However, I think I should point out to Meta that if you are saying that because of that no-one will ever think anything is wrong, that is not correct and is a mis-use of Putnam's theory. Putnam was not saying that no-one could possibly ever think they were brains in vats (or pods in a Matrix). People in the Matrix would be just as capable as doing that as you or I, their brains are identical.
They would just be completely unaware that what they imagined was a brain in a vat or a pod in a Matrix is, in fact, not actually that at all, having 'failed to refer' to it.
(I always find it useful (as does someone in that area) to object* these things, where an object* is an object in the Matrix- so the real world has pigeons, but the Matrix has pigeons*. So people in the Matrix would think of brains* in vats* instead of brains in vats)
But just because they fail to refer to the object it does not mean they do not recognise the concept. If any one of us can come up with the idea of the Matrix- as obviously we have- then anyone inside a Matrix can too, even though they would be academically mistaken about its true nature. (According to Putnam, anyway. I believe him but many don't). They could still BELIEVE it, just as much as you or I could.
As you point out, we cannot know if we ARE just brains in vats, yet you and I can freely discuss the possibility of being brains in vars and accept evidence if it is offered, even if we do not truly know what a brain or vat is until we got to the 'real' world and saw it.
It does seem to be a common misunderstanding of that essay that it was saying that no-one in the Matrix could realise they were. Check again, that is not what it says. It simply says they could not coherently express the fact that they were, is all. And that isn't due to some weird mind alteration. They would say it in the same way we do- in fact, you and I would be having this conversation IN the Matrix, and would believe it or not believe it as much there as here, and probably more so if we had seen half a dozen impossible things before breakfast. It is really a very academic point, as I say- fascinating but ultimately making no difference.
So basically, Putnam was saying- Why worry about being brains in vats? You don't even know what they ARE! He has a habit of being clever like that. But the basic philisophical issue- is reality as we see it?- remains the same.
Darn it, I bet we ARE here until the end of time now...
Mind you, that one on Dreams was dreadful. AND just a plug for the writer's book. Pfft.
Anyway, we can all assume that if the Matrix can re-set itself every so often without people noticing a problem, it can clean up small problems like this as well.