Pyrrhonism was a school of skepticism founded by Pyrrho in the fourth century BCE. It is best known through the surviving works of Sextus Empiricus, writing in the late second century or early third century CE.
Pyrrho of Elis (c. 360-c. 270 BCE) usually is credited with founding this school of skeptical philosophy. He traveled to India with Alexander the Great's army and studied with the magi and the gymnosophists. Pyrrhonism as a school was either revitalized or re-founded by Aenesidemus in the first century BC.
Pyrrhonism's objective is principally psychological, although it is best known for its epistemological arguments, particularly the problem of the criterion. Through epoché (suspension of judgment) the mind is brought to ataraxia, a state of equanimity. As in Stoicism and Epicureanism, eudaimonia is the Pyrrhonist goal of life, and all three philosophies placed it in ataraxia or apatheia. According to the Pyrrhonists, it is one's opinions about non-evident matters that prevent one from attaining eudaimonia.
The main principle of Pyrrho's thought is expressed by the word acatalepsia, which connotes the ability to withhold assent from doctrines regarding the truth of things in their own nature; against every statement its contradiction may be advanced with equal justification.
Pyrrhonists withhold assent with regard to non-evident propositions, that is, dogma. They disputed that the dogmatists had found truth regarding non-evident matters. For any non-evident matter, a Pyrrhonist tries to make the arguments for and against such that the matter cannot be concluded, thus suspending belief. According to Pyrrhonism, even the statement that nothing can be known is dogmatic. They thus attempted to make their skepticism universal, and to escape the reproach of basing it upon a fresh dogmatism. Mental imperturbability (ataraxia) was the result to be attained by cultivating such a frame of mind.
Pyrrhonist practice is for the purpose of achieving epoché, i.e., suspension of judgment. The core practice is through setting argument against argument. To aid in this, the Pyrrhonist philosophers Aenesidemus and Agrippa developed sets of stock arguments.
The ten modes of Aenesidemus
- "The same impressions are not produced by the same objects owing to the differences in animals."
- The same impressions are not produced by the same objects owing to the differences among human beings.
- The same impressions are not produced by the same objects owing to the differences among the senses.
- Owing to the "circumstances, conditions or dispositions," the same objects appear different. The same temperature, as established by instrument, feels very different after an extended period of cold winter weather (it feels warm) than after mild weather in the autumn (it feels cold). Time appears slow when young and fast as aging proceeds. Honey tastes sweet to most but bitter to someone with jaundice. A person with influenza will feel cold and shiver even though she is hot with a fever.
- "Based on positions, distances, and locations; for owing to each of these the same objects appear different." The same tower appears rectangular at close distance and round from far away. The moon looks like a perfect sphere to the human eye, yet cratered from the view of a telescope.
- “We deduce that since no object strikes us entirely by itself, but along with something else, it may perhaps be possible to say what the mixture compounded out of the external object and the thing perceived with it is like, but we would not be able to say what the external object is like by itself."
- "Based, as we said, on the quantity and constitution of the underlying objects, meaning generally by "constitution" the manner of composition." So, for example, goat horn appears black when intact and appears white when ground up. Snow appears white when frozen and translucent as a liquid.
- "Since all things appear relative, we will suspend judgement about what things exist absolutely and really existent. Do things which exist "differentially" as opposed to those things that have a distinct existence of their own, differ from relative things or not? If they do not differ, then they too are relative; but if they differ, then, since everything which differs is relative to something..., things which exist absolutely are relative."
- "Based on constancy or rarity of occurrence." The sun is more amazing than a comet, but because we see and feel the warmth of the sun daily and the comet rarely, the latter commands our attention.
- "There is a Tenth Mode, which is mainly concerned with Ethics, being based on rules of conduct, habits, laws, legendary beliefs, and dogmatic conceptions."
Superordinate to these ten modes stand three other modes:
- I: that based on the subject who judges (modes 1, 2, 3 & 4).
- II: that based on the object judged (modes 7 & 10).
- III: that based on both subject who judges and object judged (modes 5, 6, 8 & 9)
Superordinate to these three modes is the mode of relation.
The five modes of Agrippa
These tropes or "modes" are given by Sextus Empiricus in his Outlines of Pyrrhonism. According to Sextus, they are attributed only "to the more recent skeptics" and it is by Diogenes Laertius that we attribute them to Agrippa. The tropes are:
- 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.
- 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 the mode deriving from dispute, we find that undecidable dissension about the matter proposed has come about both in ordinary life and among philosophers. Because of this we are not able to choose or to rule out anything, and we end up with suspension of judgement. In the mode deriving from infinite regress, we say that what is brought forward as a source of conviction for the matter proposed itself needs another such source, which itself needs another, and so ad infinitum, so that we have no point from which to begin to establish anything, and suspension of judgement follows. In the mode deriving from relativity, as we said above, the existing object appears to be such-and-such relative to the subject judging and to the things observed together with it, but we suspend judgement on what it is like in its nature. We have the mode from hypothesis when the Dogmatists, being thrown back ad infinitum, begin from something which they do not establish but claim to assume simply and without proof in virtue of a concession. The reciprocal mode occurs when what ought to be confirmatory of the object under investigation needs to be made convincing by the object under investigation; then, being unable to take either in order to establish the other, we suspend judgement about both.
With reference to these five tropes, that the first and third are a short summary of the earlier Ten Modes of Aenesidemus. The three additional ones show a progress in the Pyrrhonist system, building upon the objections derived from the fallibility of sense and opinion to more abstract and metaphysical grounds.
According to Victor Brochard “the five tropes can be regarded as the most radical and most precise formulation of skepticism that has ever been given. In a sense, they are still irresistible today.” 
Except for the works of Sextus Empiricus and Diogenes Laërtius, the texts about ancient Pyrrhonism have been lost, except for a summary of Pyrrhonian Discourses by Aenesidemus, preserved by Photius, and a summary of Pyrrho's teaching preserved by Eusebius, quoting Aristocles, quoting Pyrrho's student Timon, in what is known as the "Aristocles passage":
Whoever wants eudaimonia (to live well) must consider these three questions: First, how are pragmata (ethical matters, affairs, topics) by nature? Secondly, what attitude should we adopt towards them? Thirdly, what will be the outcome for those who have this attitude?" Pyrrho's answer is that "As for pragmata they are all adiaphora (undifferentiated by a logical differentia), astathmēta (unstable, unbalanced, not measurable), and anepikrita (unjudged, unfixed, undecidable). Therefore, neither our sense-perceptions nor our doxai (views, theories, beliefs) tell us the truth or lie; so we certainly should not rely on them. Rather, we should be adoxastous (without views), aklineis (uninclined toward this side or that), and akradantous (unwavering in our refusal to choose), saying about every single one that it no more is than it is not or it both is and is not or it neither is nor is not. The outcome for those who actually adopt this attitude, says Timon, will be first aphasia (speechlessness, non-assertion) and then ataraxia (freedom from disturbance), and Aenesidemus says pleasure. 
Similarities with Buddhism
Adiaphora, astathmēta, and anepikrita are strikingly similar to the Buddhist Three marks of existence, suggesting that Pyrrho's teaching is based on what he learned in India, which is what Diogenes Laertius reported.
Other similarities between Pyrrhonism and Buddhism include a version of the tetralemma among the Pyrrhonist maxims and a parallel with the Buddhist Two Truths Doctrine. In Pyrrhonism the Buddhist concept of "ultimate" (paramārtha) truth corresponds with truth as defined via the criterion of truth, which in Pyrrhonism is seen as undemonstrated, and therefore nothing can be called "true" with respect of it being an account of reality. The Buddhist concept of "conventional" or "provisional" (saṁvṛti) truth corresponds in Pyrrhonism to truth defined via the Pyrrhonist criterion of action, which is used for making decisions about what to do.
The Pyrrhonist school influenced and had substantial overlap with the Empiric school of medicine. Many of the well-known Pyrrhonist teachers were also Empirics, including: Sextus Empiricus, Herodotus of Tarsus, Heraclides, Theodas, and Menodotus. However, Sextus Empiricus said that Pyrrhonism had more in common with the Methodic school in that it “follow[s] the appearances and take[s] from these whatever seems expedient.”
Because of the high degree of similarity between Nagarjuna's philosophy and Pyrrhonism, particularly the surviving works of Sextus Empiricus Thomas McEvilley suspects that Nagarjuna was influenced by Greek Pyrrhonist texts imported into India.
A revival of the use of "Pyrrhonism" as a synonym for "skepticism" occurred during the seventeenth century.
Fallibilism is a modern, fundamental perspective of the scientific method, as put forth by Karl Popper and Charles Sanders Peirce, that all knowledge is, at best, an approximation, and that any scientist always must stipulate this in her or his research and findings. It is, in effect, a modernized extension of Pyrrhonism. Indeed, historic Pyrrhonists sometimes are described by modern authors as fallibilists and modern fallibilists sometimes are described as Pyrrhonists.
List of Pyrrhonist Philosophers
- Agrippa the Skeptic
- Robert Fogelin
- Heraclides of Tarentum
- Herodotus of Tarsus
- Benson Mates
- Menodotus of Nicomedia
- Michel de Montaigne
- Sextus Empiricus
- Theodas of Laodicea
- Academic Skepticism
- Apophatic theology
- Nassim Nicholas Taleb
- One or more of the preceding sentences incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "Scepticism". Encyclopædia Britannica (11th ed.). Cambridge University Press.
- Pulleyn, William (1830). The Etymological Compendium, Or, Portfolio of Origins and Inventions. T. Tegg. p. 353.
- Sextus Empiricus, Outlines of Pyrrhonism, Trans. R.G. Bury, Harvard University Press, Cambridge, Massachusetts, 1933, p. 27
- Sextus Empiricus, Outlines of Pyrrhonism, Trans. R.G. Bury, Harvard University Press, Cambridge, Massachusetts, 1933, p. 47
- Sextus Empiricus, Outlines of Pyrrhonism, Trans. R.G. Bury, Harvard University Press, Cambridge, Massachusetts, 1933, p. 55
- Sextus Empiricus, Outlines of Pyrrhonism, Trans. R.G. Bury, Harvard University Press, Cambridge, Massachusetts, 1933, p.61
- Sextus Empiricus, Outlines of Pyrrhonism, Trans. R.G. Bury, Harvard University Press, Cambridge, Massachusetts, 1933, p.69
- Sextus Empiricus, Outlines of Pyrrhonism, Trans. R.G. Bury, Harvard University Press, Cambridge, Massachusetts, 1933, p.73
- Sextus Empiricus, Outlines of Pyrrhonism, Trans. R.G. Bury, Harvard University Press, Cambridge, Massachusetts, 1933, p.77
- Sextus Empiricus, Outlines of Pyrrhonism, Trans. R.G. Bury, Harvard University Press, Cambridge, Massachusetts, 1933, p. 79
- Sextus Empiricus, Outlines of Pyrrhonism, Trans. R.G. Bury, Harvard University Press, Cambridge, Massachusetts, 1933, p. 81
- Sextus Empiricus, Outlines of Pyrrhonism, Trans. R.G. Bury, Harvard University Press, Cambridge, Massachusetts, 1933, p. 83
- Sextus Empiricus, Outlines of Pyrrhonism, Trans. R.G. Bury, Harvard University Press, Cambridge, Massachusetts, 1933, p. 85
- Sextus Empiricus, Outlines of Pyrrhonism, Trans. R.G. Bury, Harvard University Press, Cambridge, Massachusetts, 1933, pp. 25–27
- Diogenes Laërtius, ix.
- Sextus Empiricus, Pyrrhōneioi hypotypōseis i., from Annas, J., Outlines of Scepticism Cambridge University Press. (2000).
- Brochard, V., The Greek Skeptics.
- Beckwith, Christopher I. (2015). Greek Buddha: Pyrrho's Encounter with Early Buddhism in Central Asia (PDF). Princeton University Press. pp. 22–23. ISBN 9781400866328.
- Beckwith, Christopher I. (2015). Greek Buddha: Pyrrho's Encounter with Early Buddhism in Central Asia (PDF). Princeton University Press. p. 28. ISBN 9781400866328.
- "The Lives and Opinions of Eminent Philosophers". Peithô's Web. Retrieved March 23, 2016.
- Sextus Empricus, Outlines of Pyrrhonism Book 1, Section 19
- McEvilley, Thomas (2002). The Shape of Ancient Thought. Allworth Communications. ISBN 1-58115-203-5., p. 474
- Sextus Empiricus, Outlines of Pyrrhonism I.237, trans. Etheridge (Scepticism, Man, and God, Wesleyan University Press, 1964, p. 98).
- Adrian Kuzminski, Pyrrhonism: How the Ancient Greeks Reinvented Buddhism 2008
- Thomas McEvilley, The Shape of Ancient Thought 2002 pp499-505
- Dialogues Concerning Natural Religion, page 7, section 23.
- Powell, Thomas C. "Fallibilism and Organizational Research: The Third Epistemology", Journal of Management Research 4, 2001, pp. 201–219.
- "Ancient Greek Skepticism" at the Internet Encyclopedia of Philosophy