Ancient Greek skepticism , marble head, Roman copy, Archeological Museum of Corfu Ancient Greek skeptics were not "skeptics" in the contemporary sense of selective, localized doubt. Their concerns were epistemological, noting that truth claims could not be adequately supported, and psychotherapeutic, noting that beliefs caused mental perturbation. The
Western tradition of systematic
skepticism goes back at least as far as
Pyrrho of
Elis (b. ) and arguably to
Xenophanes (b. ). Parts of skepticism also appear among the "5th century
sophists [who] develop forms of debate which are ancestors of skeptical argumentation. They take pride in arguing in a persuasive fashion for both sides of an issue." In
Hellenistic philosophy,
Pyrrhonism and
Academic Skepticism were the two schools of skeptical philosophy. Subsequently, the words
Academic and
Pyrrhonist were often used to mean
skeptic.
Pyrrhonism Like other
Hellenistic philosophies, the goal of Pyrrhonism was
eudaimonia, which the Pyrrhonists sought through achieving
ataraxia (an untroubled state of mind), which they found could be induced by producing a state of
epoché (suspension of judgment) regarding non-evident matters. Epoché could be produced by pitting one
dogma against another to undermine belief, and by questioning whether a belief could be justified. In support of this questioning Pyrrhonists developed the
skeptical arguments cited above (the
Ten Modes of Aenesidemus and the
Five Modes of Agrippa) demonstrating that beliefs cannot be justified:
Pyrrho of Elis According to an account of Pyrrho's life by his student
Timon of Phlius, Pyrrho extolled a way to become happy and tranquil: 'The things themselves are equally indifferent, and unstable, and indeterminate, and therefore neither our senses nor our opinions are either true or false. For this reason then we must not trust them, but be without opinions, and without bias, and without wavering, saying of every single thing that it no more is than is not, or both is and is not, or neither is nor is not.
Aenesidemus Pyrrhonism faded as a movement following the death of Pyrrho's student Timon. The Academy became slowly more dogmatic such that in the first century BCE Aenesidemus denounced the Academics as "Stoics fighting against Stoics", breaking with the Academy to revive Pyrrhonism. Sextus compiled and further developed the Pyrrhonists' skeptical arguments, most of which were directed against the
Stoics but included arguments against all of the schools of
Hellenistic philosophy, including the Academic skeptics. Sextus, as the most systematic author of the works by Hellenistic skeptics which have survived, noted that there are at least
ten modes of skepticism. These modes may be broken down into three categories: one may be skeptical of
the subjective perceiver, of the objective world, and
the relation between perceiver and the world. His arguments are as follows.
Subjectively, the powers of the senses and reasoning may vary among different people. And since knowledge is a product of one or the other, and since neither are reliable, knowledge would seem to be in trouble. For instance, a
color-blind person sees the world quite differently from everyone else. Moreover, one cannot even give preference based on the power of reason, i.e., by treating the rational animal as a carrier of greater knowledge than the irrational animal, since the irrational animal is still adept at navigating their environment, which suggests the ability to "know" about some aspects of the environment. Secondly, the personality of the individual might also influence what they observe, since (it is argued) preferences are based on sense-impressions, differences in preferences can be attributed to differences in the way that people are affected by the object. (Empiricus: 56) Third, the perceptions of each individual sense seemingly have nothing in common with the other senses: i.e., the color "red" has little to do with the feeling of touching a red object. This is manifest when our senses "disagree" with each other: for example, a mirage presents certain visible features, but is not responsive to any other kind of sense. In that case, our other senses defeat the impressions of sight. But one may also be lacking enough powers of sense to understand the world in its entirety: if one had an extra sense, then one might know of things in a way that the present five senses are unable to advise us of. Given that our senses can be shown to be unreliable by appealing to other senses, and so our senses may be incomplete (relative to some more perfect sense that one lacks), then it follows that all of our senses may be unreliable. (Empiricus: 58) Fourth, our circumstances when one perceives anything may be either natural or unnatural, i.e., one may be either in a state of wakefulness or sleep. But it is entirely possible that things in the world really are exactly as they appear to be to those in unnatural states (i.e., if everything were an elaborate dream). (Empiricus: 59) One can have reasons for doubt that are based on the
relationship between objective "facts" and subjective experience. The positions, distances, and places of objects would seem to affect how they are perceived by the person: for instance, the portico may appear tapered when viewed from one end, but symmetrical when viewed at the other; and these features are different. Because they are different features, to believe the object has both properties at the same time is to believe it has two contradictory properties. Since this is absurd, one must suspend judgment about what properties it possesses due to the contradictory experiences. (Empiricus: 63) One may also observe that the things one perceives are, in a sense, polluted by experience. Any given perception—say, of a chair—will always be perceived within some context or other (i.e., next to a table, on a mat, etc.) Since this is the case, one often only speaks of ideas as they occur in the context of the other things that are paired with it, and therefore, one can never know of the true nature of the thing, but only how it appears to us in context. (Empiricus: 64) Along the same lines, the skeptic may insist that all things are relative, by arguing that: • Absolute appearances either differ from relative appearances, or they do not. • If absolutes do not differ from relatives, then they are themselves relative. • But if absolutes do differ from relatives, then they are relative, because all things that differ must differ from something; and to "differ" from something is to be relative to something. (Empiricus: 67) Finally, one has reason to disbelieve that one knows anything by looking at problems in understanding objects by themselves. Things, when taken individually, may appear to be very different from when they are in mass quantities: for instance, the shavings of a goat's horn are white when taken alone, yet the horn intact is black.
Skeptical arguments The ancient Greek
Pyrrhonists developed sets of arguments to demonstrate that claims about reality cannot be adequately justified. Two sets of these arguments are well known. The oldest set is known as
the ten tropes of Aenesidemus—although whether he invented the
tropes or just systematized them from prior Pyrrhonist works is unknown. The tropes represent reasons for
epoché (
suspension of judgment). These are as follows: • Different animals manifest different modes of perception; • Similar differences are seen among individual men; • For the same man, information perceived with the senses is self-contradictory • Furthermore, it varies from time to time with physical changes • In addition, this data differs according to local relations • Objects are known only indirectly through the medium of air, moisture, etc. • These objects are in a condition of perpetual change in color, temperature, size and motion • All perceptions are relative and interact one upon another • Our impressions become less critical through repetition and custom • All men are brought up with different beliefs, under different laws and social conditions Another set are known as
the five tropes of Agrippa: •
Dissent – The uncertainty demonstrated by the differences of opinions among philosophers and people in general. •
Progress ad infinitum – All proof rests on matters themselves in need of proof, and so on to infinity, i.e, the
regress argument. •
Relation – All things are changed as their relations become changed, or, as we look upon them from different points of view. •
Assumption – The truth asserted is based on an unsupported assumption. •
Circularity – The truth asserted involves a circularity of proofs. According to
Victor Brochard "the five tropes can be regarded as the most radical and most precise formulation of philosophical skepticism that has ever been given. In a sense, they are still irresistible today."
Academic skepticism Pyrrho's thinking subsequently influenced the
Platonic Academy, arising first in the
Academic skepticism of the
Middle Academy under
Arcesilaus (c. 315 – 241 BCE) and then the
New Academy under
Carneades (c. 213–129 BCE).
Clitomachus, a student of Carneades, interpreted his teacher's philosophy as suggesting an account of knowledge based on truth-likeness. The Roman politician and philosopher,
Cicero, was also an adherent of the skepticism of the New Academy, even though a return to a more
dogmatic orientation of the school was already beginning to take place.
Augustine on skepticism In 386 CE,
Augustine published
Contra Academicos (Against the Academic Skeptics), which argued against claims made by the Academic Skeptics (266–90 BCE) on the following grounds: • Objection from Error: Through logic, Augustine argues that philosophical skepticism does not lead to happiness like the Academic Skeptics claim. His arguments is summarized as: • A wise man lives according to reason, and thus is able to be happy. • One who is searching for knowledge but never finds it is in error. • Imperfection objection: People in error are not happy, because being in error is an imperfection, and people cannot be happy with an imperfection. • Conclusion: One who is still seeking knowledge cannot be happy. • Error of Non-Assent: Augustine's argument that suspending belief does not fully prevent one from error. His argument is summarized below. • Introduction of the error: Let P be true. If a person fails to believe P due to
suspension of belief in order to avoid error, the person is also committing an error. • The Anecdote of the Two Travelers: Travelers A and B are trying to reach the same destination. At a fork in the road, a poor shepherd tells them to go left. Traveler A immediately believes him and reaches the correct destination. Traveler B suspends belief, and instead believes in the advice of a well-dressed townsman to go right, because his advice seems more persuasive. However, the townsman is actually a
samardocus (con man) so Traveler B never reaches the correct destination. • The Anecdote of the Adulterer: A man suspends belief that adultery is bad, and commits adultery with another man's wife because it is persuasive to him. Under Academic Skepticism, this man cannot be charged because he acted on what was persuasive to him without assenting belief. • Conclusion: Suspending belief exposes individuals to an error as defined by the Academic Skeptics. •
Skepticism's revival in the sixteenth century Francisco Sanches's
That Nothing is Known (published in 1581 as
Quod nihil scitur) is one of the crucial texts of
Renaissance skepticism.
Michel de Montaigne (1533–1592) The most notable figure of the Skepticism revival in the 1500s,
Michel de Montaigne wrote about his studies of
Academic Skepticism and
Pyrrhonism through his
Essais. His most notable writings on skepticism occurred in an essay written mostly in 1575–1576, "Apologie de Raimond Sebond", when he was reading
Sextus Empiricus and trying to translate
Raimond Sebond's writing, including his proof of
Christianity's natural existence. The reception to Montaigne's translations included some criticisms of Sebond's proof. Montaigne responded to some of them in
Apologie, including a defense for Sebond's logic that is skeptical in nature and similar to Pyrrhonism. His refutation is as follows: • Critics claiming Sebond's arguments are weak show how egoistic humans believe that their logic is superior to others'. • Many animals can be observed to be superior to humans in certain respects. To argue this point, Montaigne even writes about dogs who are logical and creates their own
syllogisms to understand the world around them. This was an example used in
Sextus Empiricus. • Since animals also have rationality, the over-glorification of man's mental capabilities is a trap—man's folly. One man's reason cannot be assuredly better than another's as a result. • Ignorance is even recommended by religion so that an individual can reach faith through obediently following divine instructions to learn, not by one's logic.
Marin Mersenne (1588–1648) Marin Mersenne was an author, mathematician, scientist, and philosopher. He wrote in defense of science and Christianity against atheists and
Pyrrhonists before retiring to encourage development of science and the "new philosophy", which includes philosophers like
Gassendi,
Descartes,
Galileo, and
Hobbes. A major work of his in relation to Skepticism is
La Verité des Sciences, in which he argues that although we may not be able to know the true nature of things, we can still formulate certain laws and rules for sense-perceptions through science. Additionally, he points out that we do not doubt everything because: • Humans do agree about some things, for example, an ant is smaller than an elephant • There are natural laws governing our sense-perceptions, such as optics, which allow us to eliminate inaccuracies • Man created tools such as rulers and scales to measure things and eliminate doubts such as bent oars, pigeons' necks, and round towers. A Pyrrhonist might refute these points by saying that senses deceive, and thus knowledge turns into infinite regress or circular logic. Thus Mersenne argues that this cannot be the case, since commonly agreed upon rules of thumb can be hypothesized and tested over time to ensure that they continue to hold. Furthermore, if everything can be doubted, the doubt can also be doubted, so on and so forth. Thus, according to Mersenne, something has to be true. Finally, Mersenne writes about all the mathematical, physical, and other scientific knowledge that is true by repeated testing, and has practical use value. Notably, Mersenne was one of the few philosophers who accepted
Hobbes's radical ideology—he saw it as a new science of man.
Baruch Spinoza and religious skepticism Baruch Spinoza was among the first European philosophers who were religious skeptics. He was quite familiar with the philosophy of
Descartes and unprecedentedly extended the application of the Cartesian method to the religious context by analyzing religious texts with it. Spinoza sought to dispute the knowledge-claims of the
Judeo-Christian-Islamic religious system by examining its two foundations: the
Scripture and the
Miracles. He claimed that all Cartesian knowledge, or the rational knowledge should be accessible to the entire population. Therefore, the Scriptures, aside from those by Jesus, should not be considered the secret knowledge attained from God but just the imagination of the prophets. The Scriptures, as a result of this claim, could not serve as a base for knowledge and were reduced to simple ancient historical texts. Moreover, Spinoza also rejected the possibility for the Miracles by simply asserting that people only considered them miraculous due to their lack of understanding of the nature. By rejecting the validity of the Scriptures and the Miracles, Spinoza demolished the foundation for religious knowledge-claim and established his understanding of the Cartesian knowledge as the sole authority of knowledge-claims. Despite being deeply skeptical of the religions, Spinoza was in fact exceedingly anti-skeptical towards reason and rationality. He steadfastly confirmed the legitimacy of reason by associating it with the acknowledgement of God, and thereby skepticism with the rational approach to knowledge was not due to problems with the rational knowledge but from the fundamental lack of understanding of God. Spinoza's religious skepticism and anti-skepticism with reason thus helped him transform epistemology by separating the theological knowledge-claims and the rational knowledge-claims.
Pierre Bayle (1647–1706) Pierre Bayle was a French philosopher in the late 17th century that was described by Richard Popkin to be a "supersceptic" who carried out the skeptic tradition to the extreme. Bayle was born in a Calvinist family in
Carla-Bayle, and during the early stage of his life, he converted into Catholicism before returning to Calvinism. This conversion between religions caused him to leave France for the more religiously tolerant Holland where he stayed and worked for the rest of his life. Bayle's real intention behind his extremely destructive works remained controversial. Some described him to be a
Fideist, while others speculated him to be a secret
Atheist. However, no matter what his original intention was, Bayle did cast significant influence on the upcoming
Age of Enlightenment with his destruction of some of the most essential theological ideas and his justification of religious tolerance Atheism in his works. He especially espoused skepticism regarding
inductive reasoning, and questioned what the foundation of
morality was, creating the
is–ought problem. His approach to skepticism is considered even more radical than
that of
Descartes. Hume argued that any coherent idea must be either a mental copy of an
impression (a direct sensory
perception) or copies of multiple impressions innovatively combined. Since certain human activities like religion, superstition, and
metaphysics are not premised on any actual sense-impressions, their claims to knowledge are logically unjustified. Furthermore, Hume even demonstrates that science is merely a psychological phenomenon based on the
association of ideas: often, specifically, an assumption of
cause-and-effect relationships that is itself not grounded in any sense-impressions. Thus, even scientific knowledge is logically unjustified, being not actually objective or provable but, rather, mere conjecture flimsily based on our minds perceiving
regular correlations between distinct events. Hume thus falls into extreme skepticism regarding the possibility of any certain knowledge. Ultimately, he offers that, at best, a
science of human nature is the "only solid foundation for the other sciences".
Immanuel Kant (1724–1804) Immanuel Kant (1724–1804) tried to provide a ground for empirical science against
David Hume's skeptical treatment of the notion of cause and effect. Hume (1711–1776) argued that for the notion of cause and effect no analysis is possible which is also acceptable to the empiricist program primarily outlined by
John Locke (1632–1704). But, Kant's attempt to give a ground to knowledge in the empirical sciences at the same time cut off the possibility of knowledge of any other knowledge, especially what Kant called "metaphysical knowledge". So, for Kant, empirical science was legitimate, but metaphysics and philosophy was mostly illegitimate. The most important exception to this demarcation of the legitimate from the illegitimate was ethics, the principles of which Kant argued can be known by pure reason without appeal to the principles required for empirical knowledge. Thus, with respect to metaphysics and philosophy in general (ethics being the exception), Kant was a skeptic. This skepticism as well as the explicit skepticism of
G. E. Schulze gave rise to a robust discussion of skepticism in
German idealistic philosophy, especially by
Hegel. Kant's idea was that the real world (the
noumenon or thing-in-itself) was inaccessible to human reason (though the empirical world of nature can be known to human understanding) and therefore we can never know anything about the ultimate reality of the world. Hegel argued against Kant that although Kant was right that using what Hegel called "finite" concepts of "the understanding" precluded knowledge of reality, we were not constrained to use only "finite" concepts and could actually acquire knowledge of reality using "infinite concepts" that arise from self-consciousness.
Skepticism in the 20th century and contemporary philosophy G. E. Moore famously presented the "
Here is one hand" argument against skepticism in his 1925 paper, "
A Defence of Common Sense". In contemporary philosophy,
Richard Popkin was a particularly influential scholar on the topic of skepticism. His account of the history of skepticism given in
The History of Scepticism from Savonarola to Bayle (first edition published as
The History of Scepticism From Erasmus to Descartes) was accepted as the standard for contemporary scholarship in the area for decades after its release in 1960.
Barry Stroud also published a number of works on philosophical skepticism, most notably his 1984 monograph,
The Significance of Philosophical Scepticism. From the mid-1990s, Stroud, alongside
Richard Fumerton, put forward influential anti-externalist arguments in favor of a position called "metaepistemological scepticism". Other contemporary philosophers known for their work on skepticism include
James Pryor,
Keith DeRose, and
Peter Klein. == History of skepticism in non-Western philosophy ==