The paradox of the surprise examination.

Lucian Wischik. July 1996.


The students set off home for the weekend. Their teacher had been cruel in his sentencing: "You shall have an examination at nine in the morning one day next week; but you will not know in advance which day-it will be a surprise."

"Suppose," thought one to herself with the desperation available only to the damned and the examinable: "Suppose we were still unexamined until Friday. I know there will be an examination some time this week, and so I would be able to expect the Friday examination. Therefore the teacher cannot possibly leave it until Friday."

"Suppose further," she thought to herself: "Suppose that I am still unexamined on Thursday. I know the examination cannot be on Friday, so it would have to come on Thursday. But then too I would have been able to predict its day. So, the teacher cannot leave it until Thursday either. Or Wednesday... or Tuesday... or Monday.. In fact, he cannot possibly set any examination at all, since no day would be a surprise!"

And when the teacher handed out exam papers at nine o'clock on Wednesday morning — wasn't she surprised!


This paradox has been the object of a steady stream of discussion since O'Conner brought it to public view in 1948. O'Conner called it a 'Class A Blackout' (class A practices being sprung on unexpecting soldiers), Quine introduced the 'Condemned Man', Shaw called it a 'Surprise Examination', Lyon had a Hand of Cards that were revealed in order with one of them known to be an Ace, and there have been many more presentations.

There have been as many proposed resolutions to the paradox as there have been authors. The paradox remains fascinating because it has for so long withstood a definitive explanation, and because all aspects of it continue to be debated. O'Conner said: "It is worthwhile for philosophers to pay a little more attention to these puzzles than they have done up to now even if their scrutiny does no more than make a little clearer the ways in which ordinary language can limit and mislead us." The paradox is also relevant to the Philosophy of Knowledge, since it would seem to suggest that either deductive reasoning is powerless to reason about the future, or that propositions regarding states of mind are not meaningful.

We present a unifying review of all existing literature on the subject up to 1970 and include a comprehensive bibliography.

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