There’ll be an exam tomorrow but you don’t know this.In this strange case, the class cannot take her words seriously because they cannot sensibly accept both parts of her announcement. Clearly, if they accept the first part, they cannot accept the second, and vice versa.
You’ll have a party tomorrow but you don’t know this.There is one difference however. Your niece’s declaration was presumably “innocent,” so that, upon hearing her words, you should presumably accept the first part and reject the second. (The party will occur but it will no longer be a surprise.)
There’ll be an exam tomorrow but you don’t know this.Notice that, confronted with such a dastardly announcement, a disgruntled student may mock it as follows:
This would be sarcasm of course; a joke of sorts. The point isn’t really that the teacher’s words can’t be true but only that the class cannot take them seriously: as soon as they do, they can “deduce” that her words are false. So this mocking retort is an artful way of protesting that the teacher’s announcement is self-defeating, or a blindspot for the class.Ah, but what you say cannot be true! For now that we know the exam is tomorrow, you are wrong to say that we don’t!
There will be an exam on one of the next five days but you won’t know which day until it arrives.And the clever student retorts:
At first sight, the protest is that the teacher’s words cannot be true but, as I hope is clear by now, it makes more sense to view the student as mocking her words instead, in exactly the same style shown above. His artful point is that the class cannot take her words seriously: as soon as they do, they can “deduce” that her words are false.Ah, but what you say cannot be true! The exam can’t be on Friday since we’d know about it by Thursday’s end. Nor can it be on Thursday since we’d know about it by Wednesday’s end, given that Friday is ruled out. Nor can it be on Wednesday, Tuesday or Monday, and so it can’t occur at all!