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 (donkey) that is equally hungry and thirsty is placed precisely midway between a stack of hay and a pail of water. Assume that the surrounding environments on both sides are also identical. Since the paradox assumes the donkey will always go to whichever is closer, it dies of both hunger and thirst since it cannot make any rational decision between the hay and water.
A common variant of the paradox substitutes two identical piles of hay for the hay and water; the donkey, unable to choose between the two, dies of hunger.
Although it bears his name, the medieval philosopher Jean Buridan did not invent this paradox, which probably originated as a parody of his theory of free will, according to which human freedom consists in the ability to defer for further consideration a choice between two apparently equally good alternatives (the will is otherwise compelled to choose what appears to be the best).
The paradox was later thought to constitute a counterexample to Leibniz’s principle of sufficient reason, one version of which states that there is an explanation or reason for every contingent event. Whether the donkey chooses one bale or the other is a contingent event, but there is apparently no reason or cause to determine the donkey’s choice. Yet the donkey will not starve.