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Metaphysics

Why Buridan’s Ass Doesn’t Starve

Does free will exist? Michael Hauskeller reasons about reasons.

Imagine you go to a restaurant. Looking at the menu, you discover that they serve your two favourite meals – say asparagus and spinach tart. What will you do? You may hesitate for a while, but then you will make your choice. You have to make a choice, don’t you? Even if you’re hungry or greedy enough to order both, you have to decide which to eat first.

Now, how do you decide? Given that you like both equally, why do you choose, say, spinach tart, and not asparagus? There are two possible general answers. You can say either that:

a) There is no reason (no cause) for your choice. You just act, and you could equally well choose the other meal. Or:

b) There is a reason, but it’s unknown to you.

The second answer seems more plausible, because it accords with a principle that’s fundamental to the way we think. This principle is commonly called Leibniz’s Law, or the Principle of Sufficient Reason. It can be stated in various ways:

• Nihil sine ratione: Nothing is without a reason.

• Nothing happens without a sufficient reason/cause.

• For each event A there is another event B (or a combination of events) that precedes it and fully explains why A had to happen.

• Ex nihilo nihil fit: Nothing comes out of nothing.

This principle is an expression of our inability to believe that something could just spring into existence or happen without a cause. Imagine that a glass on a table suddenly exploded. Even if you don’t have the slightest idea what caused the glass to behave in that manner, you’ll still assume that something must have caused it. If it isn’t necessary that events are caused by something, then anything would be possible. But if anything were possible, then we couldn’t act anymore, because in order to act effectively, we must be able to trust in the reliability of causal relations. So we’re certain that there’s a reason why the glass exploded. And if there’s nothing else, we will rather believe in supernatural causes than believe there’s no cause at all. We are certain that something must have caused it, whatever it is.

The same holds with respect to human actions. When you go to the restaurant and choose spinach tart, there must be a reason for your choice, we feel. Perhaps you don’t feel like asparagus today. Why? Perhaps you had asparagus last week. Or perhaps you feel that spinach tart goes better with the wine you’ve already ordered. But of course, there would have been reasons to choose asparagus instead (eg, you like it very much). So the fact that you did not choose asparagus does not mean that you had no reason for choosing it. Why then did you not choose it? Apparently because you didn’t have enough reasons, or you only had reasons that weren’t strong enough: the reasons in favour of spinach tart were stronger.

So let us presume that every reason has a certain weight. Imagine a pair of scales. On one scale we put all the reasons that make us inclined to choose spinach tart, on the other, all the reasons in favour of asparagus. We then make our decision according to which scale is heavier. Or rather, it is decided for us – the scales decide. Or, imagine being a piece of iron, and each reason is a magnet pulling us in a certain direction. Some magnets pull us in this direction, others in that, and we move where the most or strongest magnets are.

Supposing that decisions are made in such a mechanical way, we can imagine a situation where there’s perfect equilibrium between the opposing reasons. Here, the weights on the scales are equal, or the magnets that pull you to one side are neither stronger nor weaker than those which pull you in the other. What happens then? It seems that a rational decision would no longer be possible, because the reasons for either option are balanced.

This is the situation of Buridan’s Ass, named after philosopher John Buridan (1300-1358). A donkey finds himself halfway between two equally big and delicious piles of hay. Because he lacks a reason to choose one over the other, he cannot decide which one to eat, and so starves to death. This tale is usually taken as demonstrating that there is no free will; and indeed, it is plausible to ask whether or not free will is possible in such a situation.

In a normal food situation, where there is no perfect equilibrium between opposing reasons, a donkey will be able to move towards food because he has a reason to do so. But here it is unclear whether he moves because he freely wills it or because he is simply pulled in this direction. He wouldn’t really decide to eat, but rather would be driven by his hunger and the availability of food. (At least that is how it could be, because a donkey having its strings pulled is indistinguishable from a donkey that decides for himself.) In other words: what the donkey does he could do without free will. He simply doesn’t need it. And often we don’t, either. As long as there are reasons to do one thing rather than another, free will is dispensable as an explanatory principle.

To find out whether you’re really free, you’d have to be in a situation where no reason is stronger than any other – a situation like the one in which Buridan’s unfortunate ass finds himself. If you’re able to make a decision in such a balanced situation, you’d thereby demonstrate the freedom of your will. Buridan’s ass isn’t able to decide, so his will is obviously not free. But then, Buridan’s ass is a mere fiction. How about real donkeys? If we could find a donkey which was dumb enough to starve between two bales, we would have evidence against free will, at least as far as donkeys are concerned (or at least that particular donkey). But that’s not very likely. No matter how artfully we arrange the situation, a donkey will not hesitate very long, if at all, and will soon choose one of the piles of hay. He doesn’t care which, and he certainly won’t starve. However, even if we conducted thousands of experiments like this, and no donkey ever starved, we would still not have proved the existence of free will, because the reason no donkey ever starves in front of two equally attractive piles of hay may simply be that those piles aren’t really equally attractive. Perhaps in real life there aren’t any situations where the weighted reasons for a choice are equal.

Of course there are situations where we have difficulties in making up our minds (we sometimes have those difficulties, but not donkeys). This is often the case when much depends on our decision. But in the end we will decide one way or the other, even if only because the lapse of time changes the situation. Buridan’s ass starves because he’s imagined as timeless, as somehow removed from the passage of time. He’s frozen in a situation where there’s only him and the two piles of hay. Yet since donkeys live in time, no donkey will ever starve because he lacks free will.

Michael Hauskeller is Associate Professor of Philosophy at the University of Exeter.

• No animals were harmed in the making of this article.