From a Stoic point of view, Pyrrho found peace by admitting to ignorance and seeming to abandon the criterion by which knowledge is gained. Pyrrho's ignorance was not the ignorance of children or farm animals: it was a knowledgeable ignorance, arrived at through the application of logical reasoning and exposition of its inadequacy. The school of thought developed primarily in opposition to what was seen as the dogmatism, or ultimately unfounded assertions of the Stoics; Pyrrhonists made distinctions between "being" and "appearing" and between the identity and the sensing of a phenomenon.
Pyrrho and his school were not actually "skeptics" in the later sense of the word. They had the goal of αταραξια (ataraxia - peace of mind), and pitted one dogmatic philosophy against the next to undermine belief in the whole philosophic enterprise. The idea was to produce in the student a state of aversion towards what the Pyrrhonists considered arbitrary and inconsequential babble. 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. For example, the earth is round, and it would remain so even if everyone believed it were flat. Unless, of course, it is flat, and we all simply believe it is round.
The goal of this critique, which Pyrrho's followers realized would ultimately subvert even their own method, was to cultivate a distrust of all grand talk. They expected philosophy to collapse into itself. How far in this direction the Pyrrhonean commitment extended is a matter of debate. The Pyrrhonists confessed a belief in appearances, e.g. in hot and cold, grief and joy. It is impossible to deny, they admitted, that one seems to be in pain or seems to touch a piece of wood. Their world, thus, was completely phenomenological. An accomplished Pyrrhonist could, ideally, live as well as a dogmatist but with the added benefit of not worrying about truth and falsity, right and wrong, God's will, and so forth.
Later thinkers took up Pyrrho's approach and extended it into modern skepticism. In the process, a split appeared within the movement, never too large or well-liked among the literati to begin with. In the New or Middle Academy, Arcesilaus (c. 315-241 B.C.) and Carneades (c. 213-129 B.C.) argued from Stoic premises that the Stoics were actually committed to denying the possibility of knowledge, but seemed to maintain nothing themselves, butClitomachus, 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.
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. A.D. 200), 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 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. On the other hand, his attempt to exempt sensations and appearances from doubt or probabilistic understanding would have been considered dogmatic by most earlier hellenistic sceptics, and by many scepics and probibilists today.
Empiricus, as the most systematic and dogmatic author of the works by Helenistic sceptics which have survived, noted that there are at least ten modes of skepticism. These modes may be broken down into three categories: we may be skeptical of the subjective perceiver, of the objective world, and the relation between perceiver and the world.
Subjectively, both the powers of the senses and of reasoning may vary across persons. And since knowledge is a product of one and/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, we 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. For the irrational animal is still adept at navigating their environment, which presupposes the ability to know about some aspects of the environment.
Secondly, the personality of the individual might also have an impact on 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 we may also be lacking enough powers of sense to understand the world in its entirety: if we had an extra sense, then we 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 we lack), then it follows that all of our senses may be unreliable. (Empiricus:58)
Fourth, our circumstances when we do any perceiving may be either natural or unnatural, i.e., we may be either in a state of wakefulness or that of 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)
We 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, we must suspend judgment about what properties it possesses. (Empiricus:63)
We may also observe that the things we perceive 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, we can only speak of ideas as they occur in the context of the other things that are paired with it. We 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:
Finally, we have reason to disbelieve that we know anything by looking at problems in understanding the objects themselves. Things, when taken individually, may appear to be very different than 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 China, Wang Chong introduced a form of naturalism based on a rational critique of the superstition that was overtaking Confucianism and Daoism in the first century CE. His neo-Daoist philosophy was based on a secular, rational practice not unlike the scientific method.
Each of these seven propositions examines the complex and multifaceted reality from a relative point of view of time, space, substance and mode. To ignore the complexity of the objects is to commit the fallacy of dogmatism.
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 probibilists 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 Plato'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.
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.
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 proposition, 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.
In addition to Hume's argument for external world skepticism, there is another more famous argument. This is Descartes' famous dreaming doubt: Descartes was writing one evening in his room, and he thought to himself (paraphrasing very loosely): What if I am asleep in bed right now, and only dreaming that I am awake, and writing? Isn't that at least possible? Then he said, well surely, I can tell when I am awake and when I am asleep. I can tell the difference between wakefulness and a dream. All sorts of strange things happen in dreams; I pass unaccountably from scene to scene when I'm dreaming; I don't have any long memory of what happened in a day, when I'm dreaming; and so forth. Then Descartes said: Haven't I had those very thoughts in some of my dreams? Sometimes, when I was dreaming, I was convinced that I was awake! I even tried to test that I was awake, when I was dreaming, and the tests convinced me that I was awake! But I was wrong; I was dreaming. Isn't it quite possible that the same thing is happening to me right now? Isn't it possible that I am dreaming that I can test whether I'm awake or asleep -- and of course, in my dream, I pass the test? So it seems really vivid to me right now that I'm awake -- but in fact, I'm asleep?
Well, Descartes said to himself, I guess there aren't any definite signs, or tests, that I could use to tell whether I'm asleep or dreaming. I could, after all, be dreaming those very tests. I have experience of doing that, thinking that I passed the test for being awake, when really I was only dreaming. So there isn't any way to tell that I am awake now. I cannot possibly prove that I am awake. So, Descartes said to himself, I don't really know that I am awake now and writing in the evening. For all I really know, I could be asleep.
Now we can go on and examine this argument in more detail. For one thing, why does Descartes think that he doesn't know he's awake and writing? Well, he might be asleep. But what difference does that make? The difference that it makes is that his faculty of sense-perception would not be reliable if he were asleep. In other words, if he were asleep, it would seem to him that he is seeing, feeling, and hearing various things; but he wouldn't really be. In that case, of course, his faculty of perception wouldn't be reliable. But Descartes appears to go further than that: he appears to be saying that since he might be dreaming, since he can't rule out the hypothesis that he is dreaming right now, that also means that his faculty of perception is not reliable.
To many people, Descartes' position may seem absurd. Most people simply feel that of course they can tell that they're not dreaming. Here, though, Descartes' could reply that maybe you can, but maybe you're just dreaming that you can tell the difference. If you say you can tell the difference between being awake and being asleep, then you are assuming that you're awake, in which case you're begging the question against the skeptic.
Another common sense sort of response to Descartes' argument is that one can tell that one's sense-perception is reliable, and here's how: When one sees something, like that cow chewing on daisies, one can go over to the cow, touch it, hear it, lean on it, and so forth. That confirms that one really is seeing the cow. In the same way, when one hears something, like a marching band outside, one can step outside, and look at the marching band, talk to the members of the band, and so forth. That confirms that one heard the band outside. Throughout a person's life they've had so many experiences like this that they are practically certain that, in the more obvious cases anyway, their faculty of perception works -- it's generally reliable.
Descartes' skeptic will reply to this in much the same way as the previous objection: You might just be dreaming that you are touching, hearing, and leaning on the cow. That marching band might just be part of a dream. For that matter you might only be dreaming that your faculty of perception has been generally reliable. If you argue you're not dreaming as your faculty of perception is reliable, then you are once again begging the question. First, you must establish that you're not dreaming, and that's impossible. Thus, you can't know that your faculty of perception is reliable.
Additionally, a sharper skeptic might make another remark about seeing the cow and hearing the marching band. Because, after all, weren't you using sense-perception in order to try to argue that your faculty of perception is generally reliable? Think about that: in order to show that your sense of sight works, you use your sense of sight and other senses; in order to show that your sense of hearing works, you use your sense of hearing and other senses. And it's not like you can avoid that. It would be really bizarre (though some philosophers have actually tried it) to try to argue that your senses are reliable, without making use of your senses. But if you make use of your senses, you are begging the question again. You have to assume, or presuppose, that your senses are generally shipshape before you start using them to prove anything, including whether your senses are generally shipshape.
How can you prove that perception is reliable without using your senses? That seems impossible. But how can you use senses without assuming that perception is reliable? If you do that then you're arguing in a circle, you're begging the question. So what's the upshot? That you can't prove that perception is reliable. If you try, you beg the question, and question-begging is a logical fallacy.
Notice that this is actually a third skeptical argument, distinct from Hume's and Descartes', although it is related to both. Hume said you can't prove that your sense-data represent the external world; Descartes said that you can't even prove that you're not dreaming; and this third argument says that you can't prove that perception is reliable without assuming that your senses are reliable and thereby begging the question at issue.
Descartes himself thought it definitely did matter. Descartes wanted absolutely certain knowledge -- knowledge beyond any doubt. And so he thought that if you can raise the smallest doubt about something, then you don't really know it. For example, the dreaming doubt raises the possibility that you are not actually reading this article right now; you might be dreaming; and so Descartes would say (at that point -- later he thought he refuted this skepticism) that you don't know you're reading this right now.
So this forces us to ask ourselves: Do we have to have absolute certainty, lacking any doubt whatsoever, in order to have knowledge? That would be the absolutely strongest grade of justification possible. And then we would be saying that knowledge is not just sufficiently justified true belief, but certainly true belief.
Many philosophers don't think that such a strong degree of justification is necessary for knowledge. After all, they claim, we can know what the weather is going to be like, just by reading the morning forecast. Sometimes we're wrong; but if we're right then we have knowledge. So they are not particularly worried if they can't prove that they're not dreaming. They think it's extremely unlikely that they're dreaming, and they think they're perfectly well justified in thinking they're awake. And they don't have to know with absolute certainty that they're awake, of course, to be well-justified in believing they're awake. Note too that Descartes himself rejected his skeptical doubts in the end.
Here's a second thing you might observe about skepticism: if the skeptic makes absolute certainty a requirement for knowledge, then you could reply that this observation should be applied to skepticism itself. Is skepticism itself entirely beyond doubt? Isn't it possible to raise various kinds of objection to skepticism? So it would appear; but then no one can know that skepticism is true. So then the skeptic can't know that skepticism is true. But this is actually a bit of a weak reply, because it doesn't really refute skepticism. The skeptic, after all, may be perfectly happy to admit that no one knows that skepticism is true. The skeptic might rest content saying that skepticism is very probably true. That's not the kind of claim that most non-skeptics will be happy to allow.
However, any claim about the likelihood of a belief is open to sceptical questioning itself. The claim that a theory is likely to be true if it has good predictive success assumes that the reality that is behind our perceptions is itself predictable. The claim that the degree of coherency of the theory affects its likelihood of being true assumes that reality is itself coherent. So any attempt to deny the relevance of scepticism on the basis that it is unlikely cannot avoid begging the question in this respect too, because their account of what makes something likely can, at most, be likely to be true!
The form of Reid's argument is a dilemma, like this: if P, then Q; if not-P, then Q; either ot-P; therefore, in either case, Q. Either the skeptic is right, in which case we can't trust our ability to reason and so can't trust the skeptic's conclusion; or the skeptic is wrong, in which case again we can't trust the skeptic's conclusion. In either case we don't have to worry about skepticism! (In fact, the conclusion is of course not quite true. The dilemma demonstrates that the skeptic's conclusions do not follow from his argument, but only persists if one holds to the conclusiveness of the skeptic in the first place. If the skeptic is simply regarded as "inconclusive," lacking a conclusion altogether, the dilemma does not exist. But it is not unreasonable to reject a conclusion if the only known argument for it has been dismissed.)