If a man is offered a fact which goes against his instincts, he will scrutinize it closely, and unless the evidence is overwhelming, he will refuse to believe it. If, on the other hand, he is offered something which affords a reason for acting in accordance to his instincts, he will accept it even on the slightest evidence.
-- Bertrand Russell
As it turns out, it is perfectly rational to inspect evidence that contradicts your beliefs more closely than evidence that confirms them. If evidence goes against your beliefs it is more likely to be fake evidence. As the saying goes, your strength as a rationalist is your ability to detect fabricated evidence.
EDIT: As Ben Elliot points out in this post this argument really only applies if you happen to be a perfect Bayesian (and you aren't). In real life you're biased toward confirmatory evidence, and so often you really should check it over disconfirmat...
This is our monthly thread for collecting these little gems and pearls of wisdom, rationality-related quotes you've seen recently, or had stored in your quotesfile for ages, and which might be handy to link to in one of our discussions.