 Can one ever demonstrate of a proposition, that it can never be decided? No, because one would have to so by reductio ad absurdum. So one would have to say: assume that the proposition has been decided in sense \(a\), and from that deduce a contradiction. But then it would have been proved that not-\(a\) is true, and the proposition is decided after all. (van Dalen 2001b: 174n.a; translation mine)