ABSTRACT
This puzzle, due to Roy Sorensen, has a similar structure to The Unexpected Examination, but it is about intention rather than justified belief. It is developed from The Toxin Paradox. (Sorensen does not explain his choice of the name ‘Indy’.)
Here is one way you might proceed quite rationally. You realize you can make some money and that you can continue to do so. So
you go ahead on Monday, but postpone the decision when to stop. You know you will stop on Friday morning, since you cannot rationally form the intention to take the last exam, and that you may well give up before then if you cannot stop yourself rehearsing the backward induction. But on the preceding days there was a reason for taking an exam, namely that you could continue the sequence and continue to make money. As Friday approaches, you will have made some money, and, if you make the decision to stop one morning after you have just been given the money, when you can no longer refrain from making the backward induction, you will avoid taking an unnecessary exam.
