Buridan's Ass
A donkey stands exactly equidistant between two identical piles of hay, with no reason to prefer one over the other. What does it do?
The scenario is attributed to the 14th-century philosopher Jean Buridan, though it likely predates him. It was used to challenge purely rationalist accounts of choice: if action requires a sufficient reason to prefer one option, and no such reason exists, action should be impossible. The donkey, rationally speaking, starves.
Rescher, N. (1960). Choice Without Preference: A Study of the History and of the Logic of the Problem of 'Buridan's Ass'. Kant-Studien, 51(1-4), 142–175.
The scenario and its target
The setup is deliberately artificial. Equal piles, equal distances, equal freshness. No breeze favoring one side, no hunger-driven twitch toward the left. Everything is perfectly balanced. A purely rational decision-maker, one that requires a sufficient reason to act before acting, finds itself paralyzed. Neither option is better. Neither option is worse. The calculation returns no output.
The donkey isn't stupid. It's stuck because rationality, taken to its strict logical extreme, gives no answer when the inputs are equal.
Buridan (or whoever invented the scenario) was pressing on a real problem: what actually moves us to choose when reasons run out? The puzzle wasn't meant to be solved by pointing out that real donkeys always find reasons, because real donkeys aren't perfectly rational. It was meant to show that something besides reason has to do some of the work.
What it means for decision theory and free will
For decision theory, the case is relatively tractable. When two options have equal expected value, any tiebreaker will do. You can flip a coin, go left by default, or choose randomly. The theory doesn't require that reason single out one option; it only requires that you choose one.
But this solution highlights something the puzzle was pointing at all along: the mechanism that breaks ties isn't reason. It's something else, will, habit, arbitrary impulse, quantum noise. Decision theory can tell you what's rational to do given your preferences. It can't tell you what to do when preferences are genuinely tied. Something outside the calculus has to move you.
For free will, the picture is stranger. Leibniz used the scenario against what he saw as the principle of sufficient reason: if everything happens for a reason, what's the reason for choosing left over right when they're identical? He thought this showed the scenario was impossible, that perfect symmetry could never exist in the real world. That's probably right, but it doesn't solve the philosophical problem. It just relocates it.
What actually moves us
The interesting question Buridan's Ass opens is not "what should the donkey do?" but "what do we do when reasons give out?"
The honest answer is that most of the time, we just act. The deliberation doesn't run to completion; it stops when we're ready to move, which happens for reasons that aren't fully transparent to us. We choose left. We don't know exactly why.
Existentialists found this significant. For Sartre, the fact that you can always choose without a sufficient reason is the source of both freedom and anxiety. You're never compelled. You always have to just decide. The donkey that starves for want of a reason has made its own choice: to wait for certainty that never comes.
Discussion questions
- When you genuinely cannot decide between two equally good options, what finally tips you one way?
- Is pure rationality sometimes a liability in decision-making?
- Can you think of a time when flipping a coin was the wisest thing you could have done?
Take it to the dinner table.
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