 This answer is given in a slightly different way at (§7) where Wittgenstein asks “may there not be true propositions which are written in this [Russell’s] symbolism, but are not provable in Russell’s system?”, and then answers “‘True propositions’, hence propositions which are true in another system, i.e. can rightly be asserted in another game”. In light of what he says in (§§5, 6, and 8), Wittgenstein’s (§7) point is that if a proposition is ‘written’ in “Russell’s symbolism” and it is true, it must be proved/provable in another system, since that is what “mathematical truth” is. Analogously (§8), “if the proposition is supposed to be false in some other than the Russell sense, then it does not contradict this for it to be proved in Russell’s sense”, for “[w]hat is called ‘losing’ in chess may constitute winning in another game”. This textual evidence certainly suggests, as Anderson almost said, that Wittgenstein rejects a true but unprovable mathematical proposition as a contradiction-in-terms on the grounds that “true in calculus \(\Gamma\)” means nothing more (and nothing less) than “proved in calculus \(\Gamma\)”.