I have been working on Twain's aphorisms for several years, including
compiling and categorizing all from his Notebooks and Diaries, as well as
ones that appear in his lectures and other places, and I can't
find this particular one anywhere. I doubt very much if it's Twain's, but
perhaps it is. But I am all but certain that it does not appear in any of
his Notebooks and Diaries.