Mathematics Asked by Lereau on November 2, 2021
Minimal logic does not assume any falsity $bot$ or negation $neg$, so the above mentioned laws can (apart from Peirce’s) not be stated as usual. However, if we fix some propositional variable $F$, we can use it to define a kind of negation by $dotneg A := A rightarrow F$. We can then define
begin{align}
mathsf{LEM} &:= forall A. ~vdash_m A ~lor~ dotneg A
\ mathsf{DN} & := forall A. ~vdash_m dotnegdotneg A rightarrow A
\ mathsf{CP} & := forall A~B. ~vdash_m (dotneg B rightarrow dotneg A) rightarrow (A rightarrow B)
\ mathsf{Peirce} & := forall A ~B. ~vdash_m ((A rightarrow B) rightarrow A)rightarrow A
end{align}
where $A, B$ are propositions$^{(ast)}$ and $vdash_m$ stands for derivability in minimal logic. In intuitionistic logic (taking $F = bot$ and $vdash_i$ instead) they can all be shown to be equivalent.
In minimal logic, I succeeded in proving:
$$
mathsf{DN} leftrightarrow mathsf{CP} ~~~,~~~
mathsf{CP} rightarrow mathsf{Peirce} ~~~,~~~
mathsf{Peirce} rightarrow mathsf{LEM}
$$
The intuitionistic proofs I did for the other implications all needed the explosion principle and, at least to me, there seems to be no way of avoiding this. I don’t know much about the semantics of minimal logic, so my question comes down to:
Can the other implications be shown or is there some semantics showing the impossibility?
I did the proofs in part on paper and checked all of them in Coq by formalizing the deduction system for propositional minimal logic. (There is also MINLOG, but I have not worked with it so far)
$(ast)$ The quantification here is not supposed to be internal to the logic. I am only considering propositional minimal logic here. So for example, $mathsf{LEM} rightarrow mathsf{DN}$ should be understood as "adding every instance of $A lor dotneg A$ as an axiom, I can derive $dotneg dotneg B rightarrow B$ for every proposition $B$".
Update (5. April 2021): Today I found this paper
Classifying Material Implications over Minimal Logic (Hannes Diener and Maarten McKubre-Jordens)
Which pretty much sums up everything I wanted to know and more.
Using the results form the paper mentioned in the update, there is another way we can argue why $mathsf{Peirce} rightarrow mathsf{Explosion}$ can not be possible.
Assume it holds, then it means we have a way of deducing $forall A. vdash_m F rightarrow A$ from $mathsf{Peirce}$. Since $F$ does not appear in $mathsf{Peirce}$, this means we can use practically the same deduction to show $forall A. vdash_m B rightarrow A$ for any propositional variable $B$, not only the particular choice $B = F$. So we get $$ forall B~A. ~vdash_m B rightarrow A $$ This implies, that for any $X$ we have $vdash_m (X rightarrow X) rightarrow X$ which in turn implies $vdash_m X$. So we would have the highly problematic $forall X. vdash_m X$.
Answered by Lereau on November 2, 2021
$mathsf{Peirce}$ is stronger than $mathsf{LEM}$, but it happens to be interderivable with generalised excluded middle $(mathsf{GEM})$ $$ mathsf{GEM} := forall A~B. ~vdash_m A ~lor~ (A rightarrow B). $$
A weak form of Pierce's law is interderivable with $mathsf{LEM}$ $$ mathsf{WPierce} := forall A. ~vdash_m (dotneg A rightarrow A) rightarrow A. $$
None of these four principles are enough to derive $mathsf{Explosion}$. These results, as well as ones that you mention in your question body, are listed as proposition 3 in Minimal Classical Logic and Control Operators by Zena M. Ariola and Hugo Herbelin
Answered by Potato44 on November 2, 2021
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