Philosophical skepticism (UK spelling scepticism; from Greek σκέψις skepsis, "inquiry") is a philosophical school of thought that questions the possibility of certainty in knowledge. Skeptic philosphers from different historical periods adopted different principles and arguments, but their ideology can be generalized as either (1) the denial of possibility of all knowledge or (2) the suspension of judgement due to the inadequacy of evidence. 
- 1 Philosophy of Skepticism
- 2 History of Western Tradition of Skepticism
- 2.1 Ancient Greek skepticism
- 2.2 Skepticism's Revival in the Sixteenth Century
- 2.3 Skepticism in the Seventeenth Century
- 2.4 Kant's skepticism and its influence on German philosophy
- 2.5 Emerging Discussion after the Death of Richard Popkin
- 3 History of Skepticism in Non-Western Philosophy
- 4 See also
- 5 References
- 6 Further reading
- 7 External links
Philosophy of Skepticism
Skepticism is not a single position but covers a range of different positions; in the ancient world there were two main skeptical traditions. Academic skepticism took the dogmatic position that knowledge was not possible; Pyrrhonian skeptics refused to take a dogmatic position on any issue—including skepticism. Radical skepticism ends in the paradoxical claim that one cannot know anything—including that one cannot know about knowing anything.
Skepticism can be classified according to its scope. Local skepticism involves being skeptical about particular areas of knowledge, e.g. moral skepticism, skepticism about the external world, or skepticism about other minds, whereas global skepticism is skeptical about the possibility of any knowledge at all.
Skepticism can also be classified according to its method; in the Western tradition there are two basic approaches to skepticism. Cartesian skepticism —named somewhat misleadingly after René Descartes, who was not a skeptic but used some traditional skeptical arguments in his Meditations to help establish his rationalist approach to knowledge— attempts to show that any proposed knowledge claim can be doubted. Agrippan skepticism focuses on the process of justification rather than the possibility of doubt. According to this view there are three ways in which one might attempt to justify a claim but none of them are adequate: one can keep on providing further justification but this leads to an infinite regress; one can stop at a dogmatic assertion; or one can argue in circular reasoning, never reaching a viable conclusion.
Philosophical skepticism is distinguished from methodological skepticism in that philosophical skepticism is an approach that questions the possibility of certainty in knowledge, whereas methodological skepticism is an approach that subjects all knowledge claims to scrutiny with the goal of sorting out true from false claims.
Philosophical skepticism begins with the claim that the skeptic currently does not have knowledge, some adherents maintain that knowledge is, in theory, possible. It could be argued that Socrates held that view, he appears to have thought that if people continue to ask questions they might eventually come to have knowledge; but that they did not have it yet. Some skeptics have gone further and claimed that true knowledge is impossible, for example the Academic school in Ancient Greece well after the time of Carneades. A third skeptical approach would be neither to accept nor reject the possibility of knowledge.
Skepticism can be either about everything or about particular areas. A 'global' skeptic argues that he does not absolutely know anything to be either true or false. Academic global skepticism has great difficulty in supporting this claim while maintaining philosophical rigor, since it seems to require that nothing can be known—except for the knowledge that nothing can be known, though in its probabilistic form it can use and support the notion of weight of evidence. Thus, some probabilists avoid extreme skepticism by maintaining that they merely are 'reasonably certain' (or 'largely believe') some things are real or true, as for using probabilistic arguments to defend skepticism, in a sense this enlarges or increases scepticism, while the defence of empiricism by Empiricus weakens skepticism and strengthens dogmatism by alleging that sensory appearances are beyond doubt. Much later, Kant would re-define "dogmatism" to make indirect realism about the external world seem objectionable. While many Hellenists, outside of Empiricus, would maintain that everyone who is not sceptical about everything is a dogmatist, this position would seem too extreme for most later philosophers.
Nevertheless, A Pyrrhonian global skeptic labors under no such modern constraint, since he only alleged that he, personally, did not know anything and made no statement about the possibility of knowledge. Nor did Arcesilaus feel bound, since he merely corrected Socrates's "I only know that I know nothing" by adding "I don't even know that", thus more fully rejecting dogmatism.
Local skeptics deny that people do or can have knowledge of a particular area. They may be skeptical about the possibility of one form of knowledge without doubting other forms. Different kinds of local skepticism may emerge, depending on the area. A person may doubt the truth value of different types of journalism, for example, depending on the types of media they trust.
Epistemology and skepticism
Skepticism, as an epistemological argument, poses the question of whether knowledge, in the first place, is possible. Skeptics argue that the belief in something does not necessarily justify an assertion of knowledge of it; in this, skeptics oppose dogmatic foundationalism, which states that there have to be some basic positions that are self-justified or beyond justification, without reference to others. (One example of such foundationalism may be found in Spinoza's Ethics.) The skeptical response to this can take several approaches. First, claiming that "basic positions" must exist amounts to the logical fallacy of argument from ignorance combined with the slippery slope.
Among other arguments, skeptics used Agrippa's trilemma, named after Agrippa the Sceptic, to claim no certain belief could be achieved. Foundationalists have used the same trilemma as a justification for demanding the validity of basic beliefs.
This skeptical approach is rarely taken to its pyrrhonean extreme by most practitioners. Several modifications have arisen over the years, including the following :
Fictionalism would not claim to have knowledge but will adhere to conclusions on some criterion such as utility, aesthetics, or other personal criteria without claiming that any conclusion is actually "true".
Philosophical fideism (as opposed to religious Fideism) would assert the truth of some propositions, but does so without asserting certainty.
Some forms of pragmatism would accept utility as a provisional guide to truth but not necessarily a universal decision-maker.
There are two different categories of epistemological skepticism, which can be referred to as mitigated and unmitigated skepticism, the two forms are contrasting but are still true forms of skepticism. Mitigated skepticism does not accept "strong" or "strict" knowledge claims but does, however, approve specific weaker ones, these weaker claims can be assigned the title of "virtual knowledge", but must be to justified belief. Unmitigated skepticism rejects both claims of virtual knowledge and strong knowledge. Characterising knowledge as strong, weak, virtual or genuine can be determined differently depending on a person's viewpoint as well as their characterisation of knowledge.
Criticism of skepticism
Most philosophies have weaknesses and can be criticized and this is a general principle of progression in philosophy, the philosophy of skepticism asserts that no truth is knowable or only probable. Some say the scientific method also asserts probable findings, because the number of cases tested is always limited and they constitute perceptual observations. Another criticism is the proposition that “no truth is knowable” is knowably true is contradictory, the here is one hand argument is also another relatively simple criticism that reverses the skeptic's proposals and supports common sense.
Pierre Le Morvan (2011) has distinguished between three broad philosophical approaches to skepticism, the first he calls the "Foil Approach." According to the latter, skepticism is treated as a problem to be solved, or challenge to be met, or threat to be parried; skepticism‘s value on this view, insofar as it is deemed to have one, accrues from its role as a foil contrastively illuminating what is required for knowledge and justified belief. The second he calls the "Bypass Approach" according to which skepticism is bypassed as a central concern of epistemology. Le Morvan advocates a third approach—he dubs it the "Health Approach"—that explores when skepticism is healthy and when it is not, or when it is virtuous and when it is vicious.
A skeptical hypothesis is a hypothetical situation which can be used in an argument for skepticism about a particular claim or class of claims. Usually the hypothesis posits the existence of a deceptive power that deceives our senses and undermines the justification of knowledge otherwise accepted as justified. Skeptical hypotheses have received much attention in modern Western philosophy.
The first skeptical hypothesis in modern Western philosophy appears in René Descartes' Meditations on First Philosophy. At the end of the first Meditation Descartes writes: "I will suppose... that some evil demon of the utmost power and cunning has employed all his energies to deceive me."
- The "brain in a vat" hypothesis is cast in scientific terms. It supposes that one might be a disembodied brain kept alive in a vat, and fed false sensory signals, by a mad scientist.
- The "dream argument" of Descartes and Zhuangzi supposes reality to be indistinguishable from a dream.
- Descartes' evil demon is a being "as clever and deceitful as he is powerful, who has directed his entire effort to misleading me".
- The five minute hypothesis (or omphalos hypothesis or Last Thursdayism) suggests that the world was created recently together with records and traces indicating a greater age.
- The simulated reality hypothesis or "Matrix hypothesis" suggest that everyone, or even the entire universe, might be inside a computer simulation or virtual reality.
History of Western Tradition of Skepticism
Ancient Greek skepticism
The Western tradition of systematic skepticism goes back at least as far as Pyrrho of Elis (b. circa 360 BCE). However, "The 5th century sophists 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." There were many disputes that could be found within the philosophical schools of his day, and according to a later account of his life by his student Timon of Phlius, Pyrrho extolled a way to become happy and tranquil.
"Whoever wants to live well (eudaimonia) 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 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 are not "skeptics" in the modern, common sense of the term, meaning prone to disbelief. They had the goal of αταραξια (ataraxia - peace of mind), and pitted one dogma against another to undermine belief in dogmatic propositions. The idea was to produce in the student a state of indifference towards ideas about non-evident matters, since no one can observe or otherwise experience causation, external world (its "externality"), ultimate purpose of the universe or life, justice, divinity, soul, etc., they declared no need to believe in such things. The Pyrrhonists pointed out that, despite claims that such notions were necessary, some people ignorant of them get by just fine before learning about them, they further noted that science does not require belief and that faith in intelligible realities is different from pragmatic convention for the sake of experiment. For each intuitive notion (e.g. the existence of an external world), the Pyrrhonists cited a contrary opinion to negate it. They added that consensus indicates neither truth nor even probability.
Pyrrho's thinking subsequently influenced Plato's Academy, creating the Academic skepticism of the New or Middle Academy, Arcesilaus (c. 315 – 241 BCE) and Carneades (c. 213–129 BCE) argued from Stoic premises that the Stoics were actually committed to denying the possibility of knowledge, but seemed to maintain nothing themselves, but Clitomachus, a student of Carneades, interpreted his teacher's philosophy as suggesting an early probabilistic account of knowledge. The Roman politician and philosopher, Cicero, also seems to have been a supporter of the probabilistic position attributed to the Middle Academy, even if the return to a more dogmatic orientation of that school was already beginning to take place.
- Some things give animals pleasure which give other animals pain. What is useful to one animal is harmful to another.
- Each human has a different assortment of preferences, abilities and interests.
- Each sense gives a different impression of the same object.
- There is no reason to think one is sane while others are insane—the opposite could be true.
- Cultures disagree regarding beauty, truth, goodness, religion, life and justice.
- There is no consistency in perception. (His examples were that the color purple will show different tints depending on the lighting, a person looks different between noon and sunset, and a very heavy rock on land is lighter when in water)
- The senses can be shown to be deceptive. (From a distance, the square tower looks round and the sun looks small)
- Things that strengthen in moderation will weaken when taken in excess, like wine and food.
- When a thing is rare, it surprises people. When a thing is common, it does not surprise people.
- Inter-relations among things are of course relative, and by themselves are unknowable. (e.g. to know 'parent' you must know 'child,' and to know 'child' you must know 'parent.' Neither can be known by itself.)
In the centuries to come, the words Academician and Pyrrhonist would often be used to mean generally skeptic, often ignoring historical changes and distinctions between denial of knowledge and avoidance of belief, between degree of belief and absolute belief, and between possibility and probability.
Sextus Empiricus (c. 200 CE), the main authority for Pyrrhonian skepticism, worked outside the Academy, which by his time had ceased to be a skeptical or probabilistic school, and argued in a different direction, incorporating aspects of empiricism into the basis for evaluating knowledge, but without the insistence on experience as the absolute standard of it. Sextus' empiricism was limited to the "absolute minimum" already mentioned — that there seem to be appearances, he developed this basic thought of Pyrrho's into lengthy arguments, most of them directed against Stoics and Epicureans, but also the Academic skeptics. The common anti-skeptical argument is that if one knows nothing, one cannot know that one knows nothing, and so may know something after all, it is worth noting that such an argument only succeeds against the complete denial of the possibility of knowledge. Considering dogmatic the claims both to know and not to know, Sextus and his followers claimed neither. Instead, despite the apparent conflict with the goal of ataraxia, they claimed to continue searching for something that might be knowable.
Empiricus, as the most systematic author of the works by Hellenistic sceptics 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, both the powers of the senses and of 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 on the basis of 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.
In 386 CE, Augustine published Contra Academicos (Against the Skeptics), which counters many claims that Academic Skeptics make:
- Objection from Error: Through logic, Augustine proves that Skepticism does not lead to happiness like the academic skeptics claim. His proof is summarized below.
- 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 proof that suspending belief does not fully prevent one from error. His proof 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, instead believing 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
Michel de Montaigne (1533-1592)
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 recommend 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)
Mersenne was an author, a mathematician, a scientist, and a 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' radical ideology-- he saw it as a new science of man. 
Skepticism in the Seventeenth Century
Thomas Hobbes (1588-1679)
During his long stay in Paris, Thomas Hobbes was actively involved in the cricle of major skeptics like Gassendi and Mersenne who focus on the study of skepticism and epistemology. Unlike his fellow skeptic friends, Hobbes never treated skepticism as a main topic for discussion in his works. Nonetheless, Hobbes was still labeled as a religious skeptic by his contemporaries for raising doubts about Mosaic authorship of the Pentateuch and his political and psychological explanation of the religions, although Hobbes himself did not go further to challenge other religious principles, his suspicion for the Mosaic authorship did siginificant damage to the religious traditions and paved the way for later religious skeptics like Spinoza and Isaac La Peyrère to further question some of the fundamental beliefs of the Judeo-Christian religious system. Hobbes' answer to skepticism and epsitmology was innvatively political: he believed that moral knowledge and religious knowledge were in their nature relative, and there was no absolute standard of truth governing them, as a result, it was out of political reasons that certain truth standards about religions and ethics were devised and established in order to form functioning government and stable society. 
Baruch Spinoza and Religious Skepticism (1632-1677)
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-Charitian-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 sceptic tradition to the extreme. Bayle was born in a Calvnist 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 believed that truth cannot be obtained through reason and that all human endeavor to acquire absolute knowledge would inevitably lead to failure. Bayle's main approach was highly skeptical and destructive: he sought to examine and analyze all existing theories in all fields of human knowledge in order to show the faults in their reasoning and thus the absurdity of the theories themselves; in his magnum opus, Dictionnaire Historique et Critique (Historical and Critical Dictionary), Bayle painstakingly identified the logical flaws in several works throughout the history in order to emphasize the absolute futility of rationality. Bayle's complete nullification of reason led him to conclude that faith is the final and only way to truth. 
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. 
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 classical German 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.
Emerging Discussion after the Death of Richard Popkin
Because Richard Popkin was one of the founding fathers of study in this area, the account of the history of Skepticism in his books are accepted as the standard. However, recent scholars have been suggesting an addition to Popkin's account. Instead of centering the history of Skepticism around specific figures who wrote key skeptical works, Skepticism is proposed to be a continuous engagement with works by ancients like Sextus Empiricus to modern thinkers like Hume, the engagement with previous works were probably due to unwanted doubts about accepted episteme instead of purely due to classical writings becoming available at any specific time. 
History of Skepticism in Non-Western Philosophy
Ancient Indian skepticism
Ajñana (literally 'non-knowledge') were the skeptical school of ancient Indian philosophy, it was a śramaṇa movement and a major rival of early Buddhism and Jainism. They have been recorded in Buddhist and Jain texts, they held that it was impossible to obtain knowledge of metaphysical nature or ascertain the truth value of philosophical propositions; and even if knowledge was possible, it was useless and disadvantageous for final salvation.
The historical Buddha asserted certain doctrines as true, such as the possibility of nirvana, however he also upheld a form of skepticism with regards to certain questions which he left "un-expounded" (avyākata) and some he saw as "incomprehensible" (acinteyya). Because the Buddha saw these questions (which tend to be of metaphysical topics) as unhelpful on the path and merely leading to confusion and "a thicket of views", he promoted suspension of judgment towards them, this allowed him to carve out an epistemic middle way between what he saw as the extremes of claiming absolute objectivity (associated with the claims to omniscience of the Jain Mahavira) and extreme skepticism (associated with the Ajñana thinker Sanjaya Belatthiputta).
Later Buddhist Philosophy remained highly skeptical of Indian metaphysical arguments, the Buddhist philosopher Nagarjuna in particular has been seen as the founder of the Madhyamaka school, which has been in turn compared with Greek Skepticism. Nagarjuna's statement that he has "no thesis" (pratijña) has parallels in the statements of Sextus Empiricus of having "no position". Nagarjuna famously opens his magnum opus, the Mulamadhyamakakarika, with the statement that the Buddha claimed that true happiness was found through dispelling 'vain thinking' (prapañca, also "conceptual proliferation").
...in both early Buddhism and in the Skeptics one can find the view put forward that man's pursuit of happiness, the highest good, is obstructed by his tenacity in holding ungrounded and unnecessary opinions about all manner of things. Much of Buddhist philosophy, I shall argue, can be seen as an attempt to break this habit of holding on to opinions.
The Cārvāka (Sanskrit: चार्वाक) school of materialism, also known as Lokāyata, is a distinct branch of Indian philosophy, the school is named after Cārvāka, author of the Bārhaspatya-sūtras and was founded in approximately 500 BC. Cārvāka is classified as a "heterodox" (nāstika) system, characterized as a materialistic and atheistic school of thought, this school was also known for being strongly skeptical of the claims of Indian religions, such as reincarnation and karma.
While Jain philosophy claims that is it possible to achieve omniscience, absolute knowledge (Kevala Jnana), at the moment of enlightenment, their theory of anekāntavāda or 'many sided-ness', also known as the principle of relative pluralism, allows for a practical form of skeptical thought regarding philosophical and religious doctrines (for un-enlightened beings, not all-knowing arihants).
According to this theory, the truth or the reality is perceived differently from different points of view, and that no single point of view is the complete truth. Jain doctrine states that, an object has infinite modes of existence and qualities and, as such, they cannot be completely perceived in all its aspects and manifestations, due to inherent limitations of the humans. Anekāntavāda is literally the doctrine of non-onesidedness or manifoldness; it is often translated as "non-absolutism". Syādvāda is the theory of conditioned predication which provides an expression to anekānta by recommending that epithet “Syād” be attached to every expression. Syādvāda is not only an extension of Anekānta ontology, but a separate system of logic capable of standing on its own force, as reality is complex, no single proposition can express the nature of reality fully. Thus the term “syāt” should be prefixed before each proposition giving it a conditional point of view and thus removing any dogmatism in the statement, for Jains, fully enlightened beings are able to see reality from all sides and thus have ultimate knowledge of all things. This idea of omniscience was criticized by Buddhists such as Dharmakirti.
Ancient Chinese philosophy
Zhuang Zhou (~369-286 BC)
Zhuang Zhou (庄子，"Master Zhuang") was a famous ancient Chinese Taoism philospher during the Hundred Schools of Thought period. Zhuang Zhou demonstrated his skeptical thinking through several anecdotes in the preeminent work Zhuangzi that was attributed to him:
- "The Debate on the Joy of Fish" (知鱼之乐) : In this anecdote, Zhuang Zhou argued with his fellow philosopher Hui Shi on if they knew the fish in the pond was happy or not, and Zhuang Zhou said the famous sentence that "You are not I. How do you know that I do not know that the fish are happy?"  ( Autumn Floods 秋水篇, Zhuangzi)
- "The Butterfly of the Dream"(周公梦蝶) : The paradox of "Butterfly Dream" described Zhuang Zhou's confusion after dreaming himself to be a butterfly: "But he didn't know if he was Zhuang Zhou who had dreamt he was a butterfly, or a butterfly dreaming that he was Zhuang Zhou."  (Discussion on Making All Things Equal 齐物篇, Zhuangzi)
Through these anecdotes in Zhuangzi, Zhuang Zhou indicated his belief in the limitation of language and human communication and the inaccessibility of universal truth which established himself as an skeptic, but Zhuang Zhou was by no means a radical skeptic, since he only applied skeptical methods partially in some of his arguments to demonstrate his Taoism beliefs while adopting these Taoism beliefs in a dogmatic fashion. 
Wang Chong (27– ~100 AD)
Wang Chong (王充) was the leading figure of the skeptic branch of the Confucianism school in China during the first century A.D. He introduced a method of rational critique and applied it to the wide-spread dogmatism thinking of his age like phenomenology (the main contemporary Confucianism ideology that linked all natural phenomenons with human ethics), state-led cults, and popular superstition, his own philosophy incorporated both Taoism and Confucianism thinkings, and it was based on a secular, rational practice of developing hypotheses based on natural events to explain the universe which exemplified a form of naturalism that resembled the philosophical idea of Epicureans like Lucretius. 
Medieval Arabic philosophy
In Islamic theology and Islamic philosophy, the scholar Al-Ghazali (1058–1111) is considered a pioneer of methodic doubt and skepticism.[not in citation given] His 11th century book titled The Incoherence of the Philosophers marks a major turn in Islamic epistemology, as Ghazali effectively discovered a methodic form of philosophical skepticism that would not be commonly seen in the West until René Descartes, George Berkeley and David Hume. The encounter with skepticism led Ghazali to embrace a form of theological occasionalism, or the belief that all causal events and interactions are not the product of material conjunctions but rather the immediate and present will of God. While he himself was a critic of the philosophers, Ghazali was a master in the art of philosophy and had immensely studied the field, after such a long education in philosophy, as well as a long process of reflection, he had criticized the philosophical method.
The autobiography Ghazali wrote towards the end of his life, The Deliverance From Error (Al-munqidh min al-ḍalāl; several English translations) is considered a work of major importance. In it, Ghazali recounts how, once a crisis of epistemological skepticism was resolved by "a light which God Most High cast into my breast...the key to most knowledge," he studied and mastered the arguments of Kalam, Islamic philosophy, and Ismailism. Though appreciating what was valid in the first two of these, at least, he determined that all three approaches were inadequate and found ultimate value only in the mystical experience and spiritual insight (Spiritual intuitive thought – Firasa and Nur) he attained as a result of following Sufi practices. William James, in Varieties of Religious Experience, considered the autobiography an important document for "the purely literary student who would like to become acquainted with the inwardness of religions other than the Christian", comparing it to recorded personal religious confessions and autobiographical literature in the Christian tradition.
Scholars have noted the similarities between Descartes' Discourse on Method and Ghazali's work and the writer George Henry Lewes went even further by claiming that "had any translation of it [The Revival of Religious Sciences] in the days of Descartes existed, everyone would have cried out against the plagiarism."[not in citation given]
Recordings of Aztec philosophy suggest that the elite classes believed in an essentially panentheistic worldview, in which teotl represents an unified, underlying universal force. Human beings cannot truly perceive teotl due to its chaotic, constantly changing nature, just the "masks"/facets it is manifested as.
- Benson Mates
- Brain in a vat
- Celia Green
- David Hume
- Dream argument
- Five minute hypothesis
- Münchhausen trilemma
- Pierre Bayle
- Problem of the criterion
- Problem of induction
- Sextus Empiricus
- Simulated reality
- Trivialism (opposite of skepticism)
- Zhuang Zhou
- 1923-, Popkin, Richard Henry, (2003). The history of scepticism : from Savonarola to Bayle. Popkin, Richard Henry, 1923- (Rev. and expanded ed ed.). Oxford: Oxford University Press. ISBN 9780195355390. OCLC 65192690.
- Williams, Michael (2001). "Chapter 5: Agrippa's Trilemma". Problems of Knowledge: A Critical Introduction to Epistemology. Oxford University Press. p. 61. ISBN 978-0192892560.
- Francisco Sanchez, That Nothing is Known, Cambridge University Press, 1989.
- "SKEPTICISM". Encyclopedia of Empiricism. 1997.
- Popkin, H and Stroll, A. Philosophy Made Simple’’ Broadway Books, NY, NY 1993
- Kreeft, Peter & Tacelli, R. K Handbook of Christian Apologetics, IVP Academic, Ill. 1994, p. 367
- Popkin, p. 205
- Popkin, p. 230
- Kreeft p. 373
- Stanford Encyclopedia of Philosophy: Ancient Skepticism.
- Beckwith, Christopher I. (2015). Greek Buddha: Pyrrho's Encounter with Early Buddhism in Central Asia (PDF). Princeton University Press. pp. 22–23. ISBN 9781400866328.
- Diogenes Laërtius 9:80–88
- On the ten modes, see Sextus Empiricus. Outlines of Skepticism I.35–164.
- 1962-, Dutton, Blake D.,. Augustine and academic skepticism : a philosophical study. Ithaca. ISBN 9781501703553. OCLC 951625897.
- 1922-2012,, Hick, John, (). Classical and contemporary readings in the philosophy of religion (2d ed ed.). Englewood Cliffs, N.J.,: Prentice-Hall. ISBN 0131352695. OCLC 90682. Check date values in:
- M.,, Clarke, Desmond. French philosophy, 1572-1675 (First edition ed.). Oxford, United Kingdom. ISBN 9780198749578. OCLC 923850410.
- Skepticism in the modern age : building on the work of Richard Popkin. Maia Neto, José Raimundo, 1959-, Paganini, Gianni, 1950-, Laursen, John Christian., Skepticism from the Renaissance to the Enlightenment: a Conference in Memory of Richard H. Popkin (1923-2005) (2007 : Belo Horizonte, Brazil). Leiden: Brill. 2009. ISBN 9789047431909. OCLC 700517388.
- A companion to early modern philosophy. Nadler, Steven M., 1958-. Malden, Mass.: Blackwell Pub. 2002. ISBN 0631218009. OCLC 49394201.
- Grotius to Gassendi. Chappell, V. C. (Vere Claiborne), 1930-. New York: Garland Pub. 1992. ISBN 0815305761. OCLC 24846343.
- Missner, Marshall (1983). "Skepticism and Hobbes's Political Philosophy". Journal of the History of Ideas. 44 (3): 407–427. doi:10.2307/2709174.
- Black, Sam (1997). "Science and Moral Skepticism in Hobbes". Canadian Journal of Philosophy. 27 (2): 173–207.
- Skepticism and political thought in the seventeenth and eighteenth centuries. Laursen, John Christian,, Paganini, Gianni, 1950-. Toronto. ISBN 9781442649217. OCLC 904548214.
- Batnitzky, Leora (December 2003). "SPINOZA'S CRITIQUE OF RELIGIOUS AUTHORITY: SPINOZA'S CRITIQUE OF MIRACLES". Cardozo Law Review. 25: 57 – via ebscohost.
- 1985-, Matytsin, Anton M.,. The specter of skepticism in the age of Enlightenment. Baltimore. ISBN 9781421420523. OCLC 960048885.
- Lennon, Thomas M. (2002). "What Kind of a Skeptic Was Bayle?". Midwest Studies In Philosophy. 26–1: 258–279 – via Ebscohost.
- David Hume, A Treatise of Human Nature (1739), Book I, "Of the Understanding" and David Hume, An Enquiry Concerning Human Understanding (1748).
- See G. E. Schulze, Aenesidemus (1792), excerpted in Between Kant and Hegel: Texts in the Development of Post-Kantian Idealism, Translated with Introductions by George di Giovanni and H. S. Harris, Indianapolis, Indiana: Hackett Publishing, 2000. See also Frederick C. Beiser, The Fate of Reason: German Philosophy from Kant to Fichte, Cambridge, Massachusetts: Harvard University Press, 1987; Chapter 9, "Schulze's Skepticism".
- See (1) H. S. Harris, "Skepticism, Dogmatism and Speculation in the Critical Journal" (1985), in Between Kant and Hegel: Texts in the Development of Post-Kantian Idealism, Translated with Introductions by George di Giovanni and H. S. Harris, Indianapolis, Indiana: Hackett Publishing, 2000; (2) G. W. F. Hegel, "On the Relationship of Skepticism to Philosophy, Exposition of its Different Modifications and Comparison of the Latest Form with the Ancient One", Translated by H. S. Harris, in di Giovanni and Harris (2000) (cited just above); and (3) Michael N. Forster, Hegel and Skepticism, Cambridge, Massachusetts: Harvard University Press, 1989.
- Georg Wilhelm Friedrich Hegel, The Encyclopedia Logic (1830), § 28, pp. 65–68, Translated by T. F. Garaets, W. A. Suchting, and H. S. Harris, Indianapolis, Indiana: Hackett Publishing, 1991.
- 1968-, Sedley, David Louis, (2005). Sublimity and skepticism in Montaigne and Milton. Ann Arbor: University of Michigan Press. ISBN 9780472115280. OCLC 60715259.
- Kalupahana, David J. A History of Buddhist Philosophy: Continuities and Discontinuities, page 21.
- The Cowherds. Moonshadows: Conventional Truth in Buddhist Philosophy, Oxford University Press, 2011, page 115-116.
- Hayes, Richard P. Dignāga on the interpretation of signs, page 53.
- Hayes, Richard P. Dignāga on the interpretation of signs, page 35.
- Kuzminski, Adrian. Pyrrhonism: How the Ancient Greeks Reinvented Buddhism (Studies in Comparative Philosophy and Religion), 2008.
- Dundas, Paul; John Hinnels ed. (2002). The Jains. London: Routledge. ISBN 0-415-26606-8.
- Koller, John M. (July 2000). "Syādvāda as the epistemological key to the Jaina middle way metaphysics of Anekāntavāda". Philosophy East and West. Honolulu. 50 (3): 400–7. ISSN 0031-8221. JSTOR 1400182. Retrieved 2007-10-01.
- Chatterjea, Tara (2001). Knowledge and Freedom in Indian Philosophy. Lanham, MD: Lexington Books. ISBN 0739106929. pp. 77–87
- Zhuangzi. The Complete works of Zhuangzi. Watson, Burton, 1925-2017. New York. ISBN 9780231164740. OCLC 826640070.
- Berthel, Ken (2015-12-01). "Language in Zhuangzi: A Theme that Reveals the Nature of its Relativism and Skepticism". Journal of Chinese Philosophy. 42: 562–576. doi:10.1111/1540-6253.12215. ISSN 1540-6253.
- Xuerong, Ouyang (December 2003). "略论王充的怀疑主义". JOURNAL OF KAIFENG UNIVERSITY. 17-04: 11–13 – via CNKI.
- 1900-1995., Needham, Joseph, (1978-<1995>). The shorter Science and civilisation in China : an abridgement of Joseph Needham's original text. Ronan, Colin A. Cambridge: Cambridge University Press. ISBN 9780521235822. OCLC 3345021. Check date values in:
- Najm, Sami M. (July–October 1966). "The Place and Function of Doubt in the Philosophies of Descartes and Al-Ghazali". Philosophy East and West. Philosophy East and West, Vol. 16, No. 3/4. 16 (3–4): 133–411. doi:10.2307/1397536. JSTOR 1397536.
- Annotated translations by Richard Joseph McCarthy (Freedom and Fulfillment, Boston: Twayne, 1980; Deliverance From Error, Louisville, Ky.: Fons Vitae, 1999) and George F. McLean (Deliverance from error and mystical union with the Almighty, Washington, D.C.: Council for Research in Values and Philosophy, 2001). An earlier translation by William Montgomery Watt was first published in 1953 (The faith and practice of al-Ghazālī, London: G. Allen and Unwin).
- Gerhard Böwering, Encyclopedia Iranica, s.v. Ghazali.
- McCarthy 1980, p. 66
- William James, Varieties of Religious Experience, Harvard University Press, 1985, p. 319 [= 2002 Modern Library Paperback Edition, p. 438].
- Lewes, George Henry (1867). The History of Philosophy from Thales to Comte, Vol. 2: Modern Philosophy. London: Longmans, Green, and Co.
- James Maffie (2005). "Aztec Philosophy". Internet Encyclopedia of Philosophy.
- James Maffie, Aztec Philosophy: Understanding a World in Motion, University Press of Colorado, 15/03/2014
- Popkin, Richard H.. 2003. The History of Scepticism from Savonarola to Bayle. New York, New York: Oxford University Press.
- Popkin, Richard H. and J. R. Maia Neto, eds. 2007. Skepticism: An Anthology. New York, New York: Prometheus Books.
- Beiser, Frederick C. 1987. The Fate of Reason: German Philosophy from Kant to Fichte. Cambridge, Massachusetts: Harvard University Press.
- Breker, Christian. 2011. Einführender Kommentar zu Sextus Empiricus' "Grundriss der pyrrhonischen Skepsis", Mainz, 2011: electr. publication, University of Mainz. available online (comment on Sextus Empiricus’ “Outlines of Pyrrhonism” in German language)
- di Giovanni, George and H. S. Harris, eds. 2000. Between Kant and Hegel: Texts in the Development of Post-Kantian Idealism. Translated with Introductions by George di Giovanni and H. S. Harris. Indianapolis, Indiana: Hackett Publishing.
- Forster, Michael N. 1989. Hegel and Skepticism. Cambridge, Massachusetts: Harvard University Press.
- Harris, H. S. 1985. "Skepticism, Dogmatism and Speculation in the Critical Journal". In di Giovanni and Harris 2000.
- Georg Wilhelm Friedrich Hegel. 1802. "On the Relationship of Skepticism to Philosophy, Exposition of its Different Modifications and Comparison of the Latest Form with the Ancient One". Translated by H. S. Harris; in di Giovanni and Harris 2000.
- Leavitt, Fred. Dancing with Absurdity: Your Most Cherished Beliefs (and All Your Others) are Probably Wrong. (2015) Peter Lang Publishers.
- Thorsrud, Harald. 2009. Ancient Scepticism. Berkeley, California: University of California Press.
- Unger, Peter. 1975. Ignorance: A Case for Scepticism. Oxford, England: Oxford University Press. 2002.
- Zeller, Eduard and Oswald J. Reichel. 1892. The Stoics, Epicureans and Sceptics. London, England: Longmans, Green, and Co.
- Klein, Peter. "Skepticism". In Zalta, Edward N. Stanford Encyclopedia of Philosophy.
- Ancient Greek Skepticism entry in the Internet Encyclopedia of Philosophy
- Contemporary Skepticism entry in the Internet Encyclopedia of Philosophy
- Responses to skepticism by Keith DeRose
- Article: Skepticism and Denial by Stephen Novella MD, The New England Journal of Skepticism
- Classical Skepticism by Peter Suber
- Review and summary of Skepticism and the Veil of Perception by Michael Huemer