Credit for popularizing the phrase "extraordinary claims demand extraordinary evidence" is usually given to astronomer Carl Sagan, with many sources clarifying that Sagan paraphrased this slogan from an earlier saying attributed to sociologist (and founding Skeptical Inquirer editor) Marcello Truzzi. A few sources, looking deeper, note that similar sentiments were earlier expressed by French mathematician Pierre-Simon Laplace6 and Scottish philosopher David Hume.
MORE: http://www.skepticblog.org/2012/12/11/wonderful-phenomena-demand-wonderful-evidence/
