I checked Harnsberger & Rasmussen & the Twain World CD and find no reference
to that quote. I have seen this quote many times, and I do remember seeing
it attributed to Lincoln. I've checked a few other less reliable sources,
but it still does not show up. I can't prove a negative, but it doesn't seem
likely Twain is the original source. If you want to use this quote, your
next step is to prove it was uttered by Honest Abe. But if you want
something by Twain, Harnsberger does have some good quotes by Twain on fools
and being foolish.

As for Mensa... in 1980 I sold an old Dodge V-8 to a nice couple, both proud
Mensa members, and used the money to buy a new Mazda. So if I were you, I'd
go ahead and tell them the quote is by Twain. They'll believe anything.

Sincerely,

Kevin Mac Donnell
Mac Donnell Rare Books