An updated version, due to Putnam, of Descartes's thought experiment with the malin génie (see method of doubt). We are invited to consider the sceptical possibility that our experience is actually produced by a brain suspended in a life-preserving medium (a vat) and stimulated electrically in such a way as to give us the delusive experience of living the life with which we are familiar. One reaction is to deny that this is even a bare logical possibility; another is to argue that even if it is a bare logical possibility, we know that it is not the way things are. A controversial argument of Putnam's attempts to show that the hypothesis that I am a brain in a vat self-destructs, since if I were one, were I to put it to myself ‘I am a brain in a vat’, I would not be referring to brains and vats, and hence would not express appreciation of my true situation.