See R. H. Popkin, The History of Skepticism from Erasmus to Descartes (rev. ed. 1968); C. L. Stough, Greek Skepticism (1969); M. Burnyeat, ed., The Skeptical Tradition (1983); B. Stroud, The Significance of Philosophical Skepticism (1984).
In religion, skepticism refers to "doubt concerning basic religious principles (as immortality, providence, and revelation)." (Merriam–Webster)
The word skepticism can characterize a position on a single claim, but in scholastic circles more frequently describes a lasting mind-set and an approach to accepting or rejecting new information. Individuals who proclaim to have a skeptical outlook are frequently called skeptics, often without regard to whether it is philosophical skepticism or empirical skepticism that they profess.
In philosophical skepticism, pyrrhonism is a position that refrains from making truth claims. A philosophical skeptic does not claim that truth is impossible (which would be a truth claim). The label is commonly used to describe other philosophies which appear similar to philosophical skepticism, such as "academic" skepticism, an ancient variant of Platonism that claimed knowledge of truth was impossible. Empiricism is a closely related, but not identical, position to philosophical skepticism. Empiricists see empiricism as a pragmatic compromise between philosophical skepticism and nomothetic science; philosophical skepticism is in turn sometimes referred to as "radical empiricism."
Philosophical skepticism (in the West) originated in ancient Greek philosophy. One of its first proponents was Pyrrho of Elis (c. 360-275 B.C.), who traveled and studied as far as India and propounded the adoption of "practical" skepticism. Subsequently, in the "New Academy" Arcesilaus (c. 315-241 B.C.) and Carneades (c. 213-129 B.C.) developed more theoretical perspectives, by which conceptions of absolute truth and falsity were refuted as uncertain. Carneades criticized the views of the Dogmatists, especially supporters of Stoicism, asserting that absolute certainty of knowledge is impossible. Sextus Empiricus (c. A.D. 200), the main authority for Greek skepticism, developed the position further, incorporating aspects of empiricism into the basis for asserting knowledge.
Greek skeptics criticized the Stoics, accusing them of dogmatism. For the skeptics, the logical mode of argument was untenable, as it relied on propositions which could not be said to be either true or false without relying on further propositions. This was the regress argument, whereby every proposition must rely on other propositions in order to maintain its validity (see the five tropes of Agrippa the Sceptic). In addition, the skeptics argued that two propositions could not rely on each other, as this would create a circular argument (as p implies q and q implies p). For the skeptics, such logic was thus an inadequate measure of truth and could create as many problems as it claimed to have solved. Truth was not, however, necessarily unobtainable, but rather an idea which did not yet exist in a pure form. Although skepticism was accused of denying the possibility of truth, in fact it appears to have mainly been a critical school which merely claimed that logicians had not discovered truth.
René Descartes is credited for developing a global skepticism as a thought experiment in his attempt to find absolute certainty on which to base the foundation of his philosophy. David Hume has also been described as a global skeptic. However, Descartes was not himself a skeptic and developed his theory of an absolute certainty to disprove other skeptics who argued that there is no certainty.
A scientific (or empirical) skeptic is one who questions the reliability of certain kinds of claims by subjecting them to a systematic investigation. The scientific method details the specific process by which this investigation of reality is conducted. Considering the rigor of the scientific method, science itself may simply be thought of as an organized form of skepticism. This does not mean that the scientific skeptic is necessarily a scientist who conducts live experiments (though this may be the case), but that the skeptic generally accepts claims that are in his/her view likely to be true based on testable hypotheses and critical thinking.
Common topics that scientifically-skeptical literature questions include health claims surrounding certain foods, procedures, and medicines, such as homeopathy, Reiki, Thought Field Therapy (TFT), vertebral subluxations; the plausibility of supernatural entities (such as ghosts, poltergeists, angels, and gods as well as the existence of ESP/telekinesis, psychic powers, and telepathy (and thus the credibility of parapsychology); topics in cryptozoology, Bigfoot, the Loch Ness monster, alien visitations, UFOs, crop circles, astrology, repressed memories, creationism, intelligent design, dowsing, conspiracy theories, and other claims the skeptic sees as unlikely to be true on scientific grounds.
Empirical or scientific skeptics do not profess philosophical skepticism. Whereas a philosophical skeptic may deny the very existence of knowledge, an empirical skeptic merely seeks likely proof before accepting that knowledge.
Because debunkers often challenge popular ideas, many are not strangers to controversy. Critics of debunkers sometimes accuse them of robbing others of hope. Debunkers frequently reply that it is the claimant, whom they many times accuse of exploiting public gullibility, who is guilty of abuse.
Religious skepticism is skepticism regarding faith-based claims. Religious skeptics may focus on the core tenets of religions, such as the existence of divine beings or reports of earthly miracles. A religious skeptic is not necessarily an atheist or agnostic.