[davnola]

The truthmaker for whether it is a surprise or not clearly depends on their own belief states. They are reasoning about what they would or would not believe in different circumstances, so they should include a premise about their belief states in their induction, otherwise their reasoning is unsound.

They reason, if there is going to be an exam, and it hasn't happened by Thursday, then it will happen Friday, and we would believe it would happen on Friday and it cannot be a surprise.

They should reason like this: if there is going to be an exam and we believe there will be an exam, and it has not happened on Thursday &c then it cannot be a surprise.

But, they stopped believing the exam was happening. Doh.

EDIT: formatting