Skepticism (also spelled "Scepticism") is the philosophical attitude of doubting knowledge claims set forth in various areas. Skeptics have challenged the adequacy or reliability of these claims by asking what they are based upon or what they actually establish. They have raised the question whether such claims about the world are either indubitable or necessarily true, and they have challenged the alleged grounds of accepted assumptions. Practically everyone is skeptical about some knowledge claims; but the skeptics have raised doubts about any knowledge beyond 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...