“”What song the Syrens sang, or what name Achilles assumed when he hid himself among women, although puzzling questions, are not beyond all conjecture.
|—Sir Thomas Browne quoted as the epigraph to The Murders in the Rue Morgue|
“”A skeptic is one who prefers beliefs and conclusions that are reliable and valid to ones that are comforting or convenient, and therefore rigorously and openly applies the methods of science and reason to all empirical claims, especially their own. A skeptic provisionally proportions acceptance of any claim to valid logic and a fair and thorough assessment of available evidence, and studies the pitfalls of human reason and the mechanisms of deception so as to avoid being deceived by others or themselves. Skepticism values method over any particular conclusion.
|This might be|
|But we're not sure|
Skepticism, also known as scientific skepticism, is the art of constantly questioning and doubting claims and assertions, and holding that the accumulation of evidence is of fundamental importance. Skeptics use the methods and tools of science and critical thinking to determine what is true. These methods are generally packaged with a scientific "attitude" or set of virtues like open-mindedness, intellectual charity, curiosity, and honesty. To the skeptic, the strength of belief ought to be proportionate to the strength of the evidence which supports it. As such, they tend to be quite critical of woo like conspiracy theories, pseudoscience, alternative medicine, and the paranormal.
Skepticism should not be confused with pseudoskepticism, which is really a form of denialism. Steven Novella has made a better definition of skepticism that can differentiate pseudoskepticism and real skepticism which appears to be rather widespread.
“”To kill an error is as good a service as, and sometimes even better than, the establishing of a new truth or fact.
|—Charles Darwin, in correspondence to Wilson (1879)|
While there were skeptics like Harry Houdini and Mark Twain challenging snake oil salesmen and beliefs like spiritualism in the 19th and early 20th century, the modern skeptical movement started in the middle of the 20th century. Martin Gardner's 1952 work, Fads and Fallacies in the Name of Science, was one of the earliest comprehensive works of skepticism and is considered a classic. In it, Gardner criticizes weird things like UFOs, homeopathy, and Scientology on the grounds that they lack supporting evidence. Another watershed moment in the skeptical movement's early history was the 1976 founding of the Committee for Skeptical Inquiry. This non-profit, which was founded by Paul Kurtz, James Randi, Martin Gardner, Carl Sagan, Isaac Asimov, B.F. Skinner, and others, "promotes science and scientific inquiry, critical thinking, science education, and the use of reason in examining important issues." In the 1990s, several other noteworthy works of skepticism were released. These included James Randi's Flim-Flam! (1991), Carl Sagan's The Demon-Haunted World (1995), and Michael Shermer's Why People Believe Weird Things (1997).
Public skeptics like Bill Nye and Richard Dawkins spend considerable time appearing in popular media and news channels combating pseudoscience and denialism like creationism and global warming denial. Sometimes, this involves their being opposed for the sake of balance by a total crank. Some skeptics, such as Brian Dunning and Dawkins, think that this is a bad idea because it creates the image that science and bullshit are equal, opposing views. Others, such as Bill Nye and Michael Shermer, are not opposed to such debates and have had many high-profile ones with pseudoscientists like Marc Morano and Kent Hovind.
“”Skepticism is like sex and pizza: when it's good, it's very very good, and when it's bad, it's still pretty good.
|—Yvain, The Skeptic's Trilemma|
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.
“”Being thankful when someone on your moral or political team has their under-evidenced beliefs pointed out is a type of intellectual honesty.
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 is thus 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.
The increasing popularity of labeling oneself a "skeptic" has led to a surge in people who have a clear misunderstanding of the difference between healthy skepticism and denialism.
Such denialists mislabel themselves as skeptics, but skepticism by definition requires questioning and taking part in a constructive debate over the topic at hand. Refusing to accept any evidence put forward is not skepticism, but is a form of bias based on fear: a defense mechanism for dealing or coping with anxiety, if you will.
Other types of skepticism
Historically, there have also been other various flavors of skepticism besides scientific skepticism. These have been endorsed by great (and not-so-great) thinkers throughout history, some with varying degrees of compatibility with the scientific method.
Philosophical skepticism, which is often associated with radical skepticism, denies the possibility of having justified beliefs. The school of thought traces back to antiquity, most notably in Pyrrhonism. Pyrrho of Elis, the school's founder, was an ancient skeptic that believed any viewpoint could be opposed by arguments which are just as compelling. Given this, he thought that assent to all beliefs should be withheld. Pyrrho, however, did not consider the lack of justified beliefs as a bad thing. He thought that accepting that nothing is justified would lead to a state of mental tranquility known as "ataraxia". The Roman skeptic, Sextus Empiricus, later built on Pyrrho's ideas and created a rigorous skeptical methodology.  In modern times, something like this was famously espoused in the "epistemological anarchism" of Paul Feyerabend. Many postmodernists have also argued in favor of radical forms of relativism in regards to knowledge.
Moral skepticism is a group of meta-ethical theories which hold that there are no objective moral truths or that no one has any moral knowledge. While this is a minority position among philosophers who study ethics, many prominent atheist philosophers have subscribed to some version of moral skepticism. This includes both David Hume and Friedrich Nietzsche. A famous argument against the existence of moral truths, the argument from queerness, was given by atheist philosopher, J.L. Mackie. This argument states that
“”if there were objective values, then they would be entities or qualities or relations of a very strange sort, utterly different from anything else in the universe.
This argument hinges on David Hume's "is-ought" distinction.  Since moral facts are "oughts" rather than "is", they would have to be totally different from the types of entities that exist in the world around us. This radical difference, Mackie thought, should make us question their existence.
In 1999, Skeptical Inquirer magazine named their top ten and other notable skeptics of the 20th Century.
- James Randi — Former stage illusionist turned skeptical speaker. Founded the James Randi Educational Foundation (JREF) and as part of his "act" often consumed an overdose of homeopathic sleeping pills to show they don't work.
- Martin Gardner — 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 explanations for the claimed phenomena.
- Philip J. Klass — 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, and philosopher of science (logical positivism). Developed the teapot argument about the burden of proof.
- Harry Houdini — 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, socialist, and all-around quote machine. Einstein is credited with redefining modern physics.
- Joe Nickell
- Steven Novella
- Richard Dawkins
- Stephen Barrett
- Michael Shermer
- Adam Savage
- Yvette d’Entremont
- Ben Goldacre
- Tim Minchin
- Derren Brown
- Harriet Hall
- Matt Dillahunty
- Brian Dunning
- Bill Nye
- Massimo Pigliucci
- Skeptics in the Pub
- The 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
- Sir Thomas Browne, from his “Hydriotaphia, Urn Burial”
- Poe, E.A., “The Murders in the Rue Morgue”, in The Murders in the Rue Morgue and Other Tales (Penguin English Library 2012) p. 100.
- Skepticism and Epistemic Virtue: Shouldn't skeptics know what they are talking about, when they are talking about it? by Massimo Pigliucci (free pdf)
- Skeptic — the Name thing again by Steven Novella (17th November 2008)
- What is a Skeptic? on Skeptics in the Pub, Oxford
- Who are we? on Skeptic Family
- What we're about by Skeptical Society of St. Louis on meetup.com
- Letter 752. To A. Stephen Wilson. Down, March 5th, 1879. More Letters of Charles Darwin — Volume II. Page 283. Project Gutenberg Etext of More Letters of Charles Darwin Vol. 2
- Home | Skeptical Inquirer
- Should Science Debate Pseudoscience? by Brian Dunning on Skeptoid
- Why Richard Dawkins Doesn't Debate Creationists on Youtube
- Bill Nye the Science Guy vs. Climate Change and Evolution Deniers | The New York Times on Youtube
- Kent Hovind vs Michael Shermer Debate on Youtube
- Move over, Deepak, there's a new woo-meister at HuffPo by Orac (6th May 2010) on ScienceBlogs
- Ancient Skepticism in the Stanford Encylopedia of Philosophy
- Mackie, John (1977). Ethics: inventing right and wrong. London. p. 38.
- Moral Skepticism in the Stanford Encyclopedia of Philosophy
- Ten Outstanding Skeptics of the Century