The Role of Disagreement in Pyrrhonian and Cartesian Skepticism* Markus Lammenranta Introduction



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The Role of Disagreement in Pyrrhonian and Cartesian Skepticism*
Markus Lammenranta
1. Introduction
In Outlines of Scepticism, Sextus Empiricus describes the mode of disagreement, which is one of the Five Modes of Agrippa, as follows:
According to the mode deriving from dispute, we find that undecidable dissension about the matter proposed has come about both in ordinary life and among philosophers. Because of this we are not able either to choose or to rule out anything, and we end up with suspension of judgement. (PH 1.15)
René Descartes writes in a similar vein in his Rules for the Direction of the Mind:
Whenever two persons make opposite judgments about the same thing, it is certain that at least one of them is mistaken, and neither, it seems, has knowledge. For if the reasoning of one of them were certain and evident, he would be able to lay it before the other in such a way as eventually to convince his intellect as well. (CSM I, 11)1
Both Sextus and Descartes thus agree that an irresolvable disagreement leads to skepticism. The difference between them is that, whereas Sextus saw such disagreements everywhere, Descartes thought that there exists a method by which we can resolve our disagreements and avoid skepticism; this is the method he describes in Meditations.

Thus disagreement seems to play an essential role in both Pyrrhonian and Cartesian skeptical reasoning. Contemporary epistemologists typically fail to see this, because they understand knowledge and justification in individualistic and non-dialectical terms. According to their view, knowledge and justified belief depend on factors pertaining to the subject possessing knowledge or a justified belief, such as the subject’s beliefs, experiences, and psychological processes. Knowledge and justified belief do not depend on what other people believe or whether they disagree.2 It is therefore no surprise that the role of disagreement in skeptical reasoning has not been fully recognized in contemporary epistemology.

It appears to me impossible to understand the appeal of Pyrrhonian and Cartesian skepticism at all if we accept this individualistic and non-dialectical epistemology and drop disagreement from the picture. However, I cannot fully defend this view here. I will instead argue more positively that we can find intuitively plausible skeptical arguments based on disagreement in both Sextus and Descartes, and that only a dialectical conception of justification can explain the intuitive plausibility of these arguments. So it is little wonder that contemporary epistemologists who reject this conception have failed to see the force of these skeptical arguments. Only if we accept the dialectical conception of justification can we explain the intuitive appeal of skepticism and the role of disagreement within it.

One could, however, deny both the dialectical conception of justification and the intuitiveness of skeptical arguments from disagreement. For this reason, I will finish by giving reasons for the dialectical conception that are independent of skepticism; the case for dialectical epistemology and the skeptical force of disagreement would be incomplete without them.


2. The Modes of Agrippa
According to Sextus, every object of investigation can be brought under the Five Modes of Agrippa, which he describes as follows:
According to the mode deriving from dispute, we find that undecidable dissension about the matter proposed has come about both in ordinary life and among philosophers. Because of this we are not able either to choose or to rule out anything, and we end up with suspension of judgement. In the mode deriving from infinite regress, we say that what is brought forward as a source of conviction for the matter proposed itself needs another such source, which itself needs another, and so on ad infinitum, so that we have no point from which to begin to establish anything, and suspension of judgement follows. In the mode deriving from relativity, as we said above, the existing object appears to be such-and-such relative to the subject judging and to the things observed together with it, but we suspend judgement on what it is like in its nature. We have the mode from hypothesis when the Dogmatists, being thrown back ad infinitum, begin from something which they do not establish but claim to assume simply and without proof in virtue of a concession. The reciprocal mode occurs when what ought to be confirmatory of the object under investigation needs to be made convincing by the object under investigation; then being unable to take either in order to establish the other, we suspend judgement about both. (PH 1.15)
Sextus describes five modes that are supposed to induce suspension of judgment or belief about any object of inquiry. We may call them the modes of (1) disagreement, (2) infinity, (3) relativity, (4) hypothesis, and (5) circularity. Although there are five modes in total, a typical interpretation offered by contemporary epistemologists and historians of philosophy utilizes only three of them: the modes of infinity, hypothesis, and circularity. The modes of disagreement and relativity are considered unnecessary for attaining a skeptical conclusion. Scholars often point out that disagreement is neither necessary nor sufficient for skepticism, and the mode of relativity is simply dismissed as irrelevant (e.g. Barnes 1990, 113–6). The problem comprising these three modes is called the regress problem or, according to Michael Williams (1999), Agrippa’s trilemma.

It should be noted that the text contradicts this interpretation. Sextus says quite explicitly that irresolvable disagreement by itself leads to suspension of judgment. Furthermore, Sextus seems to define the very idea of skepticism in terms of disagreement:


Sceptics began to do philosophy in order to decide among appearances and to apprehend which are true and which false, so as to become tranquil; but they came upon equipollent dispute, and being unable to decide this they suspended judgment. And when they suspended judgment, tranquillity in matters of opinion followed fortuitously. (PH 1.26)
Scepticism is an ability to set out oppositions among things which appear and are thought of in any way at all, an ability by which, because of the equipollence in the opposed objects and accounts, we come first to suspension of judgement and afterwards to tranquillity. (PH 1.8)
Strong textual evidence thus suggests that Sextus considered irresolvable disagreement to be the central motivation for skepticism. He even defines a skeptic as a person who is able to find such disagreement everywhere and who therefore suspends all belief.

To defend this interpretation, I shall show that the mode of disagreement alone can provide a genuine skeptical problem and that the other modes can be considered as dependent on it. Indeed, I think there is no Agrippa’s trilemma (i.e., a skeptical problem composed solely of the modes of infinity, hypothesis, and circularity). However, because it is impossible to discuss here all attempts to formulate such a problem in terms of the three modes,3 I shall just explain how the other modes are supposed to work under my interpretation.

If we take the mode of disagreement to be the central mode, it is natural to think that the purpose of the other modes is to block any attempt to resolve disagreements. Assume that there is a dogmatist who believes that p. We, the skeptics, point out that there is disagreement about the truth of p and ask whether the dogmatist thinks that the disagreement can be resolved. If she tells us that it cannot be resolved, we will point out that she should suspend belief. If she tells us that the disagreement can be resolved, we will ask how it can be resolved. If she then gives proposition r as her reason for p, we will point out that there is also disagreement about r, and repeat the same steps again. By repeating the mode of disagreement, the dogmatist falls into the so-called Agrippa’s trilemma. However, the modes of infinity, hypothesis, and circularity cannot resolve the disagreements: the modes of hypothesis and circularity are clearly question-begging, and no one can complete an infinite chain of reasons required by the mode of infinity.

If this is how the modes of Agrippa work, it is the mode of disagreement that is central. The other modes are simply supposed to support it if the irresolvability of disagreement is questioned; they are used when a premise in the argument from disagreement is denied.


3. The Mode of Disagreement
When describing the mode of disagreement, Sextus says that we have found out that there is an irresolvable disagreement about some question and that because of this we cannot decide which of the answers to the question is true and must therefore suspend judgment. So the mode seems to pose the following skeptical argument:


  1. There is a rationally irresolvable disagreement about whether p.

  2. If there is a rationally irresolvable disagreement about whether p, I should suspend judgment about p.

  3. Therefore, I should suspend judgment about p.

Like the arguments of the ancient skeptics, this argument is to be understood as an ad hominem argument: When it is put forth against one of my beliefs, I am supposed to consider the premises, to become convinced of their truth and then to conclude that I should suspend belief about p. In order for that to happen, I may need to consider some particular instance of such disagreement in more detail and to recognize that it is rationally irresolvable. In order to evaluate the dialectical effectiveness of this argument, let us rely on the skeptic's peculiar skill of finding such disagreements and concentrate on the second premise: is it plausible that I should suspend judgment about some matter if I recognize that there is a rationally irresolvable disagreement about it?

First, we need to understand what is meant by “a rationally irresolvable disagreement.” Sextus describes such disagreements thus:
When the self-satisfied Dogmatists say that they themselves should be preferred to other humans in judging things, we know that their claim is absurd. For they are themselves a part of the dispute, and if it is by preferring themselves that they judge what is apparent, then by entrusting the judging to themselves they are taking for granted the matter being investigated before beginning the judging. (PH 1.90)
For we shall not be able ourselves to decide between our own appearances and those of other animals, being ourselves a part of the dispute and for that reason more in need of someone to decide than ourselves able to judge. (PH 1.59)
Sextus makes two points: (1) we cannot resolve a disagreement by simply preferring our own beliefs to the beliefs of those who disagree with us, because we are a part of the dispute; (2) we need an impartial judge to resolve the disagreement. These points are related: if I cannot resolve a dispute given that I am a part of it, I need someone who is not a part of the dispute to resolve it for me. However, it seems unnecessary that this impartial judge be a distinct person. I can very well look myself at the disagreement from an impartial or neutral point of view and try to decide who is right and who is wrong. Sextus’s point, of course, is that, if I cannot decide this, the disagreement is rationally irresolvable.

Taking the impartial point of view, I am not allowed to use those of my beliefs that are under dispute. To do so would be to beg the question against my opponent. That is not what a sincere inquirer would do. He or she would try to resolve the disagreement by appealing to beliefs that are independent of the disagreement. If there are no such independent reasons—reasons that my opponent could accept—in favor of my position, I should give it up.

To see what Sextus seems to have in mind, let us take an example. Assume that we are in a clothing store and I say: “That is a nice green tie. I want to buy it.” You say: “No, it is not green. It is a blue tie that looks green in this light.” Now, it is clear that I cannot rationally resolve our disagreement by claiming that I see that the tie is green, which is why it cannot be a blue tie that just appears green. Nor can I appeal to the fact that the tie appears green and argue that it therefore is green and not blue. Sextus is correct that it would be absurd to respond in either of these ways, as both responses beg the question about the color of the tie. You who believe that it is indeed a blue tie that only appears green in this light would not accept as a fact that I see that the color is green. Nor would you accept the tie's appearing green as good evidence that it really is green. In the supposed conditions, it would appear green even if it were blue. I am therefore not justified in continuing to believe that the tie is green. I have no good evidence of its color.

However, let us assume that I tell you instead that I was in the same store a few days earlier and took the ties outside of the store to see how they looked there and that the ties looked exactly the same inside and outside the store. Now it seems I have good, non-question-begging evidence that is independent of the dispute. If you are reasonable, you should accept my evidence and come to agree with me.

This sort of ordinary disagreement is typically easy to resolve. If we have not already done so, we could easily take the tie outside and verify its true color. So this kind of disagreement does not represent a serious skeptical challenge to our beliefs. The real skeptical challenge comes from disagreements that cannot in principle be resolved. It is such irresolvable disagreements which Sextus appeals to. Indeed, he seems to insist that such disagreements exist for all our beliefs about the nature of reality.

In the first of the Ten Modes, Sextus (PH 1.40–61) argues that things appear differently to animals of different species because of differences in their sense organs and constitution. Let us assume that this is true. Let us also assume that beliefs based on human senses are incompatible with the perceptual beliefs of some non-human animals. So we have a disagreement between us. It is, of course, true that we cannot argue with non-human animals. For many creatures, it’s not even clear they have language. However, we can easily imagine how things would appear from the point view of these non-human animals. We can thus look at things from both sides of the dispute. Once we do this, we can see that we have no non-question-begging reasons for accepting our perceptual beliefs as true rather than the perceptual beliefs of these non-human animals. So, in contrast to ordinary disagreements, we can do nothing to acquire reasons that are independent of the dispute and that would resolve the disagreement. We cannot appeal to our own perceptual beliefs, because the truth of these beliefs is under dispute. Nor can we appeal to the reliability of our senses, because those whose senses produce perceptual beliefs incompatible with our beliefs would not consider our senses reliable. Furthermore, it seems impossible to defend the reliability of our senses without appealing to perceptual beliefs at one point or another, which would also beg the question (Alston 1993, 12–25).

Sextus is aware that “self-satisfied Dogmatists” would not take the sensory appearances of “irrational” animals seriously. That is why he goes on to defend the senses and reasoning abilities of the so-called irrational animals. For example, dogs surpass us humans in their senses of smell and hearing, which makes them better at tracking wild beasts than we are, and they do not even lack expertise and virtue. Sextus concludes that there is no reason to consider animals of certain species to be less reliable in their senses and reasoning abilities than we are—or, at least, there are no reasons that are independent of our disagreement with them (PH 1.62–78).

When skeptics speak of rationally irresolvable disagreements, we should thus not think about cases in which the disagreement is irresolvable because one party is irrational, dishonest, incompetent, or lacking access to relevant evidence. If we had reasons to suspect those who disagree with us about these vices, we would not take our inability to persuade them seriously. It is only if we have no such reasons that a rationally irresolvable disagreement is supposed to induce suspension of judgment. A rationally irresolvable disagreement is an irresolvable disagreement between rational, competent, well-informed and sincere truth-seekers.4

There is thus an intuitively solid case for skepticism based on disagreement. About ordinary unresolved but resolvable cases of disagreement, we clearly have the intuition that participants should suspend belief until they obtain further evidence. Cases of irresolvable disagreement are similar, except that in such cases there is no further evidence that could resolve the disagreement because of the more systematic nature of the disagreement. However, this does not appear to be an epistemically relevant difference between the two kinds of cases. So if we should suspend belief in unresolved ordinary cases, we should also do so in more systematic irresolvable cases. Skeptics argue that there are such disagreements about every matter of inquiry. The role of the other modes of Agrippa is merely to help us to find such irresolvable disagreements.
4. Individualism, Justification, and Defeat
The demand to look at a disagreement from an impartial point of view means that I should give weight to my opponent’s and my own beliefs equally. If I were allowed to give more weight to my own beliefs, I could easily conclude that my opponent’s belief is false. From an impartial point view, this seems wrong, because giving more weight to that side of the disagreement, which happens to be mine, appears quite arbitrary; that the belief is mine is epistemically irrelevant.

When the skeptic concludes that I should suspend belief in the face of this sort of unresolved or irresolvable disagreement, it is appropriate to understand the term “should” epistemically.5 I should suspend belief because I am not justified in persisting in my belief. Is there any account of justification that would support this conclusion and explain the plausibility of the second premise of the skeptical argument?

A theory of justification that could explain the skeptical force of the argument from disagreement must explain why I should give equal weight to both my own and my opponent’s beliefs. Only one kind of theory appears to be able to do this; it is a theory that makes justification dialectical. According to this kind of theory, to be justified in my belief, I must be able to defend my belief to someone else. I can do this only if I have reasons or evidence that would convince the other party. This depends on both my beliefs and the beliefs of the other party. In an irresolvable disagreement, neither of us has sufficient reasons to convince the other, and neither is therefore justified in persisting in the disputed beliefs.

While the dialectical account of justification is social in nature, contemporary theories of justification are typically individualistic: they make the conditions of justification concern the individual subject, thereby restricting the justifying factors to the subject’s mental states or the causal sources of those states. For example, internalist evidentialism takes justification to be a function of the subject’s experiences and beliefs, and reliabilism takes it to be a function of the causal origin of those beliefs. According to these theories, disagreement and the beliefs of other subjects are simply irrelevant to justification.

However, this does not necessarily mean that these individualistic theories are unable to explain cases in which disagreement clearly seems to have skeptical consequences, such as the tie case; they just have to explain them in terms of those individualistic factors that can affect justification. Suspension of belief must be appropriate from one's own point of view.

This means that individualistic theories are not allowed to explain the skeptical consequences of disagreement by appealing to dialectical factors, such as question-beggingness or the impartial third-person perspective. I argue here that this makes it for them very difficult to explain the skeptical consequences of disagreement at all. To do so, I focus on a case in which those consequences are most intuitive, and argue that individualism cannot explain them even in this case.6

Let us take a case, Adam Elga’s horse race (2007, 486), about which we clearly have the intuition that disagreement prevents the participants from having justification for their beliefs:
We are to judge the same contest, a race between horse A and horse B. Initially, I think that you are as good as I at judging such races, and you think the same of me, but then we realize that we disagree: I believe that horse A won the race, and you believe that horse B won.
The intuition is that neither of us is justified in persisting in his belief: we should both give them up and seek further evidence. Individualistic accounts attempt to explain the intuition by appealing to defeaters. According to this story, I first hold a justified belief that horse A won, but when I learn that you disagree, I obtain a defeater for this belief: my belief that A won is no longer justified. How is this supposed to work?

My belief that you disagree does not alone have defeating power. I must also believe that you are as good as I at judging such things; I must believe that you are my epistemic peer. Two necessary conditions for epistemic peerage are typically offered:7


Evidential Equality

Two persons are evidentially equal relative to the question of whether p if and only if they are equally familiar with the evidence relevant to the question of whether p.


Cognitive Equality

Two persons are cognitively equal relative to the question of whether p if and only if they are equally competent or reliable in assessing the evidence relevant to the question of whether p.


So what is supposed to defeat my justification for believing that p is my justified beliefs that you are my epistemic peer and you believe that not-p. If I justifiably believe this, I am no longer justified in believing that p. This is how individualism attempts to explain the intuition that disagreement prevents justification in some cases.

A defeater is a belief or some other mental state that makes some other belief lose its justification. Assume that I am justified in believing that p on the basis of evidence e. I then form a new belief d. My belief d defeats my justification for believing that p if and only if e and d do not justify me in believing that p. As John Pollock (1986, 38–9) has taught us, there are two kinds of defeaters: (1) rebutting defeaters for my belief are my reasons to believe that my belief is false, (2) undercutting defeaters are my reasons to believe that my evidence does not support or indicate the truth of my belief.

Which kind of defeater is relevant in the case of peer disagreement? Let us take first the option that believed disagreement provides an undercutting defeater. Assume that e is good evidence for p, and that I believe that p on the basis of e. When I now learn that you, whom I take to be my epistemic peer, believe that not-p on the basis of e, I obtain evidence that e is not good evidence for p. So I seem to have an undercutting defeater for my belief that p. This is unclear, however, because, as Thomas Kelly (2005, 190) points out, I also have equally strong evidence that e is good evidence for p. Being epistemic peers and equally competent in evaluating the common evidence, your believing that not-p on the basis of e is evidence that e is not good evidence for p, and my believing that p on the basis of e is evidence that e is good evidence for p. My new total evidence thus includes the following:


  1. Evidence e.

  2. e is evidence for p.

  3. e is not evidence for p.

It now seems, as Kelly points out, that (2) and (3) cancel each other out, and I am left with my original evidence, which supports my belief that p. So disagreement provides no undercutting defeater for my belief that p.

One may object to Kelly’s conclusion and insist that I still have a defeater of some kind for my belief, because my conscious suspension of judgment about the question of whether e is good evidence for p is also an undercutting defeater for my belief that p. So undercutting defeaters need not be beliefs. Other attitudes, such as suspension of judgment, can also serve as a defeater: if I consider whether my evidence supports p and I must suspend judgment about the matter, I am not justified in believing that p. (Feldman 2006, 232–3; Bergmann 2005, 426)

However, this view seems to conflict with the widely accepted view that justification depends on total evidence. My total evidence includes both positive and negative evidence, and justification depends on what my evidence on balance supports. If parts of my positive and negative evidence cancel each other out, but the rest still provides sufficient evidence for p, I am justified in believing that p. This is what occurs here. My original evidence e was sufficient for justification before I learned about our disagreement. So why should it not be sufficient after the higher-order evidence provided by the disagreement is canceled?

If we wish to deny this, we must assume that my evidence e supports p only if I am justified in believing that e supports p. Then the defeat of this higher-order justification would defeat also the first-order justification for p.8 However, this suggestion is in danger of leading to an infinite regress.

Even if the regress could be avoided, another problem emerges. The whole idea that disagreement provides undercutting defeaters for our beliefs presupposes that you and I literally share the evidence, that we possess exactly the same evidence. Assuming evidence covers private perceptual experiences, memory experiences, and a priori intuitions, clearly we cannot literally share our evidence. I do not have your experiences, nor do you have mine. So if “evidential equality” means that we share or possess the same evidence, there can be no evidential equals and no epistemic peers, and individualism fails to explain our intuitions concerning disagreement.

“Familiarity with the evidence” could be understood more loosely. It is enough that we tell each other about our evidence; this provides us both with testimonial evidence about each other’s evidence. Feldman (2006, 233) says that evidence about evidence is evidence. He seems to suggest that my evidence about your evidence for not-p is also evidence for not-p. This may be true, but it is important to bear in mind that this is testimonial evidence; I cannot attain perceptual evidence in this way.

Once we accept this view about “sharing” evidence, disagreement can no longer provide undercutting defeaters. If we base our beliefs on different evidence, it is quite possible that my evidence supports my belief while your evidence supports your contrary belief. One of us simply has misleading evidence. That your evidence supports your belief is thus not a reason for me to doubt that my evidence supports mine.

When “familiarity with the evidence” is understood loosely, disagreement can provide, at most, rebutting defeaters for our beliefs. The idea is that, when I learn that you believe that horse B won on the basis of your evidence, I obtain evidence that horse B won, even though I do not share your evidence. So I have evidence not only for the proposition that horse A won, but also for the proposition that horse B won. The evidence for the latter proposition is a rebutting defeater for my belief that horse A won.

If this suggestion is to work, my evidence for horse B must be roughly equally strong as my evidence for horse A. We may assume that your perceptual evidence for your belief is equally strong as my perceptual evidence for mine, but this does not mean that I possess equally strong evidence for both propositions, because I do not share your perceptual evidence; I have, at most, testimonial evidence about it. It seems clear that this testimonial evidence for the proposition that horse B is the winner cannot be as strong as my direct perceptual evidence for the proposition that A won. So the former cannot be a rebutting defeater for the latter. Rather, it is the other way around.

If one thinks that my perceptual evidence is not strong enough to do the work alone, I have also other evidence that supports my belief over yours. I simply have many more reasons to doubt the proposition that B won than to doubt the proposition that A won. You may be lying or joking or teasing me when you claim that B won, or, assuming that you are sincere, there are still many reasons to suspect that you made an error. You may have gotten something in your eye, you may be drunk, or perhaps your eyesight is not as good as mine. I, in contrast, have no similar reasons to doubt my own belief that A won. The point is that there is a large number of possible mistakes that I cannot rule out in your case, but that I can rule out in mine. Surely I know that there was nothing in my eye and that I was not drunk, so it is epistemically more probable that you are the mistaken party.

Let us follow Jennifer Lackey (2010, 309–10) and call evidence that I have about my own experiences, beliefs, intentions, and reliability, but that I lack about yours, “personal evidence”. Taking this into account, my evidence for p includes:


(1) Perceptual evidence for p.

  1. Testimonial evidence that you have evidence for not-p.

  2. Personal evidence for p.

Because (2) is weaker than (1) and forms the only negative evidence regarding p, it is insufficient to defeat my justification for believing that p. This is so especially because we assumed that, before I learned about the disagreement, my perceptual evidence alone provided me with sufficient justification for believing that p. So it seems that individualism cannot even explain how disagreement can provide rebutting defeaters for our beliefs.

The situation would be different if I should somehow “bracket” a part of the positive evidence. Some philosophers, such as David Christensen (2007, 198) and Adam Elga (2007, 492), have argued that my evidence for p should be independent of the disagreement. If this is true, I should not count my perceptual evidence as a part of my evidence. However, this independence requirement is completely unmotivated and ad hoc if justification is understood individualistically: Why should I disregard part of my evidence as a response to disagreement if individualism is true?

The only motivation provided for the independence requirement is that my appeal to my original evidence would beg the question against you (Christensen 2007, 198). However, as I already pointed out, an individualist cannot appeal to dialectical factors, such as question-beggingness: I beg the question when I defend my belief by reasons that you find unacceptable. An individualist cannot appeal to such a fact, because individualism allows no role for your beliefs or for what you would be ready to accept in determining my justification for believing something. My justification in believing something depends on my beliefs and my point of view, not on yours.

We can conclude that only the dialectical conception of justification can explain the skeptical force of disagreement. Only it can provide sufficient weight for beliefs of both sides of a dispute and motivate the impartial point of view that, the skeptics insist, we must take in order to rationally resolve a dispute. I now argue that we also need the dialectical conception of justification for explaining the intuitiveness of Cartesian skepticism.
5. Cartesian Skepticism
Descartes begins his Discourse on the Method by pointing out that rationality is shared by all people:
Good sense is the best distributed thing in the world: for everyone thinks himself so well endowed with it that even those who are the hardest to please in everything else do not usually desire more of it than they possess. In this it is unlikely that everyone is mistaken. It indicates rather that the power of judging well and of distinguishing the true from the false—which is what we properly call ‘good sense’ or ‘reason’—is naturally equal in all men, and consequently that the diversity of our opinions does not arise because some of us are more reasonable than others but solely because we direct our thoughts in different paths and do not attend to the same things. (CSM I, 111)
A few passages later he says about philosophy in particular:
And, considering how many diverse opinions learned men may maintain on a single question —even though it is impossible for more than one to be true—I held as well-nigh false everything that was merely probable. (CSM I, 115)
Descartes tells us that he decided to give up the beliefs about which rational people disagreed.9 So here as well as in the passage from Rules for the Direction of the Mind quoted at the beginning of this paper, Descartes seems to think that rationally irresolvable disagreement is a source of skepticism.

Descartes begins his Meditations (CSM II, 12) somewhat differently. Here he refers to the large number of falsehoods that he had accepted as true, which is why he thought he should demolish everything and start again from the foundation. However, we should ask how Descartes discovered that there are so many falsehoods among his beliefs. The obvious answer is that he realized that there were disagreements between his current self and his earlier self, if not between himself and others, so some of his beliefs must be false.

It is true that, when Descartes then provides reasons for doubt in First Meditation (CSM II, 12–5), he formulates skeptical hypotheses that describe error-possibilities rather than disagreements, which may be considered the central difference between Cartesian and Pyrrhonian skepticism. However, there are three things that one should bear in mind. (1) A skeptical hypothesis describes a situation in which my belief is false. If another really accepted the hypothesis, we would have disagreement. Therefore, also Descartes could be conceived as raising doubts by appealing to disagreements. Such disagreements would just be possible rather than actual disagreements. (2) When Descartes then attempts to refute these hypotheses, he appeals to propositions that would be true even if the hypotheses in question were true.10 In other words, he tries to find reasons that are independent of the possible dispute, reasons that are acceptable even for someone who is inclined to accept or take seriously the hypothesis. (3) Although the meditations are conducted in his own mind, Descartes thinks we should all go through the same meditations in our minds. If we were to do this, we could rationally resolve our disagreements. So it may very well be that, though there is no mention of disagreement in Meditations, at least a part of the motivation for the enterprise derives from the need to resolve disagreements in ordinary life and philosophy. It is also possible to interpret the error-possibilities Descartes appeals to as possible disagreements. This would at least explain much of what he is doing when he attempts to rule out those possibilities.

It may therefore be just a superficial difference between Pyrrhonian skepticism and Cartesian skepticism that the former is based on disagreement and the latter on error-possibilities. At a deeper level, they may be both motivated by disagreements. Assuming this, the main difference is that, while the Pyrrhonian skeptic tries to show that there are actual disagreements that we cannot resolve, the Cartesian skeptic appeals to merely possible disagreements. This renders the job of the Cartesian skeptic much easier, as she need only formulate a couple of skeptical hypotheses and show that we cannot rule them out. The Pyrrhonian skeptic must convince us that actual disagreements exist for all our beliefs about reality, and this is much harder.11 On the other hand, for someone, such as Descartes, who wants to get rid of skepticism for good, focusing on Cartesian skepticism enjoys certain advantages: though it may be more difficult to refute Cartesian skepticism, if we can succeed in doing so, we can also refute Pyrrhonian skepticism. If we can resolve all possible disagreements, surely we can also resolve all actual disagreements, or, what is better, we could even prevent further disagreements to arise (cf. Descartes CMS II, 9).

If I am on the right track, then also Descartes and the Cartesian skeptic presuppose the dialectical conception of justification. It seems that this conception of justification alone can explain the motivation for the Cartesian project to find a certain foundation for knowledge and the intuitive plausibility of Cartesian skepticism—or so I now aim to show. It is only if we assume the dialectical conception of justification that we can explain the intuitive appeal of skepticism relying on skeptical hypotheses concerning error-possibilities.

The standard formulations of the Cartesian skeptical argument in contemporary epistemology rely on the brain-in-a-vat hypothesis, which says that I am a disembodied brain kept alive in a vat and stimulated by a powerful computer to produce experiences similar to those I now have. The skeptic uses this hypothesis to cast doubt on all my beliefs about the external world, such as my belief that I have hands. She proposes the following skeptical argument:




  1. If I am justified in believing that I have hands, I am justified in believing that I am not a handless brain in a vat.

  2. I am not justified in believing that I am not a handless brain in a vat.

  3. Therefore, I am not justified in believing that I have hands.

This argument is valid, and its premises seem intuitively plausible. The first premise is based on a very plausible closure principle:


Justification Closure

If I am justified in believing that p, and I know that p entails q, I am justified in believing that q.


A few philosophers may have denied similar principles formulated in terms of knowledge, but hardly anyone has denied the Justification Closure principle.12 Because the first premise is just an instance of it (or can easily be formulated as an instance of it), the principle explains very well the plausibility of this premise.

The second premise seems plausible, because if I am a handless brain in a vat, I have exactly the same experiences that I have now. So it seems that my sense experiences do not justify me in believing that I am not a brain in a vat. Because the skeptical hypothesis is contingent, neither does there seem to be any a priori justification for denying it. Therefore, there seems to be neither empirical nor a priori justification for denying the skeptical hypothesis. The second premise also appears to be true.

However, individualistic theories of justification have serious difficulties in explaining the intuitive plausibility of such skeptical reasoning. Most of them are committed to the so-called Moorean response to skepticism, which sees nothing wrong in the following reasoning:


  1. I have hands.

  2. If I have hands, I am not a handless brain in a vat.

  3. Therefore, I am not handless brain in a vat.

It follows from standard individualistic theories of justification—both reliabilist and evidentialist—that I may very well have justification for both premises, and that this justification then transfers to the conclusion. Such theories typically allow my sense experience as such to be sufficient to provide me with prima facie justification for believing that I have hands. Reliabilism requires only that sense perception be, in fact, reliable, and internalist evidentialism requires that my experience support my belief. The second premise is an obvious necessary truth that I am justified in believing a priori. Finally, deductive inference transmits justification of the premises to the conclusion.

To avoid such an inference, which is intuitively too easy a way of attaining justification for denying the skeptical hypothesis, a few philosophers have suggested that we accept the following principle for perceptual justification:
Perceptual Justification

In order to have perceptual justification for believing that p, we must have independent justification for believing that our perceptual faculties are reliable.


Because I know that my perceptual faculties cannot be reliable if I am a brain in a vat, in order to have perceptual justification for believing anything, I must also have independent justification for denying that I am a brain in a vat (Wright 2002, White 2006).

The principle of Perceptual Justification blocks the Moorean inference. In order to have perceptual justification for believing that I have hands, I must have independent justification for believing that I am not a brain in a vat. So I cannot attain justification for denying the skeptical hypothesis by inferring from my perceptual beliefs, as these beliefs are justified only if I am already independently justified in denying the skeptical hypothesis. The principle of Perceptual Justification makes the Moorean inference viciously circular and thus explains what is wrong with it.

The Principle of Perceptual Justification may explain the intuitive plausibility of Cartesian skepticism, and some have suggested that the skeptic presupposes some such principle (Pryor 2000, 528; Schiffer 2004, 165). However, why should we accept the principle? It does not itself seem intuitively so plausible. Rather, it seems to be an ad hoc principle that is only meant to block the Moorean inference and to explain the plausibility of skepticism. Apparently, we need an independent motivation for it; the question is: What could that be?

We have the same problem as we had in the Pyrrhonian case. The individualistic accounts of justification need to be supplemented with ad hoc independence principles to explain the intuitiveness of skepticism. However, if we accept the dialectical conception of justification, no such special principles are necessary.

The dialectical conception of justification provides a principled explanation for the force of Cartesian skeptical reasoning. When I claim (to know) that I have hands, the skeptic challenges me to defend my claim by asking how I know that I am not a handless brain in a vat, and adds that if I don’t know this, I don’t know that I have hands.13 Now, I cannot respond by claiming that I see my hands and cannot therefore be a handless brain. This would beg the question against the skeptic’s hypothesis. The skeptic would not accept this as evidence against it. Nor can I answer the challenge by claiming that it at least appears to me that I have hands. This clearly does not answer the challenge, however, because it would appear this way to me even if I were a handless brain in a vat. Neither response would be a dialectically effective defense of my belief. According to the dialectical conception of justification, I cannot be justified in my belief without having such a defense available.
6. Dialectical Justification and Ordinary Epistemic Practices
I have tried to show that we can explain the appeal of both Pyrrhonian and Cartesian skepticism if we understand the role of disagreement in skeptical reasoning. I argued that this role presupposes a conception of justification that I call “dialectical”. In other words, I first showed that certain Pyrrhonian and Cartesian skeptical arguments are intuitively sound, and argued then that we can explain this only if we accept the dialectical conception of justification. Those who reject this conception fail either to explain the intuitiveness of skepticism or to see it at all. Finally, I must provide some independent reasons for accepting the dialectical conception. Otherwise, we would not fully understand the intuitive appeal of skeptical reasoning based on disagreement.

I claim that our ordinary practices related to attributions of knowledge and justified belief support the dialectical conception of justification. First of all, it is very natural to understand certain questions of knowledge and justification dialectically. When you ask me how I know that p or what justifies me in believing that p, it is natural for me to understand these questions as challenges. You ask me to defend my claim, to give reasons for it. If I cannot do this, you think that I do not really know.14 This suggests that knowledge requires justification, and that justification requires the ability to defend one's belief when appropriately challenged. So our practice seems to support the following dialectical conception of justification:


Dialectical Conception of Justification

S is justified in believing that p if and only if S can defend p against appropriate challenges.


This conception explains the skeptical significance of disagreement, because it seems that a disagreement always appropriately challenges a belief. To return to a previous case, when I tell you that a certain tie is green, you challenge me to defend my claim if you believe that the tie is blue and merely appears to be green. Appropriately, you ask me how I know that the tie is not a blue tie that appears green. Clearly, I cannot answer the challenge by claiming that I see that the tie is green or that it appears to be green even if both claims were true. Both responses would beg the question. If I had nothing else to say, it would be quite appropriate for you to conclude that I am not justified in believing that the tie is green.

There is also a very plausible hypothesis about the point of knowledge attributions that supports the view that knowledge requires dialectical justification. Edward Craig (1990, 10) suggests that the concept of knowledge is needed to pick out dependable informants: the person who knows makes a good informant. We can now ask what conditions our concept of knowledge should have in order to serve this purpose. We can answer the question by trying to determine what properties we would want our informants to have.

Clearly, we want our informants to have true beliefs about the questions we are interested in, but, as Craig (1990, 18–9) notes, we also want them to have a property by which we can identify them, and this property must be reliably connected to truth. As some reliabilists have pointed out, this idea supports a reliabilist account of knowledge: we clearly seem to choose informants by virtue of their reliability. Someone with a reliable vision and standing on a hill is a good informant for what happens in the valley.15

This seems to be right. Knowledge requires a true and reliably formed belief, but it requires more. We often are not in a position to assess the reliability of potential informants or we may even have doubts about the truth or reliability of their beliefs. In such cases, it would be useful to ask them how they know what they claim to know and whether they could defend their beliefs to us. People who can defend their beliefs and respond to our challenges make better informants.16 So Craig’s hypothesis about the point of knowledge attribution supports the view that knowledge requires dialectical justification as well as reliability.

It seems that our ordinary epistemic practices support the dialectical conception of justification, and it is therefore appropriate for both the Pyrrhonian and Cartesian skeptics to presuppose it in their skeptical arguments. Our practices explain the intuitive force of these arguments. However, we do also have the intuition that we have knowledge and justified beliefs about many things. How do we explain this intuition? Is there any way to avoid skepticism?

This paper aims to interpret both Pyrrhonian and Cartesian skepticism in a way that gives disagreement an essential role and to explain the intuitive appeal of such skepticism. It was not intended to respond to skepticism, but let me say how I think we can avoid radical skepticism: our ordinary practices seem to support the view that attributions of knowledge and justified belief are context-sensitive (Greco 2008, 432–3; Henderson 2009). Which challenges are appropriate depends on the context of attribution. In ordinary contexts, where I evaluate someone as a potential informant, it is of course my own challenges and doubts that must be answered. I don’t care whether my informant can answer the challenge that she might be a brain in a vat. I know full well that she is not; I can rule this out myself. So when people share information in ordinary contexts, they can often meet each other's challenges and make true knowledge attributions, although in skeptical contexts they may not be able to do this. This is not to say that we are safe from skepticism even in ordinary contexts. There are many topics, such as politics, philosophy, and religion, about which disagreements abound. It may be that attributions of knowledge about these matters are rarely true, so this is where skepticism based on disagreement has real bite.17


References
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––––––. (2010). “Peer Disagreement and Higher-Order Evidence,” 111–74 in R. Feldman & T. Warfield, Disagreement. Oxford: Oxford University Press.

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––––––. (2011a). “Disagreement, Skepticism, and the Dialectical Conception of Justification,” International Journal for the Study of Skepticism 1: 3–17.

––––––. (2011b). “Skepticism and Disagreement,” 203–15 in D. E. Machuca (ed.), Pyrrhonism in Ancient, Modern, and Contemporary Philosophy. Dordrecht: Springer.

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* Forthcoming in Disagreement and Skepticism, ed. by D. Machuca, Routledge, London.


1 For the Sextan and Cartesian texts, I have used the translations in Sextus Empiricus (2000) and Descartes (1984, 1985).

2 It is true that there are externalist views about testimony that take the testifier's knowledge or justified belief to be relevant to testimonial knowledge or justified belief, but it is not clear what these views would tell about disagreement. Indeed, current discussion of peer disagreement is largely conducted in evidentialist terms.

3 However, see Lammenranta (2008, 10–3).

4 Another contemporary philosopher who understands the skeptical significance of disagreement in a roughly similar way is Gary Gutting (1982, 79–108), who discusses religious disagreements.

5 For a motivation of this normative interpretation of the modes, see Lammenranta (2008, 13–5; 2011b, 204–6).

6 This argument is modified from an argument in Lammenranta (2011a).

7 See Kelly (2005, 174–5), Christensen (2007, 188–9), and Lackey (2010).

8 Jonathan Matheson (2009, 274–6) defends this view.

9 It is not clear why Descartes says that he would take merely probable propositions to be false and thus accept their denials, when suspension of judgment seems to be more appropriate.

10 The first premises—“I exist” and “I think”—from which Descartes starts his proof that there is a benevolent God and no evil demon, would be true whether the evil demon hypothesis were true or false. So they could provide non-question-begging reasons for believing that the hypothesis is false.

11 In some passages, Sextus is forced to appeal to disagreements that may occur in the past or future or in some unknown parts of the earth. See Machuca (2011) for the relevant passages and the discussion whether these passages are in conflict with Sextus' own definition of skepticism.

12 The principle concerns propositional justification that does not entail that I really believe what I am justified in believing. For doxastic justification, we need a more complex principle.

13 Because we have assumed that merely possible disagreements are relevant, we need not assume that there is any actual skeptic. I can simply go through the skeptical dialectic in my own mind.

14 Kaplan (1991) defends this view against Chisholm, and Alston (1989, 236) suggests that our concept of justification has developed from our practice of challenging and defending beliefs.

15 See, e.g., Sosa (1991, 275) and Prichard (2009, 80–5). The example is Pritchard’s.

16 Fricker (2008, 41) notes that the capacity to give reasons is an important indicator property of a good informant, not discussed by Craig.

17 I wish to thank Nathan Ballantyne, Diego Machuca and Duncan Pritchard for their very useful comments on an earlier draft of this paper.



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