J. R. Lucas serves warning that he stands ready to refute any sufficiently specific accusation that he is a machine. Let any mechanist say, to his face, that he is some particular machine M; Lucas will respond by producing forthwith a suitable Gödel sentence φM. Having produced φM, he will then argue that – given certain credible premises about himself – he could not have done so if the accusation that he was M had been true. Let the mechanist try again; Lucas will counter him again in the same way. It is not possible to accuse Lucas truly of being a machine.

I used to think that the accusing mechanist interlocutor was an expository frill, and that Lucas was really claiming to be able to do something that no machine could do. But I was wrong; Lucas insists that the interlocutor does play an essential role. He writes that “the argument is a dialectical one. It is not a direct proof that the mind is something more than a machine; but a schema of disproof for any particular version of mechanism that may be put forward. If the mechanist maintains any specific thesis, I show that a contradiction ensues. But only if. It depends on the mechanist making the first move and putting forward his claim for inspection.” Very well. I promise to take the dialectical character of Lucas's argument more seriously this time – and that shall be his downfall.