Despite these encouraging words, Sorensen in fact goes on to deny that the surprise exam may be reduced to the paradox of the heap.In light of the similarities between the two paradoxes, it is surprising that few commentators have tried to exploit the resemblance.Indeed no one has simply asserted that the following is just another instance of the sorites:The class knows that the exam won’t occur on the last day.If the class knows that the exam won’t occur on day n, then they know that it won’t occur on day n-1.Therefore, the class knows that the exam won’t occur.Why not blame the whole puzzle on the vagueness of ‘know’? … Despite its attractiveness, I have not found any clear examples of this strategy. (pp. 292-3, lightly modified)
What should we make of this?... it is clear that the analogy with the sorites is considerably weakened if the base step of the [surprise exam paradox] is uncompelling. (p. 325)
The difference is crucial because Sorensen’s objection does not touch this argument. Indeed, his complaint that the one-day announcement is a blindspot for the class actually affirms our initial premise.The one-day announcement is a blindspot for the class.If the n-day announcement is a blindspot for the class, then so is the (n+1)-day one.Therefore, the teacher’s announcement is a blindspot for the class, no matter how many days are at issue.