I think most of us intuitively understand that the final arbiter in any argument as to whether X is true or false, exists or does not exist is evidence. If a party can present incontrovertible and overwhelming evidence for its claim then the matter soon comes to a close. If neither side has any evidence then the debate can hardly end. If the particular claims do not lend themselves to being tested (and are nonfalsifiable), if the claims cannot (perhaps in principle) ever have any evidence either for or against them, then the debate will go on ad infinitum (or until the proponents die out). Without evidence no one can know definitively whether the claims are true or false.
For instance in the case of "facilitated communication," study after study after study has shown that FC does not work as it is touted. Studies have repeatedly shown that when facilitators don't know the answers to the questions then what appears on the computer monitor are incorrect. Only when they know the answers will the child apparently type out the right answers. The implication is clear. It is the facilitator doing the typing (albeit unconsciously, perhaps through the ideomotor effect), not the autistic child. The confuting evidence is so strong that the American Psychological Association in a 1994 resolution stated that "facilitated communication is a controversial and unproved communication procedure with no scientifically demonstrated support for its efficacy."
On the other hand, supporting evidence for evolution is so overwhelming no biologist in his right mind would even dare suggest that evolution theory is crap. The flood of evidence comes from various fronts: from fossil records, homologies, selection experiments, DNA functional redundancies, transposons, redundant pseudogenes, endogenous retroviruses, .... While supporting evidence is legion there is thus far no evidence that refutes evolution.
In a recent "row" in an online discussion board the matter of presenting evidence regarding certain issues vis-a-vis Mr. Eliseo Soriano and his Ang Dating Daan (The Old Path) tv program came up. "Evidence counts," declared a Christian who's hardly sympathetic to Soriano, referring to video/audio tapes being aired by rival program Ang Tamang Daan (The Right Path) that's supposedly incriminating, evidence that proves beyond the shadow of a doubt that Soriano said/did so and so. Failing to firm up his allegations a member of Soriano's group retorted, "you cannot say things [on this forum] without evidence."
Clearly these Christians know that evidence is vital in showing whether a claim is true and not just idle (or libelous) claims. The producers of the Iglesia ni Cristo (trans. Church of God) program Ang Tamang Daan likewise know it is essential that they have proof Soriano said or did whatever it is they're claiming he said or did; thus they present (ad nauseam) a litany of video and audio clips in each of their episodes to make their case. (As to whether these AV clips have been doctored or taken out of context--we won't go off tangent and take up that issue. Needless to say, fudging and doctoring evidence is most unethical. That South Korean stem cell researcher got fired and internationally disgraced for doing exactly that). If there is good evidence for a claim then that claim has a good probability of being true. If there is no evidence then one must doubt whether it is true, and should not believe until such time good evidence is presented. Ostensibly, even Christians of various denominations/sects know the value of and need for evidence.
What is so ironic, therefore, is that the Christian forumers in those discussion threads fail to apply the same test to their core and collective beliefs--that the theological claims in the bible are in fact true. They fail to ask themselves: What objective evidence is there--outside the bible of course--that lend support to the theological claims therein, that evince they are more than just claims/beliefs by ancient writers? How do we know that those theological claims/beliefs actually have a basis in objective reality? Where's the evidence? If there is no good/testable/persuasive/compelling evidence should we believe these claims or must such lack give us pause? Should we ask for evidence only for certain matters while exempting others--specifically our treasured beliefs--from rules of evidence? Why? And what could possibly be the bases/criteria for deciding which matters are to be exempted from the need for evidence? How do we justify these criteria? If there is no evidence or no good evidence for claim Z and if Z is an extraordinary claim which does not cohere with the best of our current fund of knowledge (justified, evidence-based understanding) or if Z is not implied by the same, then is it rational to believe in Z? If so why?
The bias is pretty conspicuous. Evidence--and that means evidence with substance, evidence that will stand up to the rigors of critique (and intersubjective testing/verification)--is sought only for those beliefs that do not accord with what we want to believe in. For those beliefs that we cherish and have invested our hearts in, we relax the demands and requisites, and may not even ask for anything more than having others believing in the same. For instance, if we believe that aliens are in our midst then the flimsiest of evidence--e.g. I saw an unidentified blip streaking across the sky last night! Mr. To Pak said he fought off a couple of aliens trying to abduct him--is used to substantiate the belief. Then when confronted with the fact that such anecdotes don't prove anything and must be critically perused and investigated to rule out various other more parsimonious explanations, we resort to any number of fallacious arguments, or we declare, "Irrelevant. It doesn't matter. Because I believe in them!" And when told that mere belief (faith) does not make something true, that merely believing without evidence is wishful thinking and delusory, we unleash a storm of ad hominems and ad baculums and consign and damn our interrogator to extraterrestrial hell.
Such is the power of (strongly-held) beliefs. The power to derail and cloud our faculty to think clearly, critically, rationally, objectively.