A paradox by the German mathematician Martin Löb: Let A be any sentence. Let B be the sentence: ‘If this sentence is true, then A.’ Then a contradiction arises. Here’s the contradiction. B makes the assertion “If B is true, then A.” Now consider this argument. Assume B is true. Then, by B, since B is true, A is true. This argument shows that, if B is true, then A. But that’s exactly what B had asserted! So B is true. And therefore, by B, since B is true, A is true. And thus every sentence is true, which is...