The acronym "FiLCHeRS, ignoring vowels, helps in remembering the six rules of evidential reasoning: Falsifiability, Logic, Comprehensiveness, Honesty, Replicability, and Sufficiency. Apply these rules and no one will ever be able to sneak up on you and steal your belief. You'll be filch-proof.
Falsifiability -- It must be possible to conceive of evidence that would prove the claim false. If nothing conceivable could ever disprove a claim, it is meaningless. There are two principle ways this rule is violated:
– By the undeclared claim: a statement so broad or vague that it lacks propositional content, such as the claim that quartz crystals can restore balance and harmony to a person's spiritual energy. How could you disprove that? The undeclared claim has the advantage that virtually any evidence that could be adduced may be interpreted as confirming the claim. It is especially popular with paranormalists.
– By the multiple out, which is an inexhaustible series of excuses intended to explain away evidence that would seem to falsify the claim. Psychic healers, for example, will attribute failure to a person's lack of faith. The multiple out means, in effect, “Heads I win, tails you lose.”
Logic – Any argument in support of a claim must be both valid and sound. To be valid, the arguments premises must be true. To be sound, the rules of logic must be correctly used to reach conclusions based on such premises.
Comprehensiveness – The evidence must be exhaustive--that is, all of the available evidence must be considered. The successes of psychics, for example, are cited without reference to their much more numerous failures.
Honesty – The evidence must be evaluated without self-deception. Parapsychologists violate this rule when they conclude, after failure to replicate an initially positive result, that psi must be an elusive phenomenon. The more honest conclusion would be that the original result must have been a coincidence.
Replicability – If the evidence for a claim is based upon an experimental result, or if the evidence offered in support of a claim could logically be explained as coincidental, then the result must be repeated in subsequent experiments or trials.
The rule of replicability, which requires independent persons to follow the same procedures and achieve the same results, is an effective way of correcting bias, error, or fraud in experiments. When I correctly predict the roll of the dice, is it psychic ability or coincidence? You should demand that I repeat feat a convincing number of times.
Sufficiency – The evidence offered in support of a claim must be adequate to establish the truth of that claim:
– The burden of proof for any claim rests on the claimant. UFO buffs argue that UFO sightings not explained by skeptics must be extraterrestrial spacecraft. Hitler is alive and well in Argentina. Is it true just because you can't prove me wrong?
– Extraordinary claims demand extraordinary evidence. If I claim that it rained on my way to work last Tuesday, you would be justified in assigning that claim considerable credibility. But if I claim that I was abducted by Martians, you would want better evidence.
– Evidence based on authority and/or testimony is inadequate for any extraordinary claim. No amount of expertise in a field is a guarantee against human fallibility, nor does expertise preclude the motivation to lie.
Passing all six tests does not assure that a claim is true (there may be contrary evidence tomorrow), but it does mean that you have sold your conviction for a fair price, and that it has not been filched from you.
The above is a summary of a helpful article by James Lett in the Winter 1990 issue of Skeptical Enquirer, which I published in TELICOM, The Journal of the International Society for Philosophical Enquiry, October 1991.