Skepticism is the philosophical attitude of doubting the knowledge claims set forth in various areas and asking what they are based upon, what they actually establish, and whether they are indubitable or necessarily true. Skeptics have thus challenged the alleged grounds of accepted assumptions in metaphysics, in science, in morals and manners, and especially in religion.

Skeptical philosophical attitudes are prominent throughout the course of Western philosophy; as early as the 5th century BC the Eleatic school of thinkers denied that reality could be described in terms of ordinary experience. Evidence of Skeptical thought appears even earlier in non-Western philosophy, in particular in the Upanisads, philosophic texts of the later Vedic period (c. 1000-c. 600 BC) in India.

Pyrrhon of Elis (c. 360-c. 272 BC), credited with founding Greek Skepticism, sought mental peace by avoiding commitment to any particular view; his approach gave rise in the lst century BC to Pyrrhonism, proponents of which sought to achieve epoche (suspension of judgment) by systematically opposing various kinds of knowledge claims. One of its later leaders, Sextus Empiricus (2nd or 3rd century BC), challenged the claims of dogmatic philosophers to know more than what is evident. His goal was the state of ataraxia, wherein a person willing to suspend judgment would be relieved of the frustration of not knowing reality and would live, without dogma, according to appearances, customs, and natural inclination. The Pyrrhonians criticized Academic Skepticism, first developed in Plato's Academy in Greece in the 3rd century BC; the Academics argued that nothing could be known, and that only reasonable or probable standards could be established for knowledge.

Academic Skepticism survived into the Middle Ages in Europe and was considered and refuted by St. Augustine, whose conversion to Christianity convinced him that faith could lead to understanding. Among Islamic philosophers also there arose an antirational Skepticism that encouraged the acceptance of religious truths by faith.

Modern Skepticism dates from the 16th century, when the accepted Western picture of the world was radically altered by the rediscovery of ancient learning, by newly emerging science, and by voyages of exploration, as well as by the Reformation, which manifested fundamental disagreement among Roman Catholics and Protestants about the bases and criteria of religious knowledge. Prominent among modern Skeptical philosophers is Michel de Montaigne, who in the 17th century opposed science and all other disciplines and encouraged acceptance, instead, of whatever God reveals. His view was refuted in part by Pierre Gassendi, who remained doubtful about knowledge of reality but championed science as useful and informative. Reni Descartes also refuted Montaigne's Skepticism, maintaining that by doubting all beliefs that could possibly be false, a person can discover one genuinely indubitable truth: "I think, therefore I am" (cogito ergo sum), and that from that truth one can establish the existence of God and the existence of the external world, which Descartes claimed can be known through mathematical principles. At the end of the 17th century Pierre Bayle employed Skeptical arguments to urge that rational activity be abandoned in favour of pursuit of the conscience.

In the 18th century David Hume assembled some of the most important and enduring Skeptical arguments. He claimed that the very basis of modern science, the method of induction--by which regularities observed in the past justify the prediction that they will continue--is based on the uniformity of nature, itself an unjustifiable metaphysical assumption. Hume also sought to demonstrate that the notion of causality, the identity of the self, and the existence of an external world lacked any basis. In rebuttal, Immanuel Kant maintained that, in order to have and describe even the simplest experience, certain universal and necessary conditions must prevail.

In the 19th century Soren Kierkegaard developed religious Existentialist thought from an irrational Skepticism, asserting that certainty can be found only by making an unjustifiable "leap into faith." Nonreligious Existentialist writers, such as Albert Camus in the 20th century, have claimed that rational and scientific examination of the world shows it to be unintelligible and absurd, but that it is necessary for the individual to struggle with that absurdity. In the 20th century other forms of Skepticism have been expressed among Logical Positivist and Linguistic philosophers.

Excerpt from the Encyclopedia Britannica without permission.