Skepticism or scepticism (Greek skeptomai: to consider, to examine) refers to any view involving doubt.

An attitude of doubt about whether something exists.
A doubt about whether something can be known for certain.
A doubt about whether we are correct in arguing a certain way.
A practical method of suspended judgment, systematic doubt or criticism.

People who have skepticism as an attitude, opinion, or method are called skeptics or sceptics.

The roots of skepticism 
Skepticism on philosophy has its roots in ancient Greek philosophy. The Greek Sophists of the 5th century BC, such as Protagoras of Abdera (480–411 BC), were mostly skeptics. Gorgias (485–380 BC) said, "Nothing exists; if anything does exist, it cannot be known; if anything exists and can be known, it cannot be communicated". In the 4th century BC Pyrrho of Elis (c360–275 BC), who travelled and studied as far as India, adopted practical skepticism. Carneades (c213–129 BC) disagreed that things were completely true or false. He criticized the Dogmatists, especially Stoics. He said that completely certain knowledge is impossible. Sextus Empiricus (c200 AD), the main authority for Greek skepticism, brought together empiricism into the basis for stating knowledge.

Skepticism comes from Asian philosophy, too. The Chinese philosopher Zhuangzi (c369–c286 BC) said that what seems real could be just a dream. In Indian philosophy, the Cārvāka school thought that we should neither try to infer knowledge nor trust what other people tell us. In Buddhism, Nāgārjuna said that nothing exists by itself, and that this idea is an important part of enlightenment.

The most important part of the argument is that unless we know something for certain, then we cannot know anything. The interest lies in the way great philosophers have tried to solve this problem. Some, like Descartes, retreat into the mind: "I think, therefore I am." Others, such as the British empiricists John Locke and David Hume, would rely on our sense perception. Certain kinds of knowledge may be especially vulnerable to skepticism. For example, an agnostic believes people cannot get knowledge about supernatural things. 

Skepticism is widely used as a method for research in science, and (with changes) in modern legal procedure. Here doubt, suspending judgement, careful investigation, testing and discussion come before any statement of facts. Sometimes the process takes years before consensus is reached. This systematic approach is called methodological skepticism. It is probably the most important legacy of the skeptics.

References

Further reading
Klein P. 2002. “Skepticism” in The Oxford Handbook of Epistemology. P. Moser (ed) Oxford University Press. pp336–361

Resources

Literary skeptics 
 Ambrose Bierce: The Devil's Dictionary
 Ignacy Krasicki: Fables and Parables
 Bolesław Prus: Pharaoh
 Voltaire: Candide
 Montaigne: Essays

Organizations 
 Center for Inquiry
 Committee for Skeptical Inquiry
 The Skeptics Society
 James Randi Educational Foundation
 Rationalist International
 The New England Skeptical Society
 Australian Skeptics

Media 
 Penn & Teller: Bullshit!
 MythBusters

Other websites

 New York City Skeptics
 Skeptics Canada
 Sceptics in Russia (Manifesto)
 The Skeptic Friends Network
 British Skeptic magazine
 Australian Skeptics
 Skeptical about skeptics

 
Epistemology