"Never attribute to malice that which is adequately explained by stupidity."
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See also: OccamsRazor, McCaughansRazor
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Once you've mastered this, the next step is: DontAssumeStupidity
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According to the JargonFile:
		:	 The derivation of the Hanlon eponym is not definitely known, but a very similar remark ("You have attributed conditions to villainy that simply result from stupidity.") appears in "Logic of Empire", a classic 1941 SF story by Robert A. Heinlein, who calls it the `devil theory' of sociology. Heinlein's popularity in the hacker culture makes plausible the supposition that `Hanlon' is derived from `Heinlein' by phonetic corruption. A similar epigram has been attributed to William James, but Heinlein more probably got the idea from Alfred Korzybski and other practitioners of GeneralSemantics.
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''"Never ascribe to malice, that which can be explained by incompetence"'' is a quote from Napoleon.

''It's also been attributed to Machievelli.  Hanlon as a corruption of Heinlein sounds more plausible, but this stuff is basic folk wisdom that's no doubt been repeatedly thought up and independently expressed over centuries.''
*** http://www.mindprod.com/jgloss/unmain.html