Extraordinary claims require extraordinary evidence -- Carl Sagan
(I know this quote is very much a cliche -- but, as a realized a long time after seeing it, it is not only a nice heuristic, but it also emphasizes the bayesian, probabilistic view of knowledge over the popperian one.)
A monthly thread for posting any interesting rationality-related quotes you've seen recently on the Internet, or had stored in your quotesfile for ages.