also spelled SCEPTICISM, the philosophical attitude of doubting the

knowledge claims set forth invarious areas and asking what they are based upon, what they actually establish, and whether they are indubitableor necessarily true. Skeptics have thus challenged the alleged grounds of accepted assumptions in metaphysics, in

science, in morals and manners, and especially in religion.

A brief treatment of Skepticism follows. For full treatment, see

MACROPAEDIA: Philosophical Schools and Doctrines, Western: Skepticism.

Skeptical philosophical attitudes are prominent throughout the course ofWestern 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 thou ght 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 li ve,

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 k nowledge.

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 encouragedthe acceptance of religious truths by f aith.

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 m anifested 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. Ren

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 Skept ical 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 the y 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 th e 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 Alber t 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 tostruggle with that absurdity. In the 20th century other forms of Skept icism have been expressed among Logical

Positivist and Linguistic philosophers.