Wednesday, October 21, 2009

Some liar paradoxes without truth

Let "@" be the name of the actual world.

  1. The proposition expressed by (1) in English is not entailed by the proposition that @ is actual.
  2. The proposition expressed by (2) in English is not compossible with the proposition that @ is actual.
  3. The proposition expressed by (3) in English is not necessary.
  4. The proposition expressed by (4) in English is not known by anybody.
  5. The proposition expressed by (5) in English cannot be known by anybody.

That (1) and (2) are paradoxical is obvious. That (3) is paradoxical is easy to see. For if (3) is false, then (3) is necessarily true. If (3) is true, then then it is only contingently true. But the argument that if (3) is false, then (3) is necessarily true works in all worlds. So in no world is (3) false. So (3) cannot be contingently true.

The paradoxicality of (4) is a bit more fun, though I am less sure of it. If (4) is false, then (4) is known by somebody and hence true. So, (4) cannot be false. But now that we have a logically sound argument for (4), we know (4)—or at least we could, and then we can consider the argument in the possible world where we do know it. But if we know (4), then (4) is false.

What about (5)? Well, if (5) can be known by anybody, it can be true and known. But it cannot be both true and known. So, (5) cannot be known by anybody. But this is a good argument for the truth of (5), so even if we don't know (5), somebody can know it on the basis of this argument. But then (5) is false.

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