Massimo Pigliucci doesn’t frame this recent post at his blog as a response to Unger, but…
Now, why on earth did we engage in this, ahem, academic discussion [about brain in vat or BIV skepticism]? Because I wanted to give you a flavor of how philosophy makes progress, and why it isn’t particularly fruitful to compare it with progress in the natural sciences (did you see any systematic observation or experiment peeking through the above?)….
Even to ask the question of what is the true answer to the BIV problem is, in a fundamental way, to misunderstand the whole process. If the BIV question were an empirical one — like “how many planets are there in the solar system” — then it would have one definite answer, and a bunch of bad ones. But in conceptual space there often are several reasonable ways of looking at a particular problem (“answers”), and it will not be possible to pare them down to just one. (Another way to put this is to say that conceptual space is wider than, and underdetermined by, empirical space.)
The whole thing is here. (via Philosophy Matters)