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Skepticism (or scepticism) is the art of constantly questioning and doubting claims and assertions, and holding that the accumulation of evidence is of fundamental importance. It forms part of the scientific method, which requires relentless testing and reviewing of claimed facts and theories. The "Skeptic" spelling is generally preferred on this website (but not in the liberal UK), in case people might infer that the "c" is silent, as in "science".
 Types of skepticism
There have been various flavors of skepticism endorsed by great (and not-so-great) thinkers throughout history, some with varying degrees of compatibility with the scientific method.
 Philosophical skepticism
Philosophical skepticism is often associated with radical skepticism, which denies the possibility of knowledge. The school of thought traces back to antiquity, most notably in Pyrrhonism. In modern times, it was famously espoused in the "epistemological anarchism" of Paul Feyerabend and the similarly formulated relativism of a number of postmodernists.
A weaker form of philosophical skepticism that evaluates knowledge in a probabilistic fashion has been endorsed by numerous philosophers and scientists. This form of skepticism denies absolute certainty of knowledge, but holds that the evidence for certain ideas is "good enough" to act on them as if they were true, even though they may be later overturned. This is often associated with or espoused in the form of Bayesian statistics.
 Moral skepticism
Moral skepticism holds that there is no objective morality or "natural law." David Hume's "is-ought" distinction is one of the most common arguments in favor of moral skepticism. While moral skepticism is often associated with nihilism and moral relativism, moral skeptics (like Hume) may also hold that moral and ethical systems may be justified, but not by reason alone.
 Scientific and methodological skepticism
What often goes under the banner of "skepticism" today is usually a mixture of scientific and methodological skepticism. Methodological skepticism seeks to determine true and false claims by apportioning strength of belief to the amount of evidence supporting a certain claim. Scientific skepticism is the application of this to the scientific arena. This form of skepticism is also associated with atheism and secular humanist ethics as espoused by figures like Paul Kurtz. It usually accompanies philosophical naturalism and may incorporate weaker forms of philosophical skepticism as well.
 Why skepticism
Skepticism, at its heart, holds that in order to understand the universe, it is best to actually look at it and find out. This means challenging those who merely assert their beliefs about nature, and demanding that they show how they have derived their ideas. Theories and suggestions that can be, and are, backed by evidence and are deemed logically coherent are kept, while those that fail this test are rejected - but, very importantly, nothing is ever held above this challenge. Regardless of how often an idea or theory passes the test of evidence, skepticism holds that there is always a possibility, however slight, that it might be wrong. As a result, skepticism is the true meaning of having an open mind, and in many respects is synonymous with the scientific method.
The skeptical method provides numerous protections against cons and intentional malice by refusing to accept ideas until they have been demonstrated true. It is difficult to con someone who is skeptical about the situation because they will push for more evidence until they are convinced it is genuine. The ability for a skeptic to be convinced by evidence is also proportional to the claim being made, as said by Carl Sagan's famous soundbite "extraordinary claims require extraordinary evidence". Someone claiming to have gone shopping earlier in the morning is subject to less intense scrutiny than someone claiming a dragon is living in their garage.
 Skeptical responsibility
While it's much easier to reject a theory or conjecture wholesale once a key part of it is found to be false (fruit of the poisoned tree and all) it is also a skeptic's duty to pick through individual and specific claims put in the framework and examine them for truth value. Oftentimes something will be half right or situationally right or even just pointing to something genuinely amiss. While broad theses like that of the Austrian school, strict behaviorism, Marxism, strong nanotechnology, etc. are wrong taken as a whole there's still some truth value in them if you examine them firmly enough. Hopefully people will build off what was right rather than ignoring everything in the claim.
 Opposition to skepticism
The nature of skepticism is that no-one's word can ever just be trusted and accepted without questions and without evidence. This can be a problem for people who don't like this because it is fundamentally distrustful, or the people who know that they cannot supply evidence to back up their claims. This leads to stereotypes of skeptics being humourless killjoys who hate people with "open minds" and want to spoil all of the harmless fun that can be had in believing comforting things for their own sake.
Skepticism makes ideas difficult to spread and as thus is the antithesis of authoritarianism. Someone attempting to solicit money, or control a population can do it far easier if the audience doesn't question their actions, methods, motives or reasoning. To turn a skeptic to one's ideas requires a lot of effort and evidence.
 Notable skeptics
In 1999, Skeptical Inquirer magazine named their top ten and other notable skeptics of the 20th Century.
- James Randi - a former stage illusionist turned skeptical speaker. Founded the James Randi Educational Foundation (JREF) and as part of his "act" often consumes an overdose of homeopathic sleeping pills to show they don't work.
- Martin Gardner - a mathematician that popularised mathematical tricks and games he called "recreational mathematics". Also wrote a great deal on the paranormal
- Carl Sagan - popular science presenter for the series Cosmos. Coined the phrase "Extraordinary claims require extraordinary evidence."
- Paul Kurtz - called the "the father of secular humanism" by some in the skeptical community. Founded the Committee for the Scientific Investigation of Claims of the Paranormal (CSICOP) among other things.
- Ray Hyman - member of CSICOP and has done a great deal of investigation into parapsychology and rationalist explainations for the claimed phenomena.
- Philip J. Klass - a major skeptic of UFO claims and general ufology. Like James Randi's famous prize, he offered $10,000 for anyone who could prove their claims about them. As of his death in 2005, it remained unclaimed.
- Isaac Asimov - science fiction author. Also known in the skeptical world for the concept of "wronger than wrong" and his essay The Relativity of Wrong about how knowledge and thinking work.
- Bertrand Russell - mathematician, pacifist, socialist and philosopher. Developed the teapot argument about the burden of proof.
- Harry Houdini - an illusionist and escape artist. He dedicated his later life to debunking what he called "humbug". Later inspired stage magicians Penn and Teller to do the same.
- Albert Einstein - scientist and all-round quote machine. Einstein is credited with redefining modern physics.
- Richard Feynman
- Joe Nickell
- Karl Popper
- H.L. Mencken
- Richard Dawkins
- Stephen Jay Gould
- James Alcock
- Stephen Barrett
- Mario Bunge
- Bart Bok
- Michael Shermer (Skeptic's Society)
- Kendrick Frazier
- Mark Twain
- Oscar Pfungst
- Robert. A. Baker
- John Stear
- John Thomas Sladek
- Matt Dillahunty - host of The Atheist Experience community cable TV show (in Texas, USA), and outspoken skeptic, atheist, and advocate for the separation of church and state
 See also
- Skeptics in the Pub
- Skeptics Guide to the Universe
- Essay:An opinion on the nature of the movement
- Skepticism entry at the Stanford Encyclopedia of Philosophy
- A Skeptical Manifesto
- John Mashey on how to learn about science
- Rationally Speaking on The Varieties of Skepticism