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 and describes a hypothetical experiment with a hungry donkey. If placed exactly between two mangers filled with hay, the beast will starve to death because it can'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)
