Teaching Theory of Knowledge

2. Skepticism

      This module explores the historical challenge of skepticism and some of the responses to it. It is divided into three sections. The first is devoted to Sextus Empiricus, Montaigne and what might be called the positive defense of skepticism. The second deals with the great expositions of skepticism in modern philosophy found in the writings of Descartes and Hume. The concluding section presents Reid's naturalistic reply to the skepticism in Descartes and Hume.

2.1 Sextus Empiricus and Michel de Montaigne.
2.2 Rene Descartes.
2.3 David Hume.
2.4 Thomas Reid.

2.1 Sextus Empiricus and Michel de Montaigne. Sextus writings are a late (2nd century A.D.) compilation of doctrines handed down (and probably altered and distorted) from thinkers such as Pyrrho, Timon, Aenesidemus and members of various academies (Arcesilaus, Carneades), etc. The orientation to the topic of knowledge is far closer to our own than that of any of the materials just discussed. Sextus is largely concerned with the question of whether it is possible to find evidence, argument, or any other sort of justification which provides a better reason to think a belief is objectively true (i.e., accurately represents anything external to and independent of the subject's experience, thought, and other mental processes) than to think it is not. Outlines of Pyrrhonism contains a catalogue of methods (tropoi) for use in balancing off any considerations which seem to show that a belief is true against equally plausible defeating or undermining considerations. Sextus' view is that although we must conduct our lives on the basis of how things appear to us, it is both harmful and unnecessary to assent to beliefs about how things really are. The methods for defeating or undermining putative justifications are intended to show that assent is both unnecessary and irrational. They include (and are the historical originals of) almost every skeptical argument with which we are familiar. Burnyeat's is the best commentary available in English.


Sextus Empiricus. Outlines of Pyrrhonism. In Hallie, P. (ed.), Etheridge, S. G. (tr.), Sextus Empiricus: Skepticism, Man and God: Selections from the Major Writings. Middletown, Conn: Wesleyan University Press, 1964.

Burnyeat, M. "Can the Skeptic Live His Skepticism?" In Burnyeat (ed.), The Skeptical Tradition. University of California Press, 1983. This volume includes a number of other useful essays, including an introduction by Burnyeat, and papers by Sedley, Frede, and Striker.

Scholfield, Burnyeat, Barnes (eds.). Doubt and Dogmatism: Studies in Hellenistic Epistemology, Oxford, 1980. A number of other valuable papers are collected in this volume.

      One aspect of Pyrrhonism that is often overlooked by contemporary epistemologists is its intense practical concern. Far from the destructive, life-threatening paralysis that many people associate with skepticism, the agoge, or way of life, that Sextus described was one that stressed tranquillity, stability, quietude, ataraxia. Unsettled by the myriad opinions s/he finds on virtually every subject, the skeptic sets out "to philosophize with the object of passing judgment on the sense impressions and ascertaining which of them are true and which false, so as to attain quietude thereby." Because the search produces the recognition that there are arguments on all sides and that they tend to balance each other, s/he suspends judgment. But ironically, that suspension of judgment (epoche) produces just the state (ataraxia) s/he had sought in the initial inquiry. S/he finds what s/he wanted only after abandoning the search. Or, as Sextus puts it, "the Skeptics were in hopes of gaining quietude by means of a decision regarding the disparity of the objects of sense and of thought, and being unable to effect this they suspended judgment; and they found that quietude, as if by chance, followed upon their suspense, even as a shadow follows its substance" (Outlines of Pyrrhonism I, xii).

      In contrast, the Dogmatists, with their constant struggle to attain truth, are continually "disturbed". They make conflicting claims about the hidden natures of things, and then feel compelled to engage in endless wrangling over which claim is true. But since the claims are all about non-evident qualities and essences, there is no decisive factual criterion for resolving the dispute. "For all we know," any of the claims might be true. But since we can never say with certainty which -- if any -- is true, the only appropriate response is to withhold belief from all claims that go beyond what is immediately evident to us.


Annas, J. and Barnes, J. The Modes of Skepticism: Ancient Texts and Modern Interpretations. Cambridge, Mass.: Cambridge University Press, 1985.

Chisholm, R. "Sextus Empiricus and Modern Empiricism." Philosophy of Science 8 (1941).

Hallie, P. The Scar of Montaigne: An Essay in Personal Philosophy. Middletown, Conn: Wesleyan University Press, 1966. See especially chapter 2, "Doubt and Man."

      Michel de Montaigne (1533-1592) uses Pyrrhonism with full recognition of its potential value as a therapy, a means for curing man of his various pretensions and restoring him to his natural common sense. When he writes, "I can easily maintain an opinion, but not choose one" (Essays Book II, chapter 17), he is not simply revealing some particular weakness of his own. Rather, in the classical Pyrrhonic manner, he is exposing a deficiency at the very heart of philosophy, a weakness that affects all choices, all beliefs, all justifications. The problem is not with justifying beliefs, but with the claim that the justifications somehow guarantee the truth of those beliefs. For Montaigne, no set of reasons can be decisive in the sense required to guarantee truth, but far from being a fatal conclusion, this deficiency is seen as an invitation to mental poise, balance, a peaceful existence, the suspension of all discords. If the invitation turns out also to be a denial of the possibility of rational solutions to disputes over the limits of justification or conflicting criteria (in the sense that anything less than guaranteed truth is not acceptable, no matter how reasonable it may seem), so much the worse for philosophy and its preoccupation for truth.


Hallie, P. The Scar of Montaigne. See especially chapter 3 for a detailed account of how Montaigne uses Pyrrhonic techniques in his famous "Apology for Raymond Sebond."

Montaigne, M. The Complete Essays of Montaigne. D. Fame (tr.). Stanford: Stanford University Press, 1948.

2.2 Rene Descartes. In the first Meditation, Descartes sets forth three skeptical arguments:

  1. the argument from sense perception,
  2. the dream argument, and
  3. the evil demon argument.
Reconstruction and critical evaluation of these arguments provide exciting and engaging topics for student papers on skepticism. In the second Meditation, Descartes argues that there is at least one thing of which he can be certain, namely his own existence. From there, he goes on to provide a putative foundation for human knowledge. Two important issues for contemporary epistemology arise here:
  1. epistemic circularity: in order to demonstrate the possibility of knowledge, Descartes appeals to knowledge of God's existence; and
  2. epistemic principles: Descartes introduces "clear and distinct perception" as a criterion for knowledge.
A good discussion of these issues is provided by Van Cleve's article, "Foundationalism, Epistemic Principles, and the Cartesian Circle." Van Cleve suggests a solution to the problem of the Cartesian Circle and connects the problem to the contemporary debate between coherentists and foundationalists.


Descartes. Meditations.

Van Cleve, J. "Foundationalism, Epistemic Principles and the Cartesian Circle." Philosophical Review 88 (1979), 55-91.

Frankfurt, H. G. Demons, Dreamers, and Madmen: The Defense of Reason in Descartes' Meditatlons. Indianapolis: Bobbs-Merrill 1970.

Hooker, M. Descartes: Critical and Interpretative Essays. Baltimore: Johns Hopkins University Press, 1978.

Kenny, A. Descartes: A Study of His Philosophy. New York: Random House, 1968.

Williams, B. Descartes: the Project of Pure Enquiry. London: Penguin Books, 1978.

Wilson, M. Descartes. London: Routledge & Kegan Paul, 1978.

2.3 David Hume. While Descartes' method of doubt is a method for discovering the foundations of knowledge, Hume's skepticism is designed to undermine ordinary claims to knowledge. According to Hume, knowledge of the external world can be established neither deductively nor inductively. Induction rests on the presupposition that nature is uniform, that the future will resemble the past. But this presupposition cannot be justified. Nor can knowledge of the external world be gained through deduction, for information about physical objects goes beyond what is immediately perceived. Thus matters of fact and existence cannot be objects of knowledge, because the inference which legitimizes the transition from past experience to future occurrence is not rationally defensible.

      This apparently negative (skeptical) conclusion represents only half of the project, however. If skepticism had been the aim of the work, as some commentators suggest, then Hume should have stopped at the end of Book I of the Treatise: the account he gives of why reason and experience fail to justify the truth of simple matters of fact is quite powerful. But Hume does not want to deny the sureness of our claims that the sun will rise tomorrow, or that bread nourishes. In order to preserve those convictions, he is quite willing to concede that what guarantees them is not an intellectual certainty at all, and hence, not sufficient to justify our claims to know these matters of fact. But Hume insists that it is a certainty nonetheless, and he devotes a great deal of space in the Treatise and the Inquiry to giving an account of the nature and source of that certainty.

      Hume discusses skepticism in the Treatise, Book I, Part IV, and in the Inquiry, section XII. Hume's theory of cause and effect, that is, his attack on the idea of a necessary connection between cause and effect can be found in the Treatise, Book I. Part III, Section XIV, and the Inquiry, VII, Part II.


Hume, D. A Treatise of Human Nature.

Hume, D. Inquiry Concerning Human Understanding.

Stroud, B. Hume. London: Routledge & Kegan Paul, 1977.

Chappell, V. (ed.). Hume. New York Doubleday & Company, Inc., 1966.

Bennett, J. Locke, Berkeley, Hume: Central Themes.

2.4 Thomas Reid. In both the Inquiry and the Essays Reid sees himself as responding to the Modern skeptical tradition beginning with Descartes and culminating with Hume. Reid contends that skepticism is the inevitable result of these philosophers' commitment to the erroneous theory that the immediate object of thought is always some idea or image in the mind, and consequendy, all knowledge of the external world is mediated via these ideas.

      Reid lays the groundwork for his positive epistemology by arguing that consistency requires that we regard all our cagnitive faculties as equally trustworthy (or untrustworthy). Thus, it is illegitimate for the skeptic to use one faculty to attempt to confute another faculty. Since we all do in fact trust our faculties, we all do implicitly regard them as trustworthy. He contends that knowledge has a foundational structure which is ultimately grounded in the first principles of our constitution. These first principles confer justification on the beliefs that they give rise to.

      Reid's epistemology turns on the following question: How do we know the first principles are true? His answer to this question is a bit complicated. First of all, first principles do not admit of direct proof because any purported proof would have to proceed in terms of the reasoning faculty which is itself grounded in first principles. Hence, any reasoning intended to prove the truth of the first principles already takes for granted the thing to be proved. Of course, it is also impossible to prove that the faculties are fallacious since such a proof would equally depend on the trustworthiness of the reasoning faculty.

      The final reason for trusting our faculties and the first principles according to which they work is that the following is a first principle of our nature:

That the natural faculties by which we distinguish truth from error are not fallacious.

This meta-first principle (hence, 'MP') must, like all first principles, be regarded as innocent until proven guilty. Thus in addition to its foundational structure, knowledge for Reid rests on our own natural conviction of our reliability. The skeptic cannot get a foot in the door, since even s/he must assume, if s/he is consistent, the reliability of our faculties in the attempt to prove them unreliable.


Reid, T. Inquiry and Essays. Beanblossom and Lehrer (eds.).

All of the selections taken from Reid's An Inquiry into the Human Mind on the Principles of Common Sense.

The following selections from Reid's Essays on the Intellectual Powers are most pertinent to his epistemology:

Essay One, Chapters 1 and 2
Essay Two, Chapters 5, 14 and 20 (Chapter 20 is especially important)
Essay Four, Chapter 2
Essay Six, Chapters 1, 4, 5 and 6

Van Cleve, J. "Foundationalism, Epistemic Principles, and the Cartesian Circle," Philosophical Review, 1979.

Vernier, P. "Thomas Reid on the Foundation of Knowledge and His Answer to the Skeptic." In Barker and Beauchamp (eds.), Thomas Reid: Critical Interpretations.

Cross Reference

For a discussion of the role skepticism plays in contemporary epistemology, see also "Skepticism" in the "Contemporary Sources" section.