Douglas Adams’s “the ingenuity of complete fools,” from his 1992 novel Mostly Harmless, is the version most people quote — but Quote Investigator finds the joke already running fifty years earlier, in a 1942 syndicated comic where cartoonist J. R. Williams has one character shrug that foolproof devices can’t work “because as th’ fool foolers get more brilliant th’ fools get more brilliant too.” Even that isn’t the origin: an unsigned filler item in the Arizona Republic made the same joke in May 1934, eight years before Williams drew it.