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Nik Weaver
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Obviously, adding rules of inference that tell that us we can go from $\phi$ to $T(\ulcorner\phi\urcorner)$ and back, and $\neg\phi$ to $\neg T(\ulcorner\phi\urcorner)$ and back, would be too strong and lead to contradictions in well known ways.

Actually, that's not correct. According to a theorem of Friedman and Sheard these rules (denoted by them "$T$-intro", "$T$-elim", "$\neg T$-intro", and "$\neg T$-elim") can be consistently added to PA. (See part D of their main theorem.) [Edit: to clarify, this is in the context of Hilbert-style deduction. If these rules of inference were included in a natural deduction system then they could be converted to implications using $\to$-introduction, and then you would get the Tarski biconditionals and a liar paradox.]

I may add that I have formulated an extension $S$ of Peano arithmetic which is consistent, includes the capture scheme $\phi \to T(\ulcorner \phi\urcorner)$ for every sentence $\phi$, and for which $T(\ulcorner T(\ulcorner \cdots \ulcorner\phi\urcorner \cdots \urcorner)\urcorner)$ is not a theorem, for any false arithmetical sentence $\phi$. Thus if the release scheme "infer $\phi$ from $T(\ulcorner \phi\urcorner)$" were added it would still be consistent. Incidentally, this system has the remarkable property that it proves its own soundness and consistency in the sense that it proves the sentences $$(\forall n)({\rm Prov}_S(n) \to T(n))$$ and $$T(\ulcorner{\rm Con}(S)\urcorner).$$ See this paper or my book. (Note that my system uses intuitionistic logic, although it includes the law of excluded middle for every sentence of arithmetic.)

Obviously, adding rules of inference that tell that us we can go from $\phi$ to $T(\ulcorner\phi\urcorner)$ and back, and $\neg\phi$ to $\neg T(\ulcorner\phi\urcorner)$ and back, would be too strong and lead to contradictions in well known ways.

Actually, that's not correct. According to a theorem of Friedman and Sheard these rules (denoted by them "$T$-intro", "$T$-elim", "$\neg T$-intro", and "$\neg T$-elim") can be consistently added to PA. (See part D of their main theorem.)

I may add that I have formulated an extension $S$ of Peano arithmetic which is consistent, includes the capture scheme $\phi \to T(\ulcorner \phi\urcorner)$ for every sentence $\phi$, and for which $T(\ulcorner T(\ulcorner \cdots \ulcorner\phi\urcorner \cdots \urcorner)\urcorner)$ is not a theorem, for any false arithmetical sentence $\phi$. Thus if the release scheme "infer $\phi$ from $T(\ulcorner \phi\urcorner)$" were added it would still be consistent. Incidentally, this system has the remarkable property that it proves its own soundness and consistency in the sense that it proves the sentences $$(\forall n)({\rm Prov}_S(n) \to T(n))$$ and $$T(\ulcorner{\rm Con}(S)\urcorner).$$ See this paper or my book. (Note that my system uses intuitionistic logic, although it includes the law of excluded middle for every sentence of arithmetic.)

Obviously, adding rules of inference that tell that us we can go from $\phi$ to $T(\ulcorner\phi\urcorner)$ and back, and $\neg\phi$ to $\neg T(\ulcorner\phi\urcorner)$ and back, would be too strong and lead to contradictions in well known ways.

Actually, that's not correct. According to a theorem of Friedman and Sheard these rules (denoted by them "$T$-intro", "$T$-elim", "$\neg T$-intro", and "$\neg T$-elim") can be consistently added to PA. (See part D of their main theorem.) [Edit: to clarify, this is in the context of Hilbert-style deduction. If these rules of inference were included in a natural deduction system then they could be converted to implications using $\to$-introduction, and then you would get the Tarski biconditionals and a liar paradox.]

I may add that I have formulated an extension $S$ of Peano arithmetic which is consistent, includes the capture scheme $\phi \to T(\ulcorner \phi\urcorner)$ for every sentence $\phi$, and for which $T(\ulcorner T(\ulcorner \cdots \ulcorner\phi\urcorner \cdots \urcorner)\urcorner)$ is not a theorem, for any false arithmetical sentence $\phi$. Thus if the release scheme "infer $\phi$ from $T(\ulcorner \phi\urcorner)$" were added it would still be consistent. Incidentally, this system has the remarkable property that it proves its own soundness and consistency in the sense that it proves the sentences $$(\forall n)({\rm Prov}_S(n) \to T(n))$$ and $$T(\ulcorner{\rm Con}(S)\urcorner).$$ See this paper or my book. (Note that my system uses intuitionistic logic, although it includes the law of excluded middle for every sentence of arithmetic.)

Source Link
Nik Weaver
  • 42.8k
  • 3
  • 112
  • 213

Obviously, adding rules of inference that tell that us we can go from $\phi$ to $T(\ulcorner\phi\urcorner)$ and back, and $\neg\phi$ to $\neg T(\ulcorner\phi\urcorner)$ and back, would be too strong and lead to contradictions in well known ways.

Actually, that's not correct. According to a theorem of Friedman and Sheard these rules (denoted by them "$T$-intro", "$T$-elim", "$\neg T$-intro", and "$\neg T$-elim") can be consistently added to PA. (See part D of their main theorem.)

I may add that I have formulated an extension $S$ of Peano arithmetic which is consistent, includes the capture scheme $\phi \to T(\ulcorner \phi\urcorner)$ for every sentence $\phi$, and for which $T(\ulcorner T(\ulcorner \cdots \ulcorner\phi\urcorner \cdots \urcorner)\urcorner)$ is not a theorem, for any false arithmetical sentence $\phi$. Thus if the release scheme "infer $\phi$ from $T(\ulcorner \phi\urcorner)$" were added it would still be consistent. Incidentally, this system has the remarkable property that it proves its own soundness and consistency in the sense that it proves the sentences $$(\forall n)({\rm Prov}_S(n) \to T(n))$$ and $$T(\ulcorner{\rm Con}(S)\urcorner).$$ See this paper or my book. (Note that my system uses intuitionistic logic, although it includes the law of excluded middle for every sentence of arithmetic.)