Do private equity presentations overstate an ability to persist? : “It ain’t what you don’t know that gets you into trouble, it is what you know for sure that just ain’t so.” According to Respectfully Quoted, Twain saying this “just ain’t so.” As Forbes and others have reported, the phrase originated with Josh Billings in his 1874 book Everybody’s Friend. In saying this, I am not pointing fingers at anyone and I’m certainly not knocking Mr. Twain. The phrase has just been attributed to Twain so many times that it has become as if it were so.