A professor told a class that they are to have test on what they have learned so far. This is because they are a lazy class. The class are not pleased and start muttering. ‘When is it, they asked miserably.

Professor smiles. ‘That is up to me. I may have it at any point between now and the end of term. However, let me assure you of this, when I do have the test it will be a surprise!’

One of the student Donald, is very worried as he hasn’t been studying. ‘I could pass, I’m sure,’ he says, ‘if I knew on which day the test would be, then I could learn everything the night before.’

‘Don’t worry, Donald’ say Hilary (another student of the same class) says, I think professor is joking, I don’t think there can be a test!’

And she explains that the test cannot be held on the last day of semester, because by then the class will know that it must be going to be held, and will therefore quickly memorize the material for the best the night before. ‘That’s great’ says Donald. So it’s any day between now and the second to last day of semester then?

It can’t be the second to last day of the semester either, because if it is already night before the second to last day of semester, we had all know it would have to be coming up next day!’ Hilary says.

Donald get it now. Nor the third to last day, nor the forth, nor indeed any day! Hey!

What a joke –professor trying to get us worried! –and now he can’t hold the last without having to back down on the surprise bit.

Don’t tell the others, who spend ages trying to memorize the materials. Then one day, just a week after the original announcement, professor comes in and announces the test.

‘This is not possible’ says Donald.

‘Why not?’ replies the professor, surprised, but not very.

Because it’s got to be a surprise –and you can only hold test when we’re not expecting it!’

Yes, but Donald, you’re not expecting it, and I am holding the test’ says professor.


Was Donald reasoning fallacious? Or has the professor been deceitful?




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