John Christian Laursen. New Dictionary of the History of Ideas. Editor: Maryanne Cline Horowitz. Volume 5. Detroit: Charles Scribner’s Sons, 2005. 

Skepticism is both a generalized sense of doubt and disbelief as expressed in everyday language and an identifiable school of thought in the history of ideas. In its most general sense it refers to uncertainty, doubt, disbelief, suspension of judgment, and rejection of knowledge. It is characterized by its opposition to dogmatism, which claims to know reality and the truth.

As a philosophical tradition skepticism is best understood as the product of two movements in ancient Greek philosophy. Academic skepticism can be attributed to Socrates and to Plato’s successors at the Academy in Athens (fifth century to second century B.C.E.), and Pyrrhonism can be traced back to Pyrrho of Ellis (c. 365-275 B.C.E.). Elements of skepticism can be found in many other schools of ancient Greek philosophy, from Heraclitus to the Cyrenaics and the Cynics. There are also analogies to ancient Greek and Roman skepticism in ancient Chinese, Persian, Arabic, and Indian philosophy, but they did not have the impact on modern thinking that the Mediterranean skepticisms did.

Academic Skepticism

The Roman philosopher and statesman Marcus Tullius Cicero (106-43 B.C.E.) is the chief source for Academic skepticism. His Academica (45 B.C.E.) reports on the teachings of Arcesilaus (315-240 B.C.E.) and Carneades (214-129 B.C.E.), both heads of the Academy, and he claims allegiance to the Academic school. St. Augustine of Hippo’s earliest extant work, Contra Academicos (Against the Academics; 386 C.E.), is also an important source of knowledge about Academic skepticism.

Socrates can be placed at the origins of skepticism if it is understood that he only asked questions and did not teach positive doctrines. Plato and Aristotle strayed from his path when they claimed to know the truth. Arcesilaus gave renewed vigor to skepticism, arguing against the opinions of all men, as Cicero put it. But he also showed that skeptics could make choices in accordance with the eulogon (the reasonable) in the absence of truth. Carneades, also a master of arguing on both sides of every issue, refined this into the standard of the pithanon (the credible). Cicero translated this into Latin as probabile, setting the stage for the skeptics’ claim to live by the probable in the absence of truth.

Manuscripts of Cicero’s Academica were available in the Middle Ages to figures such as John of Salisbury (1115-1180), who used it to underpin defenses of liberty of thought and speech. The text was first printed at Rome in 1471, followed by numerous commentaries and annotations. By 1600 more than 100 editions had been published.

The Dutch humanist Desiderius Erasmus (1466?-1536) admired Academic skepticism in his Praise of Folly (1511), which provoked opposition from Christians like Philipp Melanchthon (1487-1560). Gianfrancesco Pico della Mirandola’s Examen Vanitatis (1520) drew from both Cicero and Sextus Empiricus. Omer Talon emphasized the Academics’ philosophical freedom from dogmatism in his Academia of 1547, and Petrus Ramus praised their rhetoric and style in Ciceronianus of 1557. Giulio Castellani (1528-1586) defended Aristotelianism against Academic skepticism in Adversus Marci Tullii Ciceronis (1558), arguing that disagreement is not as widespread as the skeptics claimed. Johannes Rosa (1532-1571) brought out a substantial early commentary on the Academica in German in 1571, and Pedro de Valencia (1555-1620) refashioned Academic skepticism in his own Academica of 1596, published in Spain.

Publication of Sextus Empiricus’s works in the 1560s replaced Cicero as the chief source of information about ancient skepticism. After that point most authors drew their inspiration from both sources, so it is hard to speak of purely Academic skeptics from then on. One exception is David Hume (1711-1776), sometimes called an Academic skeptic, among other reasons because a character in his Dialogues concerning Natural Religion (1779) takes the role of an Academic. There has also been scholarly debate about whether other individual early modern figures were Academic skeptics or Pyrrhonians, but in this period the two traditions were often run together and few, if any, authors made a clear distinction.


The chief source for ancient Pyrrhonism is the work of the Greek physician Sextus Empiricus (second century C.E.), including Outlines of Pyrrhonism, Against the Dogmatists, and Against the Mathematicians. Once thought of as a mere compiler, many recent studies have found philosophical originality in his texts. As Sextus explained it, skepticism was not a philosophy but rather a way of life in which one opposed all claims to truth with equal opposite claims (equipollence). Standard tropes or formula arguments could be used against any certainty or truth. He attributed one set of these tropes to the Greek philosopher Aenesidemus and another to Agrippa (both first century B.C.E.). Diogenes Laertius’ Lives of the Philosophers (early third century C.E.) is also a source for information about ancient skepticism, including the tropes.

In Sextus’ account, the basic ten tropes or formula arguments show that the same thing appears differently (1) to different animals, (2) to different individuals, (3) to different senses, (4) to the same sense in different conditions, (5) in different positions or places, (6) in company with different things, (7) in different quantities, (8) in different relations, (9) if common or if rare, and (10) to people with different customs or ways of life. Thus, any claim about a thing could be matched with an equal counterclaim. Other tropes bring out the problem of the criterion (an infinite regress), unresolved disputes, problems with attributing causation, and more. The result of the skeptical tropes was that one would suspend judgment (epochē) and then find oneself in ataraxia, or tranquility, no longer disturbed by conflicting claims. One would live in accordance with the phenomena or appearances, without taking a stand on the truth or reality behind them. One would follow one’s natural impulses as well as local customs and laws.

Even in ancient times, critics of the skeptics accused them of inconsistency, incoherence, immorality, and inability to live their skepticism. These arguments were more and less sophisticated, and ranged widely from the claim that skeptics cannot be fully skeptical because they believe their own positions are true to the claim that skeptics will not make reliable friends. As late as the 1980s, a number of scholars of ancient skepticism continued to maintain these claims, but opinion turned in the 1990s as a consensus emerged that skeptics could indeed live their skepticism, and that they would not necessarily be any more immoral than followers of other philosophies.

Much of Sextus’s text consists of refutation of other dogmatic philosophies of the time. Since he quoted their ideas in order to refute them, his text has been an important source of information about ancient Stoicism, Epicureanism, and other philosophies.

Early Reception

Occasional references to the ancient Pyrrhonists can be found throughout the late Roman and early medieval periods. The oldest extant Greek manuscript of Sextus dates from the tenth century, and manuscripts of Latin translations existed in medieval collections by the fourteenth century. More manuscripts came into Italy from Byzantium in the mid-fifteenth century, when the Florentine religious leader Girolamo Savonarola (1452-1498) used Sextus to combat pagan philosophy, and the humanist scholar Pico della Mirandola drew on Sextus to fight other dogmatists. Knowledge of the materials eventually spread into France and other northern countries.

The printing press made for the most influential dissemination of these texts. Published Latin translations by Henri II Estienne (Stephanus) (1562) and Gentian Hervet (1569) provided the stimulus for a widespread “skeptical crisis.” Michel de Montaigne (1533-1592) was the most influential of the early European writers to draw on the writings of Sextus in hisEssais (Essays; 1580-1595). In his longest essay, “Defense of Raymond Sebond,” Montaigne retailed most of the skeptical tropes and all of the skeptical vocabulary from Sextus Empiricus. In this and other essays he demolished pretensions to human knowledge and argued both sides of nearly all issues. He was never pessimistic but showed people how to live a good life in spite of skepticism, which helps explain why his work was so popular.

Later thinkers often started from Montaigne. One who went beyond him in posing questions of skepticism was René Descartes (1596-1650). Without specific precedent in the ancient materials, he set out to answer the skeptical idea that there could be an all-powerful malin genie or evil demon that manipulates human perceptions and reasoning, fooling people about the world. His conclusion was that individuals know of their existence because they can think—the famous “I think therefore I am.” Explaining why one’s perceptions of thinking could not be a deception, Descartes asserts that God would not allow such deception. Religion is invoked to certify truth. Later skeptics would worry about a deceiving God.

Bishop Pierre-Daniel Huet (1630-1721) and the Huguenot refugee Pierre Bayle (1647-1706) have been described as the “master skeptics.” Huet invoked Sextus Empiricus in great detail against Descartes and many other dogmatic philosophers in his Traité philosophique de la foiblesse de l’esprit humaine (1723; Philosophical tract on the weakness of the human mind). Bayle’s massive works attacked all previous philosophy and historical scholarship but upheld moral rigorism.

Reception in and since the Enlightenment

The Scottish philosopher David Hume responded to the skeptical challenge in ways that made him central to philosophical discussion up to the twenty-first century. His Treatise of Human Nature (1739-1740) argued for skepticism about both facts and reason. His critique of causation reduces it to little more than a habit based on constant conjunction. And yet in typical skeptical fashion he showed people how to live with skepticism on the basis of probabilities and custom.

The Prussian philosopher Immanuel Kant (1724-1804) was called the “all-destroyer” because of his rejection of many other dogmatic philosophies. He adopted skeptical Greek vocabulary when he argued that one could have no knowledge of the noumena—the reality behind appearances—but only of the phenomena. He saved free will and morality from scientific determinism by reducing human knowledge of them to faith rather than knowledge. Other skeptics writing in German in his time included Salomon Maimon and Gottlob Ernst “Aenesidemus” Schulze. When Carl Friedrich Stäudlin’s Geschichte und Geist des Skepticismus (History and spirit of skepticism) of 1794 showed Hume facing Kant on the title page, it was clear that these two thinkers had posed the skeptical challenge for the age. Stäudlin denounced unphilosophical skepticism even as he demonstrated that philosophical skepticism could not be refuted.

In the nineteenth century, the German philosopher Georg Wilhelm Friedrich Hegel (1770-1831) believed that ancient skepticism was of great philosophical importance while modern skepticism had little merit. The Danish philosopher Søren Kierkegaard (1813-1855) incorporated skepticism into his theology. One of the prize questions of the Royal Academy in Paris concerned the failure of all answers to skepticism. The Swiss philologist and philosopher Friedrich Nietzsche (1844-1900) made skepticism a significant part of his philosophy. Karl Marx (1818-1883) and Sigmund Freud (1856-1939) followed in a long-established tradition of using various skepticisms against their opponents but then claiming dogmatic truth for their own positions. Their practice reflected the important distinction between partial skepticism (e.g., of claims in one domain, such as religion, or the claims of an opposing political party) and global or universal skepticism, which suspends judgment about everything.

In the twentieth century Jean Grenier (1898-1971) translated Sextus into French. His student Albert Camus (1913-1960) drew on skepticism in his work as one of the founders of existentialism. In Germany, Odo Marquard (1928-) led a self-consciously skeptical charge against the dogmatisms of thinkers such as Jürgen Habermas. Also in the same century, some analytical philosophers developed their own ahistorical definitions of skepticism and debated them with little if any reference to the traditions of skepticism. Revisionists such as Stephen Toulmin (1922-) then interpreted one of their heroes, Ludwig Wittgenstein (1889-1951), as following in the footsteps of the ancient skeptics.

Skepticism in Medicine and Science

Of all the fields that in the early twenty-first century are considered sciences, medicine has been especially intertwined with skepticism. Sextus Empiricus was a practicing physician whose work influenced his philosophy. The writings of the Greek physicians Hippocrates (c. 460-c. 377 B.C.E.) and Galen (c. 129-c. 200 C.E.) stressed the importance of skeptical observation and experience and the dangers of dogmatic theory in medicine. Their work was an important part of medical education in early modern Europe, introducing the student to both dogmatic medicine and the skeptical critique.

Several prominent early modern physicians developed the connections between skepticism and medicine. The Toulouse professor Francisco Sanches (c. 1550-1623) called himself “Carneades philosophus,” attacking Aristotelian science in his book Quod Nihil Scitur (That nothing is known; 1581). The English physician and philosopher John Locke (1632-1704) borrowed some of the skeptical elements in his philosophy from the skeptical physician Thomas Sydenham (1624-1689). Martín Martínez (1684-1734), royal physician and president of the Royal Medicine Academy of Medicine in Seville, published Medicina Sceptica (1722-1724), attacking dogmatic Galenism, and Philosophia Sceptica (1730), which introduced Descartes to Spain. The German physician Ernst Platner’s (1744-1818) skeptical writings were influential in Kant’s time.

The early natural scientist Francis Bacon (1561-1626) was convinced that the experimental method would produce absolute certainty. Skeptics like François de La Mothe Le Vayer (1583-1672) used skeptical tropes to show that science could not produce certain knowledge. Other philosopher-scientists, such as Marin Mersenne (1588-1648) and Pierre Gassendi (1592-1655) in France, rejected the need for certainty and defended experimental science on the ground that it is enough that it produces useful knowledge. This attitude prevailed at the Royal Society in London. Skepticism could sweep away the pretensions of Aristotelians and other dogmatists while leaving scientists free to continue their experiments. In this spirit, Robert Boyle (1627-1691) named his spokesman “Carneades” in The Sceptical Chymist (1661), and Joseph Glanvill (1636-1680) titled one of his books Scepsis Scientifica (1665).

By the twentieth century, natural science had pretty much left the skeptical path, claiming something close to a monopoly on truth and knowledge. But avatars of the skeptical tradition still emerge here and there in connection with the sciences. The philosopher of science Karl Popper (1902-1994) contended that scientific claims could never be absolutely verified, only falsified. Paul Feyerabend (1924-1994) was described as a Pyrrhonian for his generally skeptical attitude toward all scientific claims.

Skepticism in Law, Historiography, and Political Thought

It is no accident that one of the chief sources for Academic skepticism was a lawyer. After all, Cicero spent much of his professional life making cases for clients, regardless of which side truth was on. Montaigne also studied law and served as a magistrate, and concluded both that judges can make the law come out any way they want, and that they are often wrong. Legal realists in the twentieth century endorsed these views, concluding that the law was more an expression of social power than of truth or certainty. Legal education encourages skepticism by teaching lawyers how to argue both sides of any case.

Especially in the seventeenth century, skepticism made its way into historiography, as writers began to question the received accounts of history. La Mothe Le Vayer’s On the Small Amount of Certainty in History (1668) and Pierre Bayle’s Historical and Critical Dictionary (1697-1702) brought numerous historical errors to public attention. The only lasting solution was to learn to live with the appearances and accept lower standards for practical purposes instead of absolute certainty.

Throughout the early modern era, skepticism was used to justify a wide variety of political stances, from radical reform to quietist conservatism. The implications of many of Montaigne’s political commentaries were quite subversive of the political arrangements of his time. But his contemporary, the Dutch thinker Justus Lipsius (1547-1606), claimed that skepticism justified repression of reformers on the ground that they could not know that they were right. The English political philosopher Thomas Hobbes (1588-1679) accepted much of the skeptical critique of knowledge and concluded that, for the sake of social order, the king should define the truth and punish deviations from it. Hume drew the different political implication that people should be left alone in commercial society to define their own manners and opinions. Kant concluded that one can know what politics should be like (ethical and republican) but that one could never know if these standards are really instantiated in any concrete political arrangements.

Earlier figures from Montaigne to Kant were often aware of the genealogy of their ideas, but even later writers working in ignorance of the roots of their ideas have come up with a similarly wide range of political conclusions. Without indicating much awareness of the skeptical tradition, the British political philosophers Edmund Burke (1729-1797) and Michael Oakeshott (1901-1990), each in different ways, used skepticism to undermine dogmatic political activism. Postmodernists with generally radical or activist sympathies have also not usually been aware of how close some of their positions are to the skeptical tradition.

Skepticism and Religion

The historical scholarship of Isaac la Peyrère (1596-1676), Benedict de Spinoza (1632-1677), and Richard Simon (1638-1712) contributed to skepticism about the Bible. In response, it has been common to accuse skeptics of atheism, libertinism, and immorality. But skeptics were not necessarily atheists. One of the most common uses of skepticism was by the self-described orthodox against pagan claims to truth; by the Lutherans and Calvinists against Catholic claims to infallibility; and by Catholics against Protestant claims to truth. Many religionists believed that if all claims to truth can be demolished, one should accept traditional religion on faith. This position is known as fideism.

Various versions of fideism were widespread. Thinkers from Montaigne to Huet and Bayle wrote that skepticism cleared the way to faith by removing rationalist objections. Kant famously wrote that he had had to deny knowledge in order to make room for faith. Whether some of these figures were insincere atheists, using fideism as a defense against charges of heresy, has been the subject of debate ever since.

But there is little doubt about the sincerity of many fideists. The sixteenth-century translators of Sextus, Hervet and Stephanus, were both Christians who believed that skepticism could help them in apologetics. Blaise Pascal (1623-1662) in France Christianized skepticism by showing that, properly understood, it set the scene for Christianity. Philosophers at the Prussian Academy who translated the Greek, Latin, and British skeptics into French and German, such as Jean-Henri-Samuel Formey (1711-1797), Jean-Bernard Mérian (1723-1807), and Jean de Castillon (1709-1791), tried to draw the teeth of skepticism by adding notes that made it consistent with Christianity. The Germans Johann Georg Hamann (1730-1788) and Friedrich Jacobi (1743-1819) adopted skepticism as a propaedeutic to Christian faith. Kierkegaard claimed that skepticism was the key to proper Christianity, which required a “leap of faith” after dogmatism had been destroyed by skepticism. The Russian theologian Lev Shestov (1866-1938) even rejected mathematics in order to achieve faith. Twentieth-century theologians were also compelled to either use skepticism or refute it.

In the twenty-first century it is safe to say that the challenges of the skeptical tradition to any claims to human truth and knowledge are alive and well. Many and perhaps most modern and postmodern thinkers have internalized much of skepticism, often without full awareness of the genealogy of their ideas. The chief elements of skepticism must be adopted, adapted, or refuted by any thinker. Since no one has succeeded fully at the last of these, variations on the former prevail.