 The problem is of course quite familiar. In different guises it is  Goodman’s grue/green problem,  Kripkenstein’s plus/quus problem, Quine’s  gavagai  problem, and  Putnam’s puzzle  of the brain in a vat with true beliefs (Goodman 1955, Wittgenstein 1953, Kripke 1982, Quine 1960, Putnam 1981). One way or another it has to be solved.