Buridan's ass

Buridan's ass is an illustration of a paradox in philosophy in the conception of free will. It refers to a hypothetical situation wherein an ass (a donkey) that is equally hungry and thirsty is placed precisely midway between a stack of hay and a pail of water. Since the paradox assumes the ass will always go to whichever is closer, it will die of both hunger and thirst since it cannot make any rational decision to choose one over the other. A common variant of the paradox substitutes two identical piles of hay for the hay and water; the ass, unable to choose between the two, dies of hunger.

The paradox is named after the 14th century French philosopher Jean Buridan, whose philosophy of moral determinism it satirizes. Although the illustration is named after Buridan, philosophers have discussed the concept before him, notably Aristotle who used the example of a man equally hungry and thirsty, and Al-Ghazali who used a man faced with the choice of equally good dates.

A version of this situation appears as metastability in digital electronics, when a circuit must decide between two states when there is an input that is changing value. In digital electronics a small amount of randomness acts as a tie-breaker, and the circuits settle into one state or the other after a usually very small, but unbounded period of time.

The paradox predates Buridan; it dates to antiquity, being found in Aristotle's On the Heavens. Aristotle, in ridiculing the Sophist idea that the Earth is stationary simply because it is circular and any forces on it must be equal in all directions, says that is as ridiculous as saying that

...a man, being just as hungry as thirsty, and placed in between food and drink, must necessarily remain where he is and starve to death.

However, the Greeks only used this paradox as an analogy in the context of discussions of the equilibrium of physical forces.

The 12th century Persian Islamic scholar and philosopher al-Ghazali discusses the application of this paradox to human decision making, asking whether it is possible to make a choice between equally good courses without grounds for preference. He takes the attitude that free will can break the stalemate.

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