In Western thought, skepticism has raised basic epistemological issues. In view of the varieties of human experience, it has questioned whether it is possible to determine which experiences are veridical. The variations that occur in different perceptions of what is presumed to be one object raise the question of which view is correct. The occurrence of illusory experiences raises the question of whether it is really possible to distinguish illusions and dreams from reality. The criteria employed can be questioned and require justification. How does one know whether one has the right criteria? By other criteria? Then, are these correct? On what standards? The attempt to justify criteria must lead to an infinite regress or stop arbitrarily. If an attempt is made to justify knowledge claims by starting with first principles, what are these based upon? Can it be established that these principles cannot possibly be false? If so, is the proof itself such that it cannot be questioned? If it is claimed that the principles are self-evident, can one be sure of this, and sure that one is not mistaken? And can one be sure that one can recognize and apply the principles correctly? Through such questioning, skeptics have indicated the basic problems that an investigator would have to resolve before he could be certain of possessing knowledge—i.e., information that could not possibly be false.
Some critics of skepticism have contended that it is an untenable view, both logically and humanly. Any attempt to formulate the position is self-refuting, since it will involve at least some knowledge claims about what is supposed to be dubious. Montaigne suggested that what the skeptics needed was a nonassertive language, reflecting the claim of Sextus that the skeptic does not make assertions but only chronicles his feelings. But the strength of skepticism lies not in whether it can be stated consistently but in its effects on the arguments of dogmatic philosophers. As Hume said, skepticism may be self-refuting, but in the process of refuting itself it undermines dogmatism. Skepticism, Sextus said, is like a purge that eliminates itself as well as everything else.
Other critics have claimed that anyone who tried to be a complete skeptic, denying or suspending all judgments about ordinary beliefs, would soon be driven insane. Even Hume thought that the complete skeptic would wind up starving to death or walking into walls or out of windows. Hume, therefore, made a distinction between philosophical doubt and natural, practical human activities. Skeptical philosophizing goes on in theory, while believing occurs in practice. Sextus and the 20th-century Norwegian skeptic Arne Naess, on the other hand, argued that skepticism is a form of mental health. Instead of going mad, the skeptic—without commitment to fixed positions—can function better than the dogmatist.
Other antiskeptical thinkers, such as A.J. Ayer and John Austin, contended that skepticism is simply unnecessary. If knowledge is defined in terms of criteria that are truly meaningful, reflecting how knowledge claims are actually advanced, challenged, and justified, then knowledge is open to all. The skeptics raise false problems, since there are, as a matter of fact, criteria for distinguishing illusory experiences from veridical ones. Doubts are resolved and knowledge attained through these procedures, after which further doubt is simply meaningless. However, Naess, in his book Scepticism (1969), sought to show that, on the standards offered by Ayer and Austin, it is still possible to ask whether a given knowledge claim may turn out to be false; hence skepticism has yet to be overcome.
As noted above, skepticism throughout history has played a dynamic role in forcing dogmatic philosophers to find better or stronger bases for their views. It has forced a continued reexamination of previous knowledge claims and has stimulated creative thinkers to work out new theories to meet skeptical problems. Indeed, the history of philosophy can be seen, in part, as a struggle with skepticism. The attacks of the skeptics also have served as a check on rash speculation; the various forms of modern skepticism have gradually eroded the metaphysical and theological bases of European thought. Most contemporary thinkers have been sufficiently affected by skepticism to abandon the search for certain and indubitable foundations of human knowledge. Instead, they have sought ways of living with unresolved skeptical problems through various forms of naturalistic, scientific, or religious faiths.