Hilary Putnam’s anti-Skepticism Argument

1) If I am a brain in a vat, I express a falsehood in uttering the sentence “I am a brain in a vat.”
2) If I am not a brain in a vat, I express a falsehood in uttering the sentence “I am a brain in a vat.”
3) I am either a brain in a vat or I am not a brain in a vat.
4) In uttering the sentence “I am a brain in a vat,” I express a falsehood. (From (1), (2) and (3))
5) In uttering the sentence “I am a brain in a vat,” I utter a sentence meaning that I am not a brain in a vat.
.: I am not a brain in a vat. (from (4) and (5))

This is not a very convincing argument. His argument hinges on meaning externalism, and this is where he gets the un-utterability of “I am a brain in a vat”, which actually means something like “I am not an English-speaker in a vat.” Of course, if we actually are brains in a vat then we actually aren’t english-speakers in vats either.