 Perhaps the most compelling evidence that the later Wittgenstein maintains algorithmic decidability as his criterion for a mathematical proposition lies in the fact that, at (RFM V, §9, 1942), he says in two distinct ways that a mathematical ‘question’ can become decidable and that when this happens, a new connexion is ‘made’ which previously did not exist. Indeed, Wittgenstein cautions us against appearances by saying that “it looks as if a ground for the decision were already there”, when, in fact, “it has yet to be invented”. These passages strongly militate against the claim that the later Wittgenstein grants that proposition \(\phi\) is decidable in calculus \(\Gamma\) iff it is provable or refutable in principle. Moreover, if Wittgenstein held this position, he would claim, contra (RFM V, §9), that a question or proposition does not become decidable since it simply (always) is decidable. If it is provable, and we simply don’t yet know this to be the case, there already is a connection between, say, our axioms and rules and the proposition in question. What Wittgenstein says, however, is that the modalities provable and refutable are shadowy forms of reality—that possibility is not actuality in mathematics (PR §§141, 144, 172; PG 281, 283, 299, 371, 466, 469; LFM 139). Thus, the later Wittgenstein agrees with the intermediate Wittgenstein that the only sense in which an undecided mathematical proposition (RFM VII, §40, 1944) can be decidable is in the sense that we know how to decide it by means of an applicable decision procedure.