Skepticism in Western philosophy, is the attitude of doubting knowledge claims set forth in various areas. Skeptics have challenged the adequacy or reliability of these claims by asking what principles they are based upon or what they actually establish. They have questioned whether some such claims really are, as alleged, indubitable or necessarily true, and they have challenged the purported rational grounds of accepted assumptions. In everyday life, practically everyone is skeptical about some knowledge claims; but philosophical skeptics have doubted the possibility of any knowledge beyond that of the contents of directly felt experience. The original Greek meaning of skeptikos was “an inquirer,” someone who was unsatisfied and still looking for truth.
From ancient times onward skeptics have developed arguments to undermine the contentions of dogmatic philosophers, scientists, and theologians. The skeptical arguments and their employment against various forms of dogmatism have played an important role in shaping both the problems and the solutions offered in the course of Western philosophy. As ancient philosophy and science developed, doubts arose about various basic, widely accepted beliefs about the world. In ancient times, skeptics challenged the claims of Plato and Aristotle and their followers, as well as those of the Stoics; and during the Renaissance similar challenges were raised against the claims of Scholasticism and Calvinism. In the 17th century, skeptics attacked Cartesianism (the system established by the French philosopher and mathematician René Descartes) along with other theories that attempted to justify the scientific revolution initiated by Copernicus, Kepler, and Galileo. Later, a skeptical offensive was leveled against the Enlightenment philosopher Immanuel Kant and then against the philosophical idealist Georg Wilhelm Friedrich Hegel and his followers. Each challenge led to new attempts to resolve the skeptical difficulties. Skepticism, especially since the Enlightenment, has come to mean disbelief—primarily religious disbelief—and the skeptic has often been likened to the village atheist.