In TLP 5.525 Wittgenstein makes a tripartite distinction: "Certainty, possibility or impossibility of a state of affairs are not expressed by a proposition ['Satz'] but by the fact that an expression ['sondern dadurch dass ein Ausdruck'] is a tautology, a significant ['sinnvoll'] proposition or a contradiction". In 4.461 Wittgenstein says: "die Wahrheit der Tautologie ist gewiss, des Satzes möglich, der Kontradiktion unmöglich". Of tautologies Wittgenstein further says that they are senseless ("sinnlos"), but not nonsensical ("unsinnig"). They are a sort of extreme cases in the operation with otherwise meaningful sentences. Wittgenstein does not make a corresponding statement about contradictions - but I think we have the right to infer that they too are senseless though not nonsensical. Since a meaningful sentence is neither necessary nor contradictory, it is contingent. This means that it and its negation are both possible. It is important to note that, on the Tractatus view, meaningful sentences are contingent. I am afraid that this is something which commentators have not always clearly observed.
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