T image of the brain in the vat-quite apart from what Putnam actually intended it to do- is used as simply another presentation of the foundational challenge of modern philosophy: how do you know that you know? How do you rule out that you are a brain in a vat? We can imagine this being the case, so it must at least be possible…right?
No! No! Ten thousand times no! Our imagination, whether taken narrowly or broadly, is neither the measure nor the cause of any real possibility. There is no relation between imagination and real possibility: some imagined things are possible, others impossible, others necessary; and the same is true for those things that cannot be imagined. As soon as we allow that our imagination indicates real possibility, we allow a parallel universe of possible beings that are evident to imagination alone, and whose real existence does not follow from actual external sensation.
This kind of argument is not philosophy, but precisely the sort of thing that philosophy seeks to avoid- namely spellcasting and shamanism. The power one feels in the brain in the vat argument is the exactly the same power that voodoo witch-doctors use to cast hexes, because whether one falls prey to a hex or the BIV argument, the mechanism is the same- we think some thing is a real possibility to be feared, fought against, or even accepted simply because we can imagine it being so. How can we rule it out? Maybe we aren’t wholly convinced by the argument/spell, but isn’t it better to assume it is true just to be safe? After all, the most reputable people in the village/campus think it’s so.
How do I know I’m not a BIV? It’s not because “this is my right hand, this is my left”; or because realism is a more effective practical postulate; or because the BIV offends my intuitions; or because I ignore the question (and it is a real question that deserves a response. I call it shamanism in all respect. Shamanism has real power). I know I am not one because I have not set up imagination as indicating real possibility.