Pyrrhonic skepticism had a tremendous influence on religious debates in post‐​Reformation Europe.

George H. Smith was formerly Senior Research Fellow for the Institute for Humane Studies, a lecturer on American History for Cato Summer Seminars, and Executive Editor of Knowledge Products. Smith’s fourth and most recent book, The System of Liberty, was published by Cambridge University Press in 2013.

In 1562 the French printer Henri Estienne published the first Latin edition of Outlines of Pyrrhonism, written c. 200 CE by the Greek physician Sextus Empiricus. Estienne explained that the work by Sextus had taught him that we cannot rely on reason, a fallible and unreliable instrument, in matters of religion, because this will only pave the way for atheism. Seven years later another publisher of Sextus, the French Catholic Gentian Hervet, also maintained that the arguments of Pyrrhonic skepticism, as summarized by Sextus, could be used to defend Christianity.

Pyrrhonic skepticism is named after Pyrrho of Elis (c. 315–275 BCE), an obscure figure who is portrayed in secondary accounts as a complete doubter, especially in ethical matters. Rather than endure the mental anguish that comes from seeking certain knowledge, Pyrrho is said to have suspended judgment, thereby attaining the mental state known to Greek philosophers as ataraxia, or what today we would call contentment, or peace of mind. Pyrrho’s ideas were put into systematic form by Aenesidemus (c. 100–40 BCE), and it was Aenesidemus and his followers who first embraced the label “skeptic” (from skeptikos, meaning “thoughtful” or “doubter”).

Pyrrhonic skepticism should be distinguished from another type of skepticism that also originated in ancient Greece. Known as Academic skepticism, this school derived its name from Plato’s Academy and was inspired by a remark attributed to Socrates: “All I know is that I know nothing.” As formulated by Arcesilaus (c. 315–241 BCE) and Carneades (c. 213–129 BCE), Academic skepticism maintained that we can attain various degrees of probability, but never certainty, in our quest for knowledge. This position was based on the standard Greek distinction between knowledge (episteme) and opinion (doxa). In this scheme, if a proposition might be false, then it cannot be certain and is therefore mere opinion, not knowledge. Since both our senses and our reason are unreliable to some degree, we can never lay claim to absolute truth, or real knowledge. And since nothing can be known with certainty, we must rely instead on opinions that vary in their degrees of probability.

Pyrrhonic skeptics criticized Academic skepticism as dogmatic. The Academics were not true skeptics because they claimed to know with certainty that certainty is impossible—a claim that is self‐​contradictory and therefore self‐​refuting. The Pyrrhonists, in contrast, did not claim that knowledge is impossible; rather, they suspended judgment on all such theoretical questions, thereby avoiding the mental discomfort generated by taxing one’s brain with insoluble problems. For the Pyrrhonists skepticism was a mental attitude and a way of life, not merely an abstract philosophical doctrine. They refused to judge or to criticize the laws and customs of their society, resolving instead to accept things as they appear to be, without committing themselves to any unorthodox position. In this way they hoped to attain the mental tranquility of ataraxia.

Of these two schools of skepticism, it was Pyrrhonic skepticism that was destined to exert the most influence on the course of western philosophy. This was possibly because no major writings of the Academics survived the ravages of time; what we know of their arguments were transmitted through much later secondary accounts, such as those of Cicero and Augustine. The Pyrrhonists were more fortunate, owing to the summary written by Sextus Empiricus, Outlines of Pyrrhonism, around 200 CE. Manuscript copies of this work began to circulate during the Italian Renaissance and were eventually disseminated throughout Europe.

Pyrrhonic skepticism, as summarized by Sextus, created a sensation—some called it a crisis—among European intellectuals. Some philosophers (such as Montaigne) embraced it enthusiastically, whereas others (such as Descartes) attempted with equal enthusiasm to refute it. But so tremendous was the influence of Sextus Empiricus that by the end of the seventeenth century the “the divine Sextus” was widely hailed as the father of modern philosophy. (For a brilliant account of the impact of Pyrrhonic skepticism on modern philosophy, see Richard H. Popkin, The History of Scepticism from Erasmus to Spinoza, rev. ed., 1964.) Pyrrhonism would remain a serious topic of discussion in Enlightenment philosophy, as we see in the writings of David Hume, a major figure in freethought whom I shall discuss in a subsequent essay.

The writings of Sextus Empiricus were used mainly by French Catholics to defend a position known as fideism, which is essentially an effort to vindicate faith at the expense of reason. By stripping reason of its cognitive efficacy, the fideists appealed to faith as the sole and ultimate source of certainty. If we look to reason instead, we will sink into a morass of uncertainty where no belief can claim superiority over any other. Fideism was a popular method of argument among French Catholics for three‐​quarters of a century, one that was commonly aimed at Calvinists. Pyrrhonic skepticism also proved useful among theologians, both Catholic and Protestant, against freethinkers who sought to discredit Christian doctrines in the name of reason. By demonstrating the impotence of reason, Pyrrhonism teaches us intellectual humility and prepares us to receive the doctrines of Christ through faith.

As noted above, Pyrrhonic skepticism, unlike Academic skepticism, did not deny outright the possibility of certain knowledge; Pyrrhonists argued instead that to affirm that certainty is unattainable is a position which is itself uncertain. When examining a philosophic belief, the Pyrrhonists marshaled a battery of arguments for both sides, pro and con, in an effort to show that reason cannot justify one belief more than another. Given this dialectical standoff, the Pyrrhonists argued that we should disengage ourselves from useless philosophic controversies. Only in this way can we attain peace of mind.

It is important to understand that Pyrrhonic skepticism was inherently conservative, because this explains much of its appeal to those Catholics who were attempting to defend the authority of their church against Protestant criticisms. According to the Pyrrhonists, rather than engage in futile speculations about philosophic truth—which could be used to criticize the religious and political status quo—we should resolve instead to accept things as they appear to be, without attempting to judge them. This means that we should passively submit to the laws, customs, and traditional beliefs of our society, rather than challenging them with philosophic principles that cannot themselves be justified.

This conservative bias is what made Pyrrhonism so useful to Catholics during the Counter‐​Reformation. Pyrrhonic arguments, when directed against freethinkers and unbelievers, could be (and were) used by Catholics and Protestants alike, as a means of showing the superiority of faith over reason. But in the internecine battle between Catholics and Protestants, the conservative implications of Pyrrhonism proved to be of greater utility to the former. Protestants, after all, were the innovators—the radicals who had rejected the traditional authority of the Church, root and branch. And though Protestants did not seek to replace that authority with reason, they did appeal to personal judgment in biblical matters as the ultimate rule of faith.

The Catholic Pyrrhonists predicted that dire consequences would result from this religious individualism. The Protestants, in counseling people to rely upon their own judgment in religious matters rather than on the authority of the church, had embarked on a dangerous path. The feeble and unreliable judgments of individuals would result in diverse and conflicting religious beliefs, and terminate in atheism. As Montaigne (1533–92), the major proponent of Catholic Pyrrhonism, put it:

The mass of ordinary people lack the faculty of judging things as they are, letting themselves be carried away by chance appearances. Once you have put into their hands the foolhardiness of despising and criticizing opinions which they used to hold in the highest awe (such as those which concern their salvation), and once you have thrown into the balance of doubt and uncertainty any articles of their religion, they soon cast all the rest of their beliefs into similar uncertainty. They had no more authority for them, no more foundation, than for those you have just undermined; and so, as though it were the yoke of a tyrant, they shake off all those other concepts which had been impressed upon them by the authority of Law and the awesomeness of ancient custom….They then take it upon themselves to accept nothing on which they have not pronounced their own approval, subjecting it to their individual assent.

Montaigne applied Pyrrhonic skepticism to the great theological question of his day: What is the proper rule of faith? By what criterion should we assess religious claims and accept one scriptural interpretation over others? In this area as in others, according to Montaigne, reason is unable to arrive at definitive conclusions. Since reason cannot discern the rule of faith, to rely upon reason in this sphere will land us in a doubt that is fatal to Christian belief. The only alternative is to accept tradition, i.e., to submit to the authority of the Catholic Church.

Montaigne is one of the most ambiguous figures in modern thought. Was he a sincere defender of the Catholic faith, or was his fideism merely a smokescreen for disbelief? This question, debated for centuries, has become a cottage industry for philosophers and historians of the Counter‐​Reformation. For example, the Montaigne scholar M. A. Screech presents Montaigne as a sincere Catholic who should be taken at his word; whereas Richard Popkin, while conceding that Montaigne (and other fideists) “seem capable of both a religious and non‐​religious interpretation,” hazards the opinion that “at best, Montaigne was probably mildly religious. His attitude appears to be more that of indifference or unexcited acceptance, without any serious religious experience or involvement.”

Whatever Montaigne’s own views may have been, his writings (especially An Apology for Raymond Sebond, the longest section in his Essays) provided a mine of information and arguments for later generations of freethinkers – who, as Screech put it, “pillaged [the Apology] for anti‐​Christian arguments.” Pyrrhonism was clearly a double‐​edged sword that could be used either to defend Christianity or to attack it. This point was not lost on Catholic authorities, who in 1697 placed Montaigne’s Essays on the Index of Prohibited Books.