answer:Nobody can present a shred of evidence that I am a brain in a vat. Therefore, if I seriously entertain the notion that I might be, I have no epistemological standards. And in that case, I might as well believe in anything. Why not invisible unicorns? Ghosts? You are correct in sensing that those people you mentioned, including Emmanuel Kant, do not make sense. Often enough, they do not, and neither do their disciples. If a philosophical system is not practical to the survival of a reasoning being, it is valueless.