top of page

60. No ATM in the desert:

Parfit’s Hitchhiker


The Question:

Imagine the following scenario: during a stroll through the desert you were held up by bandits and robbed of all your belongings. Left to perish in the desert, you have only the clothes on your back and your wallet, empty except for the credit card that the brigands had not bothered to take. Suddenly, seemingly out of nowhere, an SUV drives up. The driver leans out the window: “Lost? he asks somewhat superfluously. You are barely able to nod in the affirmative, whereupon he makes you an offer: “I can give you a lift to the next town but it’s quite out of my way, so you’ll have to pay me 1,000$ for my troubles.”

Dehydrated and lost, but relieved and thankful, you accept the offer, promising to pay him with cash from an ATM as soon as you reach civilization.

The SUV-driver takes a good look at you to verify that you seem to be a rational individual, invites you into his vehicle and drives towards the nearest town.

Sounds credible?


The Paradox:

Not necessarily.

Once the driver has reached town, you are saved. So why pay him? Serves him right for being an extortionist.

Oh, but wait! When he came upon you in the desert, the driver knew exactly that once in town there would no longer be any rational reason for you to pay him. Being totally rational himself, he knew how you would react the moment you were out of danger. So why bother making the detour to the town? He was better off just driving off and leaving you to die in the desert.

And that’s what he did.

So, even though you were perfectly willing to pay a measly $1,000 to save your life, and assured the driver of this, the latter departed and you were left to die.

A paradox.



The problem has become known as “Parfit’s Hitchhiker”, after the British philosopher Derek Parfit (1942-1217) who discussed the situation in his book Reasons and Persons. In a profile about Parfit, The New Yorker described him as the “the most important moral philosopher in the English-speaking world” and the book – together with his three-volume On What Matters – as “the most important works to be written in the field in more than a century.”[1] The subject matter of his philosophical enquiries were questions about ethics, rationality and personal identity. “We have reasons for acting. We ought to act in certain ways, and some ways of acting are morally wrong. Some outcomes are good or bad, in a sense that has moral relevance.”[2]



The fact that you appeared to the driver to be a rational person did not work in your favor…at all. Rather, it was the reason for your undoing. The reason is that altough rational people, lost in the desert, would gladly pay 1,000$ to save their lives, once saved, they would not become philanthropists. “Nothing that I do now will change what happened in the desert”, you would say to yourself after reaching the city. “My paying $1,000 wouldn't provide me with any further advantage." Of course, you knew this before you died in the desert, and the driver knew that too. And so he did the rational thing: he drove off.

The root cause of the problem is that the cause-effect relationship, in which one event is the result of another, is reversed. In general, causes entail effects. For example, a shot is fired and a target is hit. Or, one pays for a service and then the service is rendered. In the scenario described here, the cause-effect situation is reversed. The driver wants to give a lift (effect) if he gets money (cause). But those two things happen in the wrong order! There is a gap that cannot be bridged. 

The problem could be solved by enacting laws and regulations, and by instituting mechanisms to enforce them. True, a lack of enforcement mechanisms may very often be to the advantage of debtors; but in this case it is to your detriment. If there were a way for you to make a binding commitment in the desert – to pay the $1,000 upon arrival – you would be saved. But since no such instrument exists in the above scenario, you are left to die.

Another enforcement mechanism would be the enhancement or diminution of a person’s trustworthiness. But this is only of significance in a scenario that repeats itself. Since the two protagonists are not likely to cross each other’s paths again, trustworthiness plays no role.


Technical supplement:

If only electorates were as rational as the SUV-driver. ‘Rational politicians’, who make campaign promises only to forget them when elected, would never be picked by ‘rational voters’. Fortunately, there is an enforcement mechanism of sorts: politicians want to get re-elected.


Of course, enforcement mechanism and contract law are no panaceae. Insolvencies may leave creditors stranded. Apparently, that was (ex-)president Donald Trump’s modus operandi: get the work done and then stiff the businesses by declaring bankruptcy. That’s what makes America great again.

Speaking of MAGA, a rational patron would obviously not tip the waiter at a fancy, expensive restaurant that she will never visit again. Accordingly, the rational waiter, seeing that the rational customer is from out of town and unlikely ever to return, will give her terrible service.

Oh, and by the way, are busses more rational than taxis? Otherwise why does one have to pay for bus trips in advance while naïve taxi drivers only charge at the end?


Parfit’s Hitchhiker is analogous to Newcomb’s Paradox (see Chapter ///): The stranded person’s character, i.e., her rationality, is evident to the SUV-driver, just as the player is predictable by Algol in Newcomb’s Paradox. The hitchhiker is aware of her situation before the driver makes his decision, which is similar to the case where the player is aware of the game before Algol makes its prediction. The driver will take the hitchhiker home if and only if he predicts that her promise will be kept, just as Algol will place $1,000,000 under the wooden box if and only if it predicts that the player will only take the wooden box.


© George Szpiro, 2019




[1] MacFarquhar, Larissa (2011), “How To Be Good: An Oxford philosopher thinks he can distill all morality into a formula. Is he right?”, The New Yorker, Sept. 5, 2011

[2] Reasons and Persons, page viii.

bottom of page