Today's post, Absurdity Heuristic, Absurdity Bias was originally published on 05 September 2007. A summary (taken from the LW wiki):

Under some circumstances, rejecting arguments on the basis of absurdity is reasonable. The absurdity heuristic can allow you to identify hypotheses that aren't worth your time. However, detailed knowledge of the underlying laws should allow you to override the absurdity heuristic. Objects fall, but helium balloons rise. The future has been consistently absurd and will likely go on being that way. When the absurdity heuristic is extended to rule out crazy-sounding things with a basis in fact, it becomes absurdity bias.

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