by John Allen PaulosA bit of information is common knowledge among a group of people if all parties know it, know that the others know it, know that the others know they know it, and so on. I discussed common knowledge in my books Once Upon a Number and A Mathematician Plays the Stock Market and illustrated it with the following parable, which suggests its power. Every woman except Mrs. A and Mrs. B knows about both these cases of infidelity. Mrs. A knows only of Mr. B’s, and Mrs. B knows only of Mr. A’s. The parable and these more contemporary speculations demonstrate what can happen when mutual knowledge becomes common knowledge.