Buridan's ass was an illustration of a problem in the philosophy of free will. It referred to Jean Buridan, a French philosopher in the 14th century who once carried out an experiment with a hungry donkey. He placed it exactly between two mangers filled with hay, and the beast starved to death because it couldn't work out which one to go for.
The Fifth and Eighth Doctors used this concept on the Eye of Orion. They both stayed equidistant from a Raston Warrior Robot as they approached it. Unable to decide on a target, the robot deactivated. (PROSE: The Eight Doctors)
