 To maintain this position, Wittgenstein distinguishes between (meaningful, genuine) mathematical propositions, which have mathematical sense, and meaningless, senseless (‘sinnlos’) expressions by stipulating that an expression is a meaningful (genuine) proposition of a mathematical calculus iff we know of a proof, a refutation, or an applicable decision procedure (PR §151; PG 452; PG 366; AWL 199–200). “Only where there’s a method of solution [a ‘logical method for finding a solution’] is there a [mathematical] problem”, he tells us (PR §§149, 152; PG 393). “We may only put a question in mathematics (or make a conjecture)”, he adds (PR §151), “where the answer runs: ‘I must work it out’”.