www.csicop.org/si/9012/critical-thinking.html (external - login to view)
And you might also usefully look here for some critical thinking lessons:
www.skepdic.com/refuge/ctlessons.html (external - login to view)
Since some folks are disinclined to follow links, I’ll summarize Lett’s essay for you.
This idea is due to Karl Popper, who used it to distinguish science from pseudoscience: Any true claim must in principle be falsifiable. It must be possible to at least imagine evidence that would prove it false. This really amounts to saying that the evidence must matter. If no conceivable evidence could ever disprove a claim, then the evidence in its favour doesn’t matter either, it’s impervious to any kind of evidence. That doesn’t mean such a claim must be therefore be true, it just means the claim is meaningless in any factual sense. Or, to use a sparkling phrase from one of my old professors, it is “propositionally vacuous.”
Any argument offered in support of a claim must be logically sound. This means the conclusion must follow unavoidably from the premises and the premises must be true. This is often hard to be sure of. It’s easy to produce an absolutely valid argument and come to a completely false conclusion if you start with false premises. Knowing whether the premises are true or not can involve a lot of work and a lot of additional information.
The evidence offered in support of a claim must be comprehensive. You must consider all the evidence, not just the material that supports the claim. You’re not allowed to pick the evidence you like and discard the rest.
The evidence must be evaluated without deception, of yourself or others; you must be honest. This is often very difficult to do, because the will to believe is so strong, and it’s very easy to deceive yourself no matter how careful you are.
Any test or experiment must be duplicated by others, with the same result. This is the safeguard against fraud, error, and coincidence. A single test result by itself is never adequate.
The evidence offered in support of a claim must be sufficient to establish its truth. In particular, testimonials are never adequate. No credentials place anybody above the risk of fallibility, and sincerity is worth nothing.
And there are some additional stipulations: the burden of proof is on the claimant, and extraordinary claims demand extraordinary evidence.
The burden of proof is on the claimant for a simple reason: failing to prove something is false is not the same as proving it’s true. For instance, we cannot prove no UFOs are alien spacecraft, the best we can do is show that it’s highly unlikely, so anybody making that claim has to prove the positive. Extraordinary claims require extraordinary evidence for reasons of balance. If I claim it rained last Tuesday where I live, you’d be justified in accepting my claim as reasonable on the basis of your own experience of the world. But if I claim I was abducted by aliens who performed a variety of humiliating and invasive medical procedures on me last Tuesday, you’re justified in demanding more substantial evidence than just my claim. The ordinary evidence of my testimony is adequate for ordinary claims, but not for bizarre ones like alien abduction.
James Lett summarized it this way: “Because human beings are often motivated to rationalize and to lie to themselves, because they are sometimes motivated to lie to others, because they can make mistakes, and because perception and memory are problematic, we must demand that the evidence for any factual claim be evaluated without self-deception, that it be carefully screened for error, fraud, and appropriateness, and that it be substantial and unequivocal.” Any claim that fails any of these six rules deserves to be rejected. Passing all six doesn’t mean something is true, but it does mean you’re justified in placing considerable confidence in it.
And just for completeness, here’s a partial list of some things that fail the tests of those six rules, in no particular order except the order they occurred to me: astrology, palmistry, graphology, iridology, reflexology, reincarnation, talking with the dead, astral projection, spirit channelling, telepathy, teleportation, telekinesis, clairvoyance, hidden codes in the Bible, anything Nostradamus ever wrote and anything anyone now thinks it might mean, Tarot, Ouija boards, homeopathy, pyramidology, dowsing, psychic surgery, alien abductions, crystal therapy, creation science, intelligent design theory, anything Erich von Daniken ever published, anything Emmanuel Velikovsky ever published, anything James Redfield’s published with the word ‘Celestine’ anywhere in it, anything ‘non-fiction’ L. Ron Hubbard ever published, anything with the name Art Bell, Whitley Streiber, or Deepak Chopra attached to it....ach, the list is endless.