Skepticism, History of

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Skepticism (also spelled "Scepticism") is the philosophical attitude of doubting knowledge claims set forth in various areas. Skeptics have challenged the adequacy or reliability of these claims by asking what they are based upon or what they actually establish. They have raised the question whether such claims about the world are either indubitable or necessarily true, and they have challenged the alleged grounds of accepted assumptions. Practically everyone is skeptical about some knowledge claims; but the skeptics have raised doubts about any knowledge beyond the contents of directly felt experience. The original Greek meaning of skeptikos was "an inquirer," someone who was unsatisfied and still looking for truth.

From ancient times onward skeptics have developed arguments to undermine the contentions of dogmatic philosophers, scientists, and theologians. The skeptical arguments and their employment against various forms of dogmatism have played an important role in shaping both the problems and the solutions offered in the course of western philosophy. As ancient philosophy and science developed, doubts arose about basic accepted views of the world. In ancient times skeptics challenged the claims of Platonism, Aristotelianism, and Stoicism, and in the Renaissance those of Scholasticism and Calvinism. After René Descartes, skeptics attacked Cartesianism and other theories justifying the "new science." Later, a skeptical offensive was leveled against Kantianism and then against Hegelianism. Each skeptical challenge led to new attempts to resolve the difficulties. Skepticism, especially since the Enlightenment, has come to mean disbeliefprimarily religious disbeliefand the skeptic has often been likened to the village atheist.

Various Senses and Applications

Skepticism developed with regard to various disciplines in which men claimed to have knowledge. It was questioned, for example, whether one could gain any certain knowledge in metaphysics (the study of the nature and significance of being as such) or in the sciences. In ancient times a chief form was medical skepticism, which questioned whether one could know with certainty either the causes or cures of diseases. In the area of ethics, doubts were raised about accepting various mores and customs and about claiming any objective basis for making value distinctions. Skepticisms about religion have questioned the doctrines of different traditions. Certain philosophies, like those of David Hume and Immanuel Kant, have seemed to show that no knowledge can be gained beyond the world of experience and that one cannot discover the causes of phenomena. Any attempt to do so, as Kant argued, leads to antinomies, contradictory knowledge claims. A dominant form of skepticism, the subject of this article, concerns knowledge in general, questioning whether anything actually can be known with complete or adequate certainty. This type is called epistemological skepticism.

Kinds of epistemological skepticism can be distinguished in terms of the areas in which doubts are raised; that is, whether they be directed toward reason, toward the senses, or toward knowledge of things-in-themselves. They can also be distinguished in terms of the motivation of the skepticwhether he or she is challenging views for ideological reasons or for pragmatic or practical ones to attain certain psychological goals. Among the chief ideological motives have been religious or antireligious concerns. Some skeptics have challenged knowledge claims so that religious ones could be substitutedon faith. Others have challenged religious knowledge claims in order to overthrow some orthodoxy. Kinds of skepticism also can be distinguished in terms of how restricted or how thoroughgoing they arewhether they apply only to certain areas and to certain kinds of knowledge claims or whether they are more general and universal.

Ancient Skepticism

Historically, skeptical philosophical attitudes began to appear in pre-Socratic thought. In the fifth century BCE, the Eleatic philosophers, known for reducing reality to a static One, questioned the reality of the sensory world, of change and plurality, and denied that reality could be described in the categories of ordinary experience. On the other hand, the Ephesian philosopher of change Heraclites and his pupil Cratylus thought that the world was in such a state of flux that no permanent, unchangeable truth about it could be found; and Xenophanes, a wandering poet and philosopher, doubted whether man could distinguish true from false knowledge.

A more developed skepticism appeared in some of Socrates' views and in several of the Sophists. Socrates, in the early Platonic dialogues, was always questioning the knowledge claims of others; and in the Apology, he said that all that he really knew was that he knew nothing. Socrates' enemy, the Sophist Protagoras, contended that man is the measure of all things. This thesis was taken as a kind of skeptical relativism: no views are ultimately true, but each is merely one man's opinion. Another Sophist, Gorgias, advanced the skeptical-nihilist thesis that nothing exists; and if something did exist, it could not be known; and if it could be known, it could not be communicated.

academic skepticism

Academic skepticism, so-called because it was formulated in the Platonic Academy in the third century BCE, developed from the Socratic observation, "All I know is that I know nothing." Its theoretical formulation is attributed to Arcesilas (c. 315241 BCE) and Carneades (c. 213129 BCE), who worked out a series of arguments, directed primarily against the knowledge claims of the Stoic philosophers, to show that nothing could be known. As these arguments have come down to us, especially in the writings of Cicero, Diogenes Laertius, and Saint Augustine, the aim of the Academic skeptical philosophers was to show, by a group of arguments and dialectical puzzles, that the dogmatic philosopher (that is, the philosopher who asserted that he knew some truth about the real nature of things), could not know with absolute certainty the propositions he said he knew. The Academics formulated a series of difficulties to show that the information we gain by means of our senses may be unreliable, that we cannot be certain that our reasoning is reliable, and that we possess no guaranteed criterion or standard for determining which of our judgments is true or false.

The basic problem at issue is that any proposition purporting to assert some knowledge about the world contains some claims that go beyond the merely empirical reports about what appears to us to be the case. If we possessed any knowledge, this would mean for the skeptics, that we knew a proposition, asserting some non-empirical, or trans-empirical claim, which we were certain could not possibly be false. If the proposition might be false, then it would not deserve the name of knowledge, but only that of opinion, i.e., that it might be the case. Since the evidence for any such proposition would be based, according to the skeptics, on either sense information or reasoning, and both of these sources are unreliable to some degree, and no guaranteed or ultimate criterion of true knowledge exists, or is known, there is always some doubt that any non-empirical or trans-empirical proposition is absolutely true and hence constitutes real knowledge. As a result, the Academic skeptics said that nothing is certain. The best information we can gain is only probable and is to be judged according to probabilities. Hence, Carneades developed a type of verification theory and a type of probabilism that is somewhat similar to the theory of scientific '"knowledge" of present-day pragmatists and positivists.

The skepticism of Arcesilas and Carneades dominated the philosophy of the Platonic Academy until the first century before Christ. In the period of Cicero's studies, the Academy changed from skepticism to the eclecticism of Philo of Larissa and Antiochus of Ascalon. The arguments of the Academics survived mainly through Cicero's presentation of them in his Academica and De Natura Deorum, and through their refutation in St. Augustine's Contra Academicos, as well as in the summary given by Diogenes Laertius. The locus of skeptical activity, however, moved from the Academy to the school of the Pyrrhonian skeptics, which was probably associated with the Methodic school of medicine in Alexandria.

the pyrrhonian school

The putative father of Greek skepticism is Pyrrho of Elis (c. 360c. 272 BCE) and his student Timon (c. 315225 BCE). He avoided committing himself to any views about what was actually going on and acted only according to appearances. In this way he sought happiness or at least mental peace. The stories about Pyrrho that are reported indicate that he was not a theoretician, but rather a living example of the complete doubter, the man who would not commit himself to any judgment that went beyond what seemed to be the case. His interests seem to have been primarily ethical and moral, and in this area he tried to avoid unhappiness that might be due to the acceptance of value theories and to judging according to them. If such value theories were to any degree doubtful, accepting them and using them could only lead to mental anguish.

Pyrrhonism, as a theoretical formulation of skepticism, is attributed to Aenesidemus (c. 10040 BCE). The Pyrrhonists considered that both the Dogmatists and the Academics asserted too much, one group saying, "Something can be known," the other that "Nothing can be known." Instead, the Pyrrhonians proposed to suspend judgment on all questions on which there seemed to be conflicting evidence, including the question whether or not something could be known.

Building on the type of arguments developed by Arcesilas and Carneades, Aenesidemus and his successors put together a series of "Tropes" or ways of proceeding to bring about suspense of judgment on various questions. In the sole surviving texts from the Pyrrhonian movement, those of Sextus Empiricus, these are presented in groups of ten, eight, five, and two tropes, each set offering reasons why one should suspend judgment about knowledge claims that go beyond appearances. The Pyrrhonian skeptics tried to avoid committing themselves on any and all questions, even as to whether their arguments were sound. Skepticism for them was an ability, or mental attitude, for opposing evidence both pro and con on any question about what was nonevident, so that one would suspend judgment on the question. This state of mind then led to a state of ataraxia, quietude, or unperturbedness, in which the skeptic was no longer concerned or worried about matters beyond appearances. Skepticism was a cure for the disease called Dogmatism or rashness. But, unlike Academic skepticism, which came to a negative dogmatic conclusion from its doubts, Pyrrhonian skepticism made no such assertion, merely saying that skepticism is a purge that eliminates everything including itself. The Pyrrhonist, then, lives undogmatically, following his natural inclinations, the appearances of which he is aware, and the laws and customs of his society, without ever committing himself to any judgment about them.

The Pyrrhonian movement flourished up to about 200 CE, the approximate date of Sextus Empiricus, and flourished mainly in the medical community around Alexandria as an antidote to the dogmatic theories, positive and negative, of other medical groups. The position has come down to us principally in the writings of Sextus Empiricus in his Hypotyposes (Outlines of Pyrrhonism) and the larger Adversus mathematicos, in which all sorts of disciplines from logic and mathematics to astrology and grammar are subjected to skeptical devastation. In his Outlines of Pyrrhonism and Adversus mathematicos, Sextus presented the tropes developed by previous Pyrrhonists. The ten tropes attributed to Aenesidemus showed the difficulties to be encountered in ascertaining the truth or reliability of judgments based on sense information, owing to the variability and differences of human and animal perceptions.

Other arguments raised difficulties in determining whether there are any reliable criteria or standardslogical, rational, or otherwisefor judging whether anything is true or false. To settle any disagreement, a criterion seems to be required. Any purported criterion, however, would appear to be based on another criterion, thus requiring an infinite regress of criteria, or else it would be based upon itself, which would be circular. Sextus offered arguments to challenge any claims of dogmatic philosophers to know more than what is evident; and in so doing he presented in one form or another practically all of the skeptical arguments that have ever appeared in subsequent philosophy.

Sextus said that his arguments were aimed at leading people to a state of ataraxia (unperturbability). People who thought that they could know reality were constantly disturbed and frustrated. If they could be led to suspend judgment, however, they would find peace of mind. In this state of suspension they would neither affirm nor deny the possibility of knowledge but would remain peaceful, still waiting to see what might develop. The Pyrrhonist did not become inactive in this state of suspense but lived undogmatically according to appearances, customs, and natural inclinations.

Medieval Skepticism

Pyrrhonism ended as a philosophical movement in the late Roman Empire, as religious concerns became paramount. In the Christian Middle Ages the main surviving form of skepticism was the Academic, described in St. Augustine's Contra academicos. Augustine, before his conversion, had found Cicero's views attractive and had overcome them only through revelation. With faith, he could seek understanding. Augustine's account of skepticism and his answer to it provided the basis for medieval discussions.

In Islamic Spain, where there was more contact with ancient learning, a form of antirational skepticism developed among Muslim and Jewish theologians. Al-Ghazālī, an Arab theologian of the eleventh and early twelfth centuries, and his Jewish contemporary Judah ha-Levi (c. 1075/c. 1085c. 1141), who was a poet and physician as well as a philosopher, offered skeptical challenges (much like those later employed by the occasionalist Nicolas Malebranche and by David Hume) against the contemporary Aristotelians in order to lead people to accept religious truths in mystical faith. This view that truth in religion is ultimately based on faith rather than on reasoning or evidencewhat is known as fideismalso appears in the late Middle Ages in the German cardinal and philosopher Nicolaus of Cusa's advocacy of learned ignorance as the way to religious knowledge.

Another line of thinking that includes skeptical elements was that of the followers of William of Ockham (12851347) in the fourteenth century, who were exploring the consequences of accepting divine omnipotence and a divine source for all knowledge. They examined puzzles about whether God could deceive mankind, regardless of the evidence, and could make all human reasoning open to question.

Modern Skepticism

Modern Skepticism emerged in part from some of the Ockhamite views but mainly from the rediscovery of the skeptical classics. Very little of the Pyrrhonnian tradition had been known in the Middle Ages, but in the fifteenth century the texts of Sextus Empiricus in Greek were brought from the Byzantine Empire into Italy. Sextus' Outlines of Pyrrhonism was published in Latin in 1562, his Adversus matematicos in 1569, and the Greek texts of both in 1621. Interest in Cicero was revived and his Academica and De natura deorum were also published in the sixteenth century.

The voyages of exploration; the humanistic rediscovery of the learning of ancient Greece, Rome, and Palestine; and the new scienceall combined to undermine confidence in man's accepted picture of the world. The religious controversy between the Protestants and Catholics raised fundamental epistemological issues about the bases and criteria of religious knowledge.

renaissance and reformation

Toward the end of the fifteenth century, there was a revival of interest in ancient skepticism among Florentine humanists. Politian was lecturing on philosophy using notes from Sextus with which he had recently become acquainted from manuscripts brought from Byzantium. Humanists, including Gianfrancesco Pico della Mirandola, were acquiring and studying Sextus' texts. Some of these manuscripts were deposited in the convent of San Marco where the Dominican friar and prophet Girolamo Savonarola was heading up an exciting intellectual forum in which ancient philosophies were being analyzed. Savonarola, who did not read Greek, asked two of his monks to prepare a Latin translation of Sextus from one of these manuscripts. This apparently was to be used as a weapon against philosophy independent of religion. Before Savonarola's project could be completed the convent was destroyed and he was executed.

Gianfrancesco Pico, one of Savonarola's disciples and the nephew of the great Pico della Mirandola, published the first work using skepticism as a way of challenging all of philosophy. Gianfrancesco Pico's Examen Vanitatis (1520) is the first work to present Sextus in Latin for the European audience. In 1562 Henri Estienne (Stephanus) published a Latin translation of the Pyrrhoniarum Hypotyposes in Paris, and in 1569 Gentian Hervet published a Latin translation of Adversus Mathematicos in Antwerp. The Greek texts were first printed at Cologne, Paris, and Geneva in 1621. Some texts of Sextus appeared in English in 1592 in a work attributed to Sir Walter Raleigh titled "The Skepticke." A full translation of Book One of Sextus appeared in 1659 in Thomas Stanley's History of Philosophy ; instead of explaining skepticism he just presented the whole book to the readers. A French translation was started by Pierre Gassendi's disciple Samuel Sorbière but was never finished or published. The first complete French translation, by Claude Huart, did not appear until 1725.

religious controversy: erasmus and luther

The skeptical issue became more central when raised in the debate between Erasmus and Martin Luther. Using Academic skeptical materials, Erasmus insisted that the issues in dispute could not be resolved and that one should therefore suspend judgment and remain with the church. In 1524, Erasmus finally published a work, De Libero Arbitrio, attacking Martin Luther's views on free will. Erasmus' general anti-intellectualism and dislike of rational theological discussions led him to suggest a kind of skeptical basis for remaining within the Catholic Church. This contempt for intellectual endeavor was coupled with his advocacy of a simple, non-theological Christian piety. Theological controversies were not Erasmus' meat, and he states that he would prefer to follow the attitude of the skeptics and suspend judgment, especially where the inviolable authority of Scripture and the decrees of the Church permit. He says he is perfectly willing to submit to the decrees, whether or not he understands them or the reasons for them.

Scripture is not as clear as Luther would have us believe, and there are some places that are just too shadowy for human beings to penetrate. Theologians have argued and argued the question without end. Luther claims he has found the right answer and has understood Scripture correctly. But how can we tell that he really has? Other interpretations can be given that seem much better than Luther's. In view of the difficulty in establishing the true meaning of Scripture concerning the problem of free will, why not accept the traditional solution offered by the Church? Why start such a fuss over something one cannot know with any certainty? For Erasmus, what is important is a simple, basic, Christian piety, a Christian spirit. The rest, the superstructure of the essential belief, is too complex for a man to judge. Hence it is easier to rest in a skeptical attitude, and accept the age-old wisdom of the Church on these matters, than to try to understand and judge for oneself.

This attempt, early in the Reformation, at a skeptical "justification" of the Catholic rule of faith brought forth a furious answer from Luther, the De Servo Arbitrio of 1525. Erasmus' book, Luther declared, was shameful and shocking, the more so since it was written so well and with so much eloquence. De Libero Arbitrio begins with the announcement that the problem of the freedom of the will is one of the most involved of labyrinths. The central error of Erasmus' book, according to Luther, was that Erasmus did not realize that a Christian cannot be a skeptic. Christianity involves the affirmation of certain truths because one's conscience is completely convinced of their veracity. The content of religious knowledge, according to Luther, is far too important to be taken on trust. One must be absolutely certain of its truth. Hence, Christianity is the complete denial of skepticism. To find the truths, one only has to consult Scripture.

Of course there are parts that are hard to understand, and there are things about God that we do not, and perhaps shall not, know. But this does not mean that we cannot find the truth in Scripture. The central religious truth can be found in clear and evident terms, and these clarify the more obscure ones. However, if many things remain obscure to some people, it is not the fault of Scripture, but of the blindness of those who have no desire to know the revealed truths. Luther's view, and later that of Calvin, proposed a new criterionthat of inner experiencewhile the Catholics of the Counter-Reformation employed Pyrrhonian and Academic arguments to undermine the criterion. Following after Erasmus, H. C. Agrippa von Nettesheim, a stormy occult philosopher and physician, employed the skeptical arguments against Scholasticism, Renaissance Naturalism, and many other views to win people to the "true religion."


Gentian Hervet, secretary to the Cardinal of Lorraine, and participant at part of the Council of Trent, linked his work on Sextus with what Gianfrancesco Pico had earlier done. During the 1560s, Hervet, a humanist, fought intellectually against the encroachments of Calvinism, challenging various Protestants to debate with him, and publishing many pamphlets against their views. He saw Sextus' work as ideal for demolishing this new form of heretical dogmatism, that of the Reformer. If nothing can be known, then, he insisted, Calvinism cannot be known. The only certainty we can have is God's Revelation. Skepticism, by controverting all human theories, will cure people from dogmatism, give them humility, and prepare them to accept the doctrine of Christ. Hervet's employment of Pyrrhonism against Calvinism was soon to be shaped into a skeptical machine of war for use by the Counter-Reformation. This view of Pyrrhonism, by one of the leaders of French Catholicism, was to set the direction of one of its major influences on the next three-quarters of a century.

montaigne and sanches

The new concern with skepticism was given a general philosophical formulation by Michel de Montaigne and his cousin Francisco Sanches. Michel de Montaigne was the most significant figure in the sixteenth century revival of ancient skepticism. Not only was he the best writer and thinker of those who were interested in the ideas of the Academics and Pyrrhonians, but he was also the one who felt most fully the impact of the Pyrrhonian arguments of complete doubtand its relevance to the religious debates of the time. Montaigne was simultaneously a creature of the Renaissance and the Reformation. He was a thorough-going humanist, with a vast interest in, and concern with, the ideas and values of Greece and Rome, and their application to the lives of men in the rapidly changing world of sixteenth-century France. Montaigne was sent to the Collège de Guyenne in 1539 when he was six years old and was there for the next seven years. The college reflected the religious tensions of the time. Two of its leaders were André de Gouvea, a Portuguese New Christian, and George Buchanan, the Scottish Latin poet.

Montaigne's 1576 essay "Apologie of Raimond Sebond" unfolds in his inimitable rambling style as a series of waves of skepticism, with occasional pauses to consider and digest various levels of doubt, but with the overriding theme an advocacy of a new form of fideismCatholic Pyrrhonism. The essay begins with a probably inaccurate account of how Montaigne came to read and translate the audacious work of the fifteenth century Spanish theologian, Raimond Sebond. Starting from a quibble about the validity of the arguments of Sebond, Montaigne moved to a general skeptical critique of the possibility of human beings understanding anything. In a rather back-handed manner, Montaigne excuses Sebond's theological rationalism by saying that although he, Montaigne, is not versed in theology, it is his view that religion is based solely on faith given to us by the Grace of God; true religion can only be based on faith, and any human foundation for religion is too weak to support divine knowledge. If human beings had the real light of faith, then human means, like the arguments of Sebond, might be of use. Montaigne explored the human epistemological situation and showed that man's knowledge claims in all areas were extremely dubious and so made pure faith the cornerstone of religion. Montaigne recommended living according to nature and custom and accepting whatever God reveals.

Sanches, in Quod nihil scitur, also written in 1576, advocated recognizing that nothing can be known and then trying to gain what limited information one can through empirical scientific means. In his book, Sanches develops his skepticism by means of an intellectual critique of Aristotelianism, rather than by an appeal to the history of human stupidity and the variety and contrariety of previous theories. Sanches begins by asserting that he does not even know if he knows nothing. Then he proceeds, step by step, to analyze the Aristotelian conception of knowledge to show why this is the case.

Every science begins with definition and definitions are nothing but names arbitrarily imposed upon things in a capricious manner, having no relation to the things named. The names keep changing, so that when we think we are saying something about the nature of things by means of combining words and definitions, we are just fooling ourselves. And if the names assigned to an object such as man, like "rational animal," all mean the same thing, then they are superfluous and do not help to explain what the object is. On the other hand, if the names mean something different from the object, then they are not the names of the object. By means of such an analysis, Sanches worked out a thorough-going nominalism.

Sanches' first conclusion was the usual fideistic one of the timethat truth can be gained only by faith. His second conclusion was to play an important role in later though: just because nothing can be known in an ultimate sense, we should not abandon all attempts at knowledge but should try to gain what knowledge we can, namely, limited, imperfect knowledge of some of those things with which we become acquainted through observation, experience and judgment. The realization that nihil scitur ("nothing is known") thus can yield some constructive results. This early formulation of "constructive" or "mitigated" skepticism was to be developed into an important explication of the new science by Marin Mersenne, Pierre Gassendi, and the leaders of the Royal Society.

The Seventeenth Century

Montaigne's skepticism was extremely influential in the early seventeenth century. His followers, Pierre Charron in De la Sagesse (1601) and Jean-Pierre Camus in Essay sceptique (1603), became most popular in the early seventeenth century, especially among the avant-garde intellectuals in Paris. The so-called libertines, including Gabriel Naudé, Mazarin's secretary; Guy Patin, rector of the Sorbonne medical school; and François La Mothe Le Vayer, teacher of the dauphin, espoused Montaigne's attitude and were often accused of being skeptical even of fundamental religious tenets. Others, like François Veron, used the arguments of Sextus and Montaigne to challenge the Calvinist claim of gaining true knowledge from reading Scripture. French Counter-Reformers, by raising skeptical epistemological problems about whether one could determine what book is the Bible, what it actually says, what it means, and so on, forced Calvinists to seek an indisputable basis for knowledge as a prelude to defending their theological views.

gassendi and mersenne

In the 1620s efforts to refute or mitigate this new skepticism appeared. Some authors simply stated that Aristotle would have resolved the difficulties by applying his theory of sense perception and knowledge to the problems raised. Others, like François Garasse, decried the irreligious tendencies they discerned in all this doubting. Still others, like Francis Bacon, tried to overcome the skeptical difficulties by appealing to new methods and new instruments that might correct errors and yield firm and unquestionable results. Herbert of Cherbury, in De Veritate (1624), offered an elaborate scheme for overcoming skepticism which combined Aristotelian and Stoic elements, and ultimately appealed to common notions, or truths known by all men, as the criteria by which reliable and indubitable judgment would be possible.

Perhaps the most forceful presentation of skepticism in the early seventeenth century is Pierre Gassendi's earliest work, Exercitationes Paradoxicae Adversus Aristoteleos (1624). A Christian Epicurean, Gassendi, himself originally a skeptic, challenged almost every aspect of Aristotle's view, as well as many other theories. He applied a battery of ancient and Renaissance skeptical arguments, concluding, "No science is possible, least of all in Aristotle's sense." In this work, Gassendi indicated in embryo what became his and Marin Mersenne's constructive solution to the skeptical crisis, the development of an empirical study of the world of appearances rather than an attempt to discover the real nature of things.

Mersenne, one of the most influential figures in the intellectual revolution of the times, while retaining epistemological doubts about knowledge of reality, yet recognized that science provided useful and important information about the world. Mersenne granted that the problems raised by Sextus could not be answered and that, in a fundamental sense, knowledge of the real nature of things cannot be attained. However, he insisted, information about appearances and deductions from hypotheses can provide an adequate guide for living in this world and can be checked by verifying predictions about futures experiences. Gassendi, in his later works, developed this constructive skepticism as a via media between complete doubt and dogmatism, and offered his atomic theory as the best hypothetical model for interpreting experience. Mersenne and Gassendi combined skepticism about metaphysical knowledge of reality with a way of gaining useful information about experience through a pragmatic scientific method. The constructive skepticisms of Gassendi and Mersenne, and later of members of the Royal Society of England like Bishop John Wilkins and Joseph Glanvill, thus developed the attitude of Sanches into a hypothetical, empirical interpretation of the new science.


René Descartes offered a fundamental refutation of the new skepticism, contending that, by applying the skeptical method of doubting all beliefs that could possibly be false (due to suffering illusions or being misled by some power), one would discover a truth that is genuinely indubitable, namely, "I think, therefore I am" (cogito ergo sum ), and that from this truth one could discover the criterion of true knowledge, namely, that whatever is clearly and distinctly conceived is true. Using this criterion, one could then establish: God's existence, that he is not a deceiver, that he guarantees our clear and distinct ideas, and that an external world exists that can be known through mathematical physics. Descartes, starting from skepticism, claimed to have found a new basis for certitude and for knowledge of reality.

replies to descartes

Throughout the seventeenth century skeptical criticsMersenne, Gassendi, the reviver of Academic philosophy Simon Foucher, and Pierre-Daniel Huet, one of the most learned men of the agesought to show that Descartes had not succeeded and that, if he sincerely followed his skeptical method, his new system could only lead to complete skepticism. They challenged whether the cogito proved anything, or whether it was indubitable; whether Descartes' method could be successfully applied, or whether it was certain; and whether any of the knowledge claims of Cartesianism were really true. Nicolas Malebranche, the developer of occasionalism, revised the Cartesian system to meet the skeptical attacks only to find his efforts challenged by the new skeptical criticisms of Foucher and by the contention of the Jansenist philosopher Antoine Arnauld that Malebranchism led to a most dangerous Pyrrhonism.

Huet's Censura Philosophae Cartesiana (1689) and his unpublished defense of it raised doubts about each element of the proposition, "I think, therefore perhaps I may be." Gassendi, Huet, and others questioned whether Descartes' criterion could determine what was true or false. Could we really tell what was clear and distinct, or could we only tell that something appeared clear and distinct to us? Mersenne pointed out that even with the criterion we could not be sure that what was clear and distinct to us, and hence true, was really true for God. Hence, in an ultimate sense, even the most certain Cartesian knowledge might be false. Gassendi, in what Descartes called the "objections of objections," pointed out that for all anyone could ascertain, the whole Cartesian system of truths might be only a subjective vision in somebody's mind and not a true picture of reality. Huet argued that since all the fundamental Cartesian data consisted of ideas, and ideas are not real physical things, the Cartesian world of ideas, even if clear and distinct, cannot represent something quite different from itself.

followers of descartes

As Cartesianism was attacked from many sides, adherents modified it in various ways. The radical revision of Nicolas Malebranche, designed partially to avoid skeptical difficulties involved in connecting the world of ideas with reality, was immediately attacked by the skeptic Simon Foucher. The orthodox Cartesian Antoine Arnaud claimed that Malebranchism could only lead to a most dangerous Pyrrhonism. Foucher, who wished to revive Academic skepticism, applied various skeptical gambits to Malebranche's theory, one of which was to be important in subsequent philosophy. He argued that the skeptical difficulties which Descartes and Malebranche used to deny that sense qualities (the so-called secondary qualitiescolor, sound heat, taste, smell) were features of real objects, applied as well to the mathematically describable primary qualities like extension and motion, which the Cartesians considered the fundamental properties of things. These mathematical qualities, as perceived, are as variable and as subjective as the others. If the skeptical arguments are sufficient to cause doubt about the ontological status of secondary qualities, Foucher contended, they are also sufficient to lead us to doubt that primary ones are genuine features of reality.

english skepticism

Various English philosophers, culminating in John Locke, tried to blunt the force of skepticism by appealing to common sense and to the "reasonable" man's inability to doubt everything. They admitted that there might not be sufficient evidence to support the knowledge claims extending beyond immediate experience. But this did not actually require that everything be doubted; by using standards of common sense, an adequate basis for many beliefs could be found.

This theory of limited certitude was articulated especially by two figures, John Wilkins and Joseph Glanvill. The theory is a development from the earlier solution to the skeptical problems advanced by Sebastian Castillio and William Chillingworth. Wilkins set forth the theory of limited certainty as both an answer to dogmatism and to excessive skepticism. Wilkins completely rejected the dogmatists' outlook, and then offered a way of defusing the potentially disastrous results of complete skepticism. In order to find a moderate skeptical stance from which religion and science could flourish, Wilkins felt it was necessary to analyze what kind of certainty human beings could actually attain. The highest level of certainty, absolute infallible certainty, which could not possibly be false, is beyond human attainment. Only God has such certainty. The highest human level Wilkins called conditional infallible certainty. This requires that "our faculties be true, and that we do not neglect the exerting of them."

Glanvill saw the reliability of our faculties as central for avoiding any ultimate and overwhelming skepticism. Glanvill, like Wilkins, saw that the kind of certainty we would need to be absolutely sure of our faculties is unattainable"for it may not be absolutely impossible, but that our Faculties may be so construed, as always to deceive us in the things we judg most certain and assured." We may not be able to attain infallible certitude, but we can attain indubitable certitudethat our faculties are true. This is indubitable in two sensesone, that we find we have to believe them, and, two, that we have no reason or cause for doubting them. In terms of this distinction, Wilkins, Glanvill, and their colleagues built up a theory of empirical science and jurisprudence for studying nature and deciding human problems within the limits of "reasonable doubt." Their limited skepticism appears in the Anglo-American theory of legal evidence and in the theory of science of the early Royal Society. They believed that by applying their probabilistic empirical method to religious questions they could justify a tolerant, latitudinarian form of Christianity.

other resolutions of skepticism

Other answers were offered to the skeptics and to their challenge of some of the basic tenets of the new philosophy. Thomas Hobbes had admitted the force of the problem of finding the criterion for judging what was genuinely true, and he insisted that the solution was ultimately politicalthe sovereign would have to decide. Blaise Pascal in his scientific works gave one of the finest expositions of the hypothetical probabilistic nature of science and mathematics. Pascal, who presented the case for skepticism most forcefully in his Pensées, still denied that there can be a complete skepticism; for nature prevents it. Lacking rational answers to complete skepticism, man's only recourse lies in turning to God for help in overcoming doubts. Spinoza, on the other hand, with his completely rational vision of the world, could not regard skepticism as a serious problem. If one had clear and adequate ideas, there would be no need or excuse for doubting. Doubt was only an indication of lack of clarity, not of basic philosophical difficulties.

The philosopher who took the skeptics most seriously was Gottfried Wilhelm von Leibniz, and he was regarded as a closer friend intellectually by the skeptics of his age than any of the other metaphysicians of the period. Leibniz, although certainly not a philosophical skeptic, agrees with some of the major contentions of the skeptics, and is willing to admit, unlike other metaphysicians of the seventeenth century, that there are general, and perhaps unanswerable, objections that can be raised against any philosophical theory. The skeptics and Leibniz could agree on the major failings of Cartesianism, although they were hardly in agreement as to what to do about them. Leibniz and the skeptics were all humanists and found great value in the tradition of man's effort to understand his universe; hence they rejected the Cartesian attitude towards the past. In his discussions, especially with Simon Foucher and Pierre Bayle, Leibniz agreed that there are first principles of philosophical reasoning that have not been satisfactorily demonstrated.

Leibniz was willing to regard metaphysics as a hypothetical enterprise, that is, as an attempt to present theories which agree with the known facts, which avoid certain difficulties in previous theories, and which give a satisfactory or adequate explanation of the world that is experienced. In the debate with Pierre Bayle over the article "Rorarius," in Bayle's Dictionnaire historique et critique, Leibniz does not argue for his theory as the true picture of reality, but rather as the most consistent hypothesis to explain the known scientific facts and the general conclusions of the "new philosophers" about the relation of the mind and the body, and to avoid the "unfortunate" complications or conclusions of the views of Descartes, Malebranche, or Spinoza. Leibniz was unwilling to see these limitations on our knowledge as a reason for skeptical despair or to see these points as constituting a radical skepticism that cast whatever knowledge we had in any serious doubt. For Leibniz, whatever merits the skeptical arguments had, they did not have to lead to negative or destructive conclusions. At best, skepticism should be a spur to constructive theorizing, and not a reason for doubting or despairing of the possibility of knowledge.

bayle and the enlightenment

The culmination of seventeenth-century skepticism appears in the writings of Pierre Bayle, especially in his monumental Dictionnaire historique et critique (16971702). Bayle, a superb dialectician, challenged philosophical, scientific, and theological theories, both ancient and modern, showing that they all led to perplexities, paradoxes, and contradictions. He argued that the theories of Descartes, Spinoza, Leibniz, and Malebranche, when skeptically analyzed, cast in doubt all information about the world, even whether a world exists. Bayle skillfully employed skeptical arguments about such things as sense information, human judgments, logical explanations, and the criteria of knowledge in order to undermine confidence in human intellectual activity in all areas. Bayle suggested that man should abandon rational activity and turn blindly to faith and revelation; he can therefore only follow his conscience without any criterion for determining true faith. Bayle showed that the interpretations of religious knowledge were so implausible that even the most heretical views, like Manichaeismknown for its cosmic dualism of good and eviland Atheism made more sense. As a result Bayle's work became "the arsenal of the Enlightenment," and he was regarded as a major enemy of religion.

Bayle, in his later works, indicated that he held some positive views even though he presented no answers to his skepticism. There is still much scholarly debate as to what his actual position was, but he influenced many people in the eighteenth century. His skeptical arguments were soon applied to traditional religion by Voltaire and others. But in place of Bayle's doubts or his appeal to faith, they offered a new way of understanding man's worldthat of Newtonian scienceand professed an inordinate optimism about what man could comprehend and accomplish through scientific examination and induction. Though Bayle remained the heroic figure who had launched the Age of Reason by criticizing all the superstitions of past philosophy and theology, the leaders of the Enlightenment, both in France and Britain, felt that his skepticism was passé and only represented the summit of human understanding before "God said, Let Newton be, and all was light."

The Eighteenth Century

Most eighteenth-century thinkers gave up the quest for metaphysical knowledge after imbibing Bayle's arguments. George Berkeley, an Empiricist and Idealist, fought skeptical doubts by identifying appearance and reality and offering a spiritualistic metaphysics. He was immediately seen as just another skeptic, since he was denying the world beyond experience.


Bayle's chief eighteenth-century successor was David Hume. Combining empirical and skeptical arguments, Hume, in the Treatise of Human Nature and the Enquiry Concerning Human Understanding, charged that neither inductive nor deductive evidence could establish the truth of any matter of fact. Knowledge could only consist of intuitively obvious matters or demonstrable relations of ideas but not of anything beyond experience; the mind can discover no necessary connections within experience nor any root causes of experience. Beliefs about the world are based not upon reason or evidence, nor even upon appeal to the uniformity of nature, but only on habit and custom. Basic beliefs cannot be justified by reasoning. Belief that there is an external world, a self, a God is common; but there is no adequate evidence for it. Although it is natural to hold these convictions, they are inconsistent and epistemologically dubious. "Philosophy would render us entirely Pyrrhonian, were not Nature too strong for it." The beliefs that a man is forced to hold enable him to describe the world scientifically, but when he tries to justify them he is led to complete skepticism. Before he goes mad with doubts, however, Nature brings him back to common sense, to unjustifiable beliefs. Hume's fideism was a natural rather than a religious one; it is only animal faith that provides relief from complete doubt. The religious context of skepticism from Montaigne to Bayle had been removed, and man was left with only his natural beliefs, which might be meaningless or valueless.

the philosophes

The French Enlightenment philosophers, the philosophes, built on the skeptical reading of Locke and Bayle, and on their interpretation of Berkeley as a radical skeptic. While they produced vast accumulations of new forms of knowledge, they also placed this alongside a skepticism about whether one could ever establish that this knowledge was about an external reality. Perhaps the most skeptical of them was the great French mathematician Marquis de Condorcet who held that mathematics, physics, and moral philosophies were all just probable. He also raised the possibility that our present mental faculties by which we judged our knowledge might change over time and, hence, that what we found true today might not be so tomorrow.

reid and the common-sense school

The central themes in Hume's skeptical analysisthe basis of induction and causality, knowledge of the external world and the self, proofs of the existence of Godbecame the key issues of later philosophy. Hume's contemporary Thomas Reid hoped to rebut Hume's skepticism by exposing it as the logical conclusion of the basic assumptions of modern philosophy from Descartes onward. Such disastrous assumptions should be abandoned for commonsensical principles that have to be believed. When the conclusions of philosophy run counter to common sense, there must be something wrong with philosophy. Since nobody could believe and act by complete skepticism, the fact that this skepticism was the consistent issue of the Cartesian and Lockean way of ideas only showed the need to start anew. Reid offered his common-sense realism as a way of avoiding Hume's skepticism by employing as basic principles the beliefs we are psychologically unable to doubt.

Hume was unimpressed by Reid's argument. As Hume and Kant saw, Reid had not answered Hume's skepticism but had only sidestepped the issue by appealing to commonsensical living. This provided, however, neither a theoretical basis for beliefs nor a refutation of the arguments that questioned them. The Scottish common-sense school of Oswald, Beattie, Stewart, Brown, and others kept reiterating its claim to have refuted Hume's skepticism by appealing to natural belief, while at the same time conceding that Hume's fundamental arguments could not be answered. Thomas Brown, an early-nineteenth-century disciple of Reid, admitted that Reid and Hume differed more in words than in opinions, saying, "'Yes,' Reid bawled out, 'we must believe in an outward world': but added in a whisper, 'we can give no reason for our belief.' Hume cries out, 'we can give no reason for such a notion': and whispers, 'I own that we cannot get rid of it.'"

the german enlightenment and kant

The Scottish school was perhaps the first to make Hume's version of modern skepticism the central view to be combated if philosophy was to make coherent sense of man's universe. The more fundamental attempt, for subsequent philosophy, to deal with Hume's skepticism was developed in Germany in the second half of the eighteenth century and culminated in Kant's critical philosophy. Such leaders of the Prussian Academy as Jean Henry Samuel Formey, Johann Bernhard Mérian, and Johann Georg Sulzer had long been arguing against Pyrrhonism. They were among the first to read, translate (into French and German), and criticize Hume's writings. They saw in the skeptical tradition up to Bayle and Huet, and in Hume's version of it, a major challenge to all man's intellectual achievements. Although their answers to skepticism were hardly equal to the threat they saw in it, these writers helped revive interest in and concern with skepticism in an age that thought it had solved, or was about to solve, all problems. Others in Germany contributed to an awareness of the force of skepticism: Johann Christoff Eschenbach by his edition of the arguments of Sextus, Berkeley, and Arthur Collier (Berkeley's contemporary) against knowledge of an external corporeal world; Ernst Platner by his skeptical aphorisms and his German edition of Hume's Dialogues on Natural Religion (1781); hosts of German professors by dissertations against skepticism; and the translators of the Scottish critics of Hume.

Kant saw that Hume had posed a most fundamental challenge to all human knowledge claims. To answer him, it had to be shown not that knowledge is possible but how it is possible. Kant combined a skepticism toward metaphysical knowledge with the contention that certain universal and necessary conditions are involved in having experience and describing it. In terms of these it is possible to have genuine knowledge about the forms of all possible experience, space and time, and about the categories in which all experience is described. Any effort to apply this beyond all possible experience, however, leads into contradictions and skepticism. It is not possible to know about things-in-themselves nor about the causes of experience.

skeptical rejoinders to kant

Though Kant thought that he had resolved the skeptical problems, some of his contemporaries saw his philosophy as commencing a new skeptical era. G. E. Schulze (or Schulze-Aenesidemus) a notable critic of Kantianism, insisted that, on Kant's theory, no one could know any objective truths about anything; he could only know the subjective necessity of his views. So Schulze, by insisting on the inability of the Kantian analysis to move from subjective data about what people have to believe to any objective claims about reality, contended that Kant had not advanced beyond Hume's skepticism, and that this failure of the Kantian revolution actually constituted a vindication of Hume's views.

Salomon Maimon contended that, though there are such things as a priori concepts, their application to experience is always problematical, and whether they apply can only be found through experience. Hence, the possibility of knowledge can never be established with certainty. Assured truth on the basis of concepts is possible only of human creations, like mathematical ideas, and it is questionable whether these have any objective truth. Thus Maimon developed a mitigated Kantianism (to some extent like that of the Neo-Kantian movement a century later) in which the reality of a priori forms of thought is granted but in which the relation of these forms to matters of fact is always in question. Knowledge (that is, propositions that are universal and necessary, rather than ones that are just psychologically indubitable) is possible in mathematics but not in sciences dealing with the world. Unlike the logical positivists, who were to claim that mathematics was true because it consisted only of vacuous logical tautologies, Maimon contended that mathematics was true because it was about creations of our mind. Its objective relevance was always problematical.

Maimon's partial skepticism exposed some of the fundamental limitations of Kant's critical philosophy as a solution to the skeptical crisis. Developing the thesis that human creativity is the basis of truth, Johann Georg Hamann posited a new way of transcending skepticism. Hamann accepted Hume's and Kant's arguments as evidence that knowledge of reality cannot be gained by rational means but only by faith. Hamann exploited the skeptical thought of these philosophies to press for a complete antirational fideism. He used Hume's analyses of miracles and of the evidence for religious knowledge to try to convince Kant of the futility of the search for truth by rational means. During the height of nineteenth-century positivism, materialism, and idealism, Hamann's type of fideism was revitalized by Kierkegaard and in France by Catholic opponents of the French Revolution and liberalismlike Joseph de Maistre and H.-F.-R. Lamennais, who used it as a critique of French liberal, empirical, and Enlightenment views and as a new defense of orthodoxy and political conservatism. Kierkegaard brilliantly combined themes from Sextus, Hume, and Hamann to attack the rationalism of the Hegelians, to develop a thoroughgoing skepticism about rational achievements, and to show the need for faith in opposition to reason. Fideism has become a major element in twentieth-century neo-orthodox and existentialist theology, which tries to show that the traditional skeptical problems still prevent us from finding an ultimate basis for our beliefs except by faith.


In the mainstream of philosophy after Kant, although skepticism continues to play a vital role, few philosophers have been willing to call themselves skeptics. The German metaphysicians, from Fichte and Hegel onward, sought to escape from the skeptical impasse produced by Hume and Kant and to reach knowledge of reality through the creative process and the recognition of historical development. They attempted to portray skepticism as a stage in the awareness and understanding of the process of events. For Fichte, skepticism made one recognize the need for commitment to a fundamental outlook about the world. The commitment to see the world in terms of creative thought processes led to a revelation of the structure of the universe as an aspect of the Absolute Ego.

For Hegel skepticism was the nadir of philosophy, actually its antithesis. According to Hegel, human knowledge is a historically developing process. At each stage of the process both our knowledge and the world itself are limited and contain contradictions, which are overcome at the next stage. Only the final, Absolute stage, when no further contradictions can be developed, permits genuine knowledge that is not partly true and partly false. Then, presumably, skepticism is no longer possible. The English Hegelian F. H. Bradley, in his Appearance and Reality (1893), used the traditional skeptical arguments to show that the world was unintelligible in terms of empirical or materialistic categories, and hence that one had to go beyond the world appearance to find true knowledge.

Recent and Contemporary Philosophy

Irrational skepticism was developed into Existentialism by Søren Kierkegaard in the nineteenth century. Using traditional skeptical themes to attack Hegelianism and liberal Christianity, Kierkegaard stressed the need for faith. Only by an unjustified and unjustifiable "leap into faith" could certainty be foundwhich would then be entirely subjective rather than objective. Modern neo-orthodox and Existentialist theologians have argued that skepticism highlights man's inability to find any ultimate truth except through faith and commitment. Nonreligious forms of this view have been developed by Existentialist writers like Albert Camus, combining the epistemological skepticism of Kierkegaard and Leon Shestov with the skepticism regarding religion and objective values of Friedrich Nietzsche.

In his Myth of Sisyphus, Camus portrays man as trying to measure the nature and meaning of an essentially absurd universe by means of questionable rational and scientific criteria. Camus regards the skeptical arguments used by Kierkegaard and Shestov as showing decisively the contradictory nature of human rational attempts to understand the world, but he rejects their fideistic solution: overcoming the skeptical crisis by "a leap into faith." Instead, he accepts Nietzsche's picture of the ultimate meaninglessness of the world because "God is dead." The rational and scientific examination of the world shows it to be unintelligible and absurd but it is necessary to struggle with it. It is thus through action and commitment that one finds whatever personal meaning one can, though it has no objective significance. The mythological Sisyphus, eternally pushing a huge rock uphill, only to have to fall to the bottom again, typifies the human situation. He does not expect to find truth, nor does he expect to end his struggle. He finds no ultimate point or value in his situation, but he perseveres with a "silent joy," realizing that his struggle has meaning only for him, in terms of his human condition. The struggle is neither sterile nor futile for him, though it is meaningless in terms of understanding or possible achievement.

George Santayana, an American critical Realist, in Scepticism and Animal Faith, presented a naturalistic skepticism. Any interpretation of immediate or intuited experience is open to question. To make life meaningful, however, men make interpretations by "animal faith," according to biological and social factors. The resulting beliefs, though unjustified and perhaps illusory, enable them to persevere and find the richness of life. When the full force of complete skepticism is realized, Santayana claimed, one can appreciate what is in fact absolutely indubitable, the immediately experienced or intuited qualities that Santayana called "essences." The interpretation of these essences leads to various questionable metaphysical systems. A thoroughgoing skepticism makes one realize the unjustifiable assumptions involved in interpreting the realm of essences, and also that we do interpret them and thereby construct meaningful pictures of the world. Santayana called the process of interpretation "animal faith," which is consistent with complete skepticism and involves following natural and social tendencies and inclinations.

Types of skepticism also appear in logical positivism and various forms of linguistic philosophy. The attack on speculative metaphysics developed by the physicist and early Positivist Ernst Mach, Bertrand Russell, and Rudolf Carnap, a leader in the Vienna Circle, where logical positivism was nourished, incorporated a skepticism about the possibility of gaining knowledge beyond experience or logical tautologies. Russell and the important philosopher of science Karl Popper have further stressed the unjustifiability of the principle of induction, and Popper has criticized theories of knowledge based upon empirical verification. A founder of linguistic analysis, Fritz Mauthner, has set forth a skepticism in which any language is merely relative to its users and thus subjective. Every attempt to tell what is true just leads one back to linguistic formulations, not to objective states of affairs. The result is a complete skepticism about realitya reality that cannot even be expressed except in terms of what he called godless mystical contemplation. Mauthner's linguistic skepticism bears some affinities to the views expressed in Ludwig Wittgenstein's Tractatus Logico-Philosophicus.

A different way of dealing with skepticism was set forth by the English philosopher, G. E. Moore at Cambridge. He contended that no matter what skeptical arguments may be they do not eliminate people's certitude about what they immediately perceive. There is a kind of "certain knowledge" that each of us has and can build on even though we know that it can be questioned in some theoretical way. Wittgenstein explored this kind of resolution in his essay On Certainty and sought to get beyond what Moore had done. Many contemporary philosophers are still writing and arguing about what constitutes knowledge and whether, in some way, we can find any basis for certainty.


A new, radical form of skepticism has developed in the last half century: postmodernism. This view challenges whether there can be any rational framework for discussing intellectual problems or whether the frameworks that people use are related to their life situations. Developing out of literary criticism and psychological investigations, the postmodernists have been undermining confidence in the investigation of the world in which we live by showing that the investigations are part of what needs to be scrutinized. Using ideas from Martin Heidegger, Michel Foucault, Jacques Derrida, Jean-François Lyotard, and Richard Rorty, they see philosophy and science as human activities to be judged in terms of their role in human life, rather than by some standard that can be said to be true or false. Rather than attempting to find a holistic truth or set of truths that are knowable and eternal, Postmodernists stress reflexivity, fragmentation, discontinuity, and ambiguity. Critics see this as a most dangerous development in that there will be no objective standpoint for evaluating theories. But that, of course, is part of the postmodernist outlook. Psychologists and sociologists have been adding to this view by stressing how intellectual outlooks vary according to sexual orientation, racial background, gender, and other fundamental features of human outlooks. Skepticism results from seeing that there is no objective standpoint from which to sort out the better or worse of these points of view.

criticism and evaluation

In Western thought, skepticism has raised basic epistemological issues. In view of the varieties of human experience, it has questioned whether it is possible to tell which are veridical. The variations that occur in different perceptions of what is presumed to be one object raise the question of which is the correct view. The occurrence of illusory experiences raises the question of whether it is really possible to distinguish illusions and dreams from reality. The criteria employed can be questioned and require justification. On what basis does one tell whether one has the right criteria? By other criteria? Then, are these correct? On what standards? The attempt to justify criteria seems either to lead to an infinite regress or to just stop arbitrarily. If an attempt is made to justify knowledge claims by starting with first principles, what are these based upon? Can it be established that these principles cannot possibly be false? If so, is the proof itself such that it cannot be questioned? If it is claimed that the principles are self-evident, can one be sure of this, sure that one is not deceived? And can one be sure that one can recognize and apply the principles correctly? Through such questioning, skeptics have indicated the basic problems that an investigator would have to resolve before he could be certain of possessing knowledge; that is, information that could not possibly be false.

Critics have contended that skepticism is both a logically and a humanly untenable view. Any attempt to formulate the position will be self-refuting since it will assert at least some knowledge claims about what is supposed to be dubious. Montaigne suggested that the skeptics needed a nonassertive language, reflecting the claim of Sextus that the skeptic does not make assertions but only chronicles his feelings. The strength of skepticism lies not in whether it can be stated consistently but upon the effects of its arguments on dogmatic philosophers. As Hume said, skepticism may be self-refuting, but in the process of refuting itself it undermines dogmatism. Skepticism, Sextus said, is like a purge that eliminates itself as well as everything else.

Critics have claimed that anyone who tried to be a complete skeptic, denying or suspending all judgments about ordinary beliefs, would soon be driven insane. Even Hume thought that the complete skeptic would have to starve to death and would walk into walls or out of windows. Hume, therefore, separated the doubting activity from natural practical activities in the world. Skeptical philosophizing went on in theory, while believing occurred in practice. Sextus and the contemporary Norwegian skeptic Arne Naess have said, on the other hand, that skepticism is a form of mental health. Instead of going mad, the skepticwithout commitment to fixed positionscan function better than the dogmatist.

Some thinkers like A. J. Ayer and J. L Austin have contended that skepticism is unnecessary. If knowledge is defined in terms of satisfying meaningful criteria, then knowledge is open to all. The skeptics have raised false problems, because it is, as a matter of fact, possible to tell that some experiences are illusory since we have criteria for distinguishing them from actual events. We do resolve doubts and reach a state of knowledge through various verification procedures, after which doubt is meaningless. Naess, in his book Scepticism, has sought to show, however, that, on the standards offered by Ayer and Austin, one can still ask if knowledge claims may not turn out to be false and hence that skepticism has still to be overcome.

Skepticism throughout history has played a dynamic role in forcing dogmatic philosophers to find better or stronger bases for their views and to find answers to the skeptical attacks. It has forced a continued reexamination of previous knowledge claims and has stimulated creative thinkers to work out new theories to meet the skeptical problems. The history of philosophy can be seen, in part, as a struggle with skepticism. The attacks of the skeptics also have served as a check on rash speculation; the various forms of modern skepticism have gradually eroded the metaphysical and theological bases of European thought. Most contemporary thinkers have been sufficiently affected by skepticism to abandon the search for certain and indubitable foundations of human knowledge. Instead, they have sought ways of living with the unresolved skeptical problems through various forms of naturalistic, scientific, or religious faiths.

See also Aenesidemus; Agrippa von Nettesheim, Henricus Cornelius; al-Ghazālī, Muhammad; Ancient Skepticism; Antiochus of Ascalon; Aristotelianism; Aristotle; Arnauld, Antoine; Augustine, St.; Augustinianism; Austin, John Langshaw; Averroism; Ayer, Alfred Jules; Bacon, Francis; Bayle, Pierre; Beattie, James; Berkeley, George; Bradley, Francis Herbert; Brown, Thomas; Calvin, John; Camus, Albert; Carnap, Rudolf; Carneades; Cartesianism; Charron, Pierre; Cicero, Marcus Tullius; Collier, Arthur; Condorcet, Marquis de; Cratylus; Derrida, Jacques; Descartes, René; Diogenes Laertius; Enlightenment; Erasmus, Desiderius; Fichte, Johann Gottlieb; Fideism; Foucault, Michel; Foucher, Simon; Gassendi, Pierre; Glanvill, Joseph; Gorgias of Leontini; Greek Academy; Halevi, Yehuda; Hamann, Johann Georg; Hegel, Georg Wilhelm Friedrich; Hegelianism; Heidegger, Martin; Herbert of Cherbury; Huet, Pierre-Daniel; Hume, David; Kant, Immanuel; Kierkegaard, Søren Aabye; Lamennais, Hugues Félicité Robert de; La Mothe Le Vayer, François de; Leibniz, Gottfried Wilhelm; Locke, John; Logical Positivism; Luther, Martin; Lyotard, Jean Francois; Mach, Ernst; Maimon, Salomon; Maistre, Comte Joseph de; Malebranche, Nicolas; Mani and Manichaeism; Medieval Philosophy; Mersenne, Marin; Montaigne, Michel Eyquem de; Moore, George Edward; Moral Skepticism; Neo-Kantianism; Nicholas of Cusa; Nietzsche, Friedrich; Ockhamism; Pascal, Blaise; Philo of Larissa; Pico della Mirandola, Count Giovanni; Pico della Mirandola, Gianfrancesco; Platonism and the Platonic Tradition; Popper, Karl Raimund; Protagoras of Abdera; Pyrrho; Reformation; Reid, Thomas; Renaissance; Rorty, Richard; Russell, Bertrand Arthur William; Sanches, Francisco; Santayana, George; Schulze, Gottlob Ernst; Sextus Empiricus; Shestov, Lev Isaakovich; Skepticism, Contemporary; Socrates; Spinoza, Benedict (Baruch) de; Stewart, Dugald; Stoicism; Sulzer, Johann Georg; Timon of Phlius; William of Ockham; Wittgenstein, Ludwig Josef Johann; Xenophanes of Colophon.


The basic statements and arguments of various forms of skepticism are given in:

Academic skepticism: Cicero, Academica and De natura deorum, both with trans. by H. Rackham, Loeb Classical Library (1956).

Pyrrhonian skepticism: Sextus Empiricus, Outlines of Scepticism, ed. by J. Annas, J. Barnes, K. Ameriks, D. M. Clarke (2000); The Skeptic Way: Sextus Empiricus's Outlines of Pyrrhonism, translated, introduction, and commentary by Benson Mates (1996); Adversus Mathematicos, with trans. by R. G. Bury, Loeb Classical Library: vol. 12, Against the Logicians and Outlines of Pyrrhonism (19331936); vol. 3, Against the Physicists, Against the Ethicists (1936); vol. 4, Against the Professors (19591960); and Scepticism, Man, and God: Selections from the Major Writings of Sextus Empiricus, ed. by P. Hallie, translated by S. G. Etheridge (1964); L. Floridi, Sextus Empiricus: The Transmission and Recovery of Pyrrhonism (2002).

Renaissance skepticism: Michel de Montaigne, "L'apologie de Raimond Sebond," in Les essais de Michel de Montaigne, edited by Pierre Villey, new ed. (1922); Fréderic Brahami, Le scepticisme de Montaigne (1997); Donald Frame, Montaigne: A Biography (1984); Elaine Limbrick and Douglas F. Thomson, That Nothing Is Known: Francisco Sanches (1988).

Skepticism and Fideism: Blaise Pascal, Pensées, edited by L. Brunschvicg (1951).

Skepticism in relation to modern philosophy: Pierre Bayle, Dictionnaire historique et critique, esp. the articles "Pyrrho" and "Zeno of Elea," both of which appear in Bayle's Historical and Critical Dictionary: Selections, translated and edited by Richard H. Popkin (1965); David Hume, Dialogues concerning Natural Religion, edited by Richard H. Popkin (1980); Enquiries concerning the Human Understanding and concerning the Principles of Morals, 2nd ed., edited by L. A. Selby-Bigge (1957), and A Treatise of Human Nature, edited by David Fate Norton and Mary J. Norton (2000); Sylvia Giocanti, Penser l'irrésolution: Montaigne, Pascal, La Mothe le Vayer, Trois itinéraires sceptiques (2001); John C. Laursen, The Politics of Scepticism in the Ancients, Montaigne, Hume and Kant (1992); Gianni Paganini, The Return of Scepticism: From Hobbes and Descartes to Bayle (2003); Antony Mckenna and Alain Mothu, La philosophie clandestine a l'âge classique ; José Maia Neto, The Christianization of Pyrrhonism: Scepticism and Faith in Pascal, Kierkegaard, and Shestov (1995); Gianluca Mori, Bayle philosophe (1999); Martin Mulsow, Moderne aus dem Untergrund: radikale frühaufklärung in Deutschland 16801720 (2002); René Pintard, Les libertinage érudits dans le première moitié du XVII siècle (1943); Don Cameron Allen, Doubt's Boundless Sea: Skepticism and Faith in the Renaissance (1964); Richard H. Popkin, The History of Scepticism from Savonarola to Bayle, rev. ed. (2003) and The High Road to Pyrrhonism (1980); Popkin, Silvia Berti, and Françoise Charles-Daubert, Heterodoxy, Spinozism, and Free Thought in Early-Eighteenth-Century Europe: Studies on the Traite Des Trois Imposteurs (1996); Popkin and Arjo Vanderjagt, Skepticism and Irreligion in the Seventeenth and Eighteenth Centuries (1993); Popkin and José Maia Neto, Skepticism in Renaissance and Post-Renaissance Thought: New Interpretations (2004); Popkin, Ezequiel Olaso, and Giorgio Tonelli, Skepticism in the Enlightenment (1997).

The standard studies of ancient skepticism are: Victor Brochard, Les Sceptiques grecs (1887); Norman Maccoll, The Greek Sceptics from Pyrrho to Sextus (1869); Mary Mills Patrick, The Greek Sceptics (1929); Leon Robin, Pyrrhon et le scepticisme grec (1944); and Eduard Zeller, The Stoics, Epicureans and Sceptics, trans. by O. J. Reichel (1880); Charlotte L. Stough, Greek Skepticism (1969); Raoul Richter, Der Skeptizismus in der Philosophie (19041908); Cornelia De Vogel, Greek Philosophy.

For general studies of skepticism, see: Richard H. Popkin, "Skepticism," Encyclopedia of Philosophy, vol. 7, pp. 449461 (1967); Arne Naess, Scepticism (1968); and Benson Mates, Skeptical Essays (1981).

Richard Popkin (1967, 2005)