I am working on this problem in which I have a theory $T$ based on language $\mathcal{L}$ and the only information we have is that T is consistent and $\vdash \lnot(A \rightarrow A)$. Given this information, how can I know if this theory is sound, complete and/or decidable?

My only guess is that I can say that $T$ is sound because since $T$ is consistent and we can derive $\lnot(A \rightarrow A)$ from axioms and inference rules, $\lnot(A \rightarrow A)$ is a theorem and because of that we can assume that $T$ is sound (because the premises and conclusions are true).

Am I correct? What else can I say about this theory?

Thank you!

  • $\begingroup$ Your problem statement makes no sense because in first-order classical logic $\lnot (A \to A)$ is false, and therefore it can only be a theorem in inconsistent theory. Please double check that you transcribed it correctly. $\endgroup$ Jul 21, 2020 at 20:59

1 Answer 1


I'll assume that we're working in the context of classical propositional logic.

The first thing to point out is that $\lnot(A \rightarrow A)$ leads to a contradiction: ($A \land \lnot A$). Since propositional logic itself is consistent, it is impossible to derive a contradiction, so $\vdash \lnot(A \rightarrow A)$ is impossible.

So perhaps your intention was $T \vdash \lnot(A \rightarrow A)$. This can only happen if $T$ is itself inconsistent.

Given that $T$ is inconsistent, $T$ is vacuously complete (since an inconsistent theory can prove any formula) and decidable (for the same reason). As for soundness, it's a property of the logical system, not any particular theory like $T$. Propositional logic is sound.


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