“Ellsberg offered several groups of people a chance to bet on drawing either a red ball or a black ball from two different urns, each holding 100 balls. Urn 1 held 50 balls of each color; the breakdown in Urn 2 was unknown. Probability theory would suggest that Urn 2 was split 50-50, for there was no basis for any other distribution.”
From page 280 of Against the Gods: The Remarkable Story of Risk.