The basic idea behind skepticism.

A skeptic does not deny that things such as ghosts, ESP, faith healing, etc. exist. They just realize that they require evidence and proof to support them, and so far, there is an incredible lack of them for a phenomenon that is supposely widespread.

Try finding even ordinary proof for the following extraordinary claims:
  • The physical universe has an existence independent of our mind.
  • Everything useful to human beings can be examined using our current scientific perspective.
  • All the thousands of experiments and meta-analyses that seem to prove beyond normal standards of scientific proof that ESP has at least some reality, came about by coincidence.

    See also CSICON.

/msg artfuldodger

None of your "extraordinary claims" has any empirical content. You're right, there are no empirical proofs for such claims.

Contrast this with a claim like the existence of horned pink dragons or ESP. These are claims with empirical substance (horned pink dragons are dangerous pests, who will eat my homework if they get a chance; ESP lets me read your mind unless you wrap tin foil around your head). These claims are similar to claims of the existence of the moon and a human ability to do maths; they're just a little more outlandish. In other words, we'd like some fairly impressive proofs of them. Strong enough that Occam's razor will make us conclude that they're more likely to be true than that the "proofs" are somehow wrong.

Invisible pink unicorns fall into the first (unempirical) category, unless I claim some real effect of their existence, which is unexplainable by any other means (including transparent blue unicorns). Of course, I shall be very careful not to make such claims if I wish to retain my credibility.

None of your 3 points has any empirical content that I can glean, it is true. So, by analogy, you'd reason it is impossible e.g. to rule out the existence of God by purely empirical means. This is indeed true. But any claim of empirical proof of God's existence will have to be extraordinary. There are other paths available to you than the scientific one; I imagine many of the religious scientists who exist take these paths. But claiming "scientific proof of God's existence" (or your favourite parapsychological phenomenom, or the like) will require scientific proof.

You don't have to play in Science's playground. But if you do decide to play there, you have to play by the rules.

"extraordinary" is subjective.

Scientific hypotheses are always open to refutation. The hypothesis "all crows are black" can be disproved if a white crow is found. However many black crows are found, the hypothesis can still be disproved by a single bird.

In reality the discovery of the white crow would be treated with scepticism. Those loyal to the black crow hypothesis would argue that it is a well known fact that all crows are black, and this has been shown by numerous studies. Therefore, the white crow is a hoax, forgery or some other kind of white bird. In short, an extraordinary claim. A comment in a birdwatchers notebook may be sufficient proof that a crow is black, but the white crow will have to be studied intensely to prove that it is truly white, and genuinely a crow. The sceptics would probably not be convinced until lots of white crows had been found and studied by scientists. Most scientists would be reluctant to have anything to do with the so called white crow, for fear of damaging their reputations

The statement that "extraordinary claims require extraordinary evidence", means in practice: "claims that I believe require less evidence than claims that I don't believe". It is a way of twisting the evidence to fit the theory.

For example, tree trunks fluctuate with the phases of the moon. This was shown recently by a paper in Nature (Ernst Zurcher, Maria-Giulia Cantiani, Francesco Sorbetti-Guerri and Denis Michel (1998) Tree stem diameters fluctuate with tide, Nature 392 16th April p.665). This particular extraordinary claim was nothing new. The experiment (requiring a calendar and tape measure) had been done several times before. Ridiculed and dismissed every time. The results were no surprise in the French timber industry, where timber is marked with a crescent or circle to indicate when it was cut

Jays are a kind of multicoloured crow.

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