Philosophy, The Matrix, and Brains in Vats

In a departure from normal Businesspundit posting, I wonder if any of you could explain this to me. The article is about philosophy and The Matrix. Here is the part I do not understand.

The first extended discussion of the idea, by Hilary Putnam of Harvard University, in 1981, sought to prove that "I am a brain in a vat" is a type of self-defeating utterance, which can never be true. This is (roughly) because terms like "brain" and "vat" could not carry their usual meanings for someone who was cut off—in the way that "The Matrix" supposes—from the everyday objects to which the terms purportedly refer. Somebody might, in theory, have the misfortune of having his brain artificially stimulated without any normal connection to the external world. But he or she would not have the language to state or even think that this was so. And thus, according to Professor Putnam, they cannot intelligibly suppose that it is so either.

A related train of thought is implied by the work of Donald Davidson, who was, until his death in August, probably the most discussed of living American philosophers. Davidson argued, ingeniously, that our basic beliefs about the world cannot be wrong through and through because otherwise we would have no reason to regard them as genuine beliefs. In other words, Davidson maintained that it is literally impossible to fool all of the people all of the time. For film directors, though, there is plainly no harm in trying.

I'll do some research on Putnam and Davidson, but right now their ideas don't make any sense to me. I do not understand why an external world allows language to develop to talk about it, but an artificially simulated external world doesn't. The brain shouldn't be able to tell the difference. What am I missing?