashioned Foundationalism. Lanham, MD: Rowman and Littlefield, 2001.

Fogelin, Robert J. Pyrrhonian Reflections on Knowledge and Justification. New York: Oxford University Press, 1994.

Klein, Peter. "Human Knowledge and the Infinite Regress of Reasons." Philosophical Perspectives 13 (1999): 297325.

Ortega y Gasset, José. Ideas y Creencias. Buenos Aires: Espasa Calpe, 1940.

Sextus Empiricus. Outlines of Scepticism. 2nd ed., edited by Julia Annas and Jonathan Barnes. New York: Cambridge University Press, 2000.

Sosa, Ernest. "How to Resolve the Pyrrhonian Problematic: A Lesson from Descartes." Philosophical Studies 85 (23) (1997): 229249.

Wittgenstein, Ludwig. On Certainty (Uber Gewissheit), edited by G. E. M. Anscombe and G. H. von Wright. Translated by Denis Paul and G. E. M. Anscombe. Oxford: Basil Blackwell, 19691975.

Juan Comesaña (2005)