Thanks to all who responded to my previous message.

Now that we have several people saying it is indeed a Twain quote and
others saying it is Lincoln's, I am even more confused.

I'm writing an article about Mensa and wanted to include it. And, of
course, I'm on a deadline.

If anyone can give me a source, please do. I will feel more confident about
attribution with a source.

And, yes, the book I was thinking of is indeed R. Kent Rasmussen's.  If
anyone has it, would you check the quote for me?

C.J. Peiffer