Routley and Meyer's relevant logic tradition denies that □¬p entails ¬◇p when modal operators are interpreted through world-relative inconsistency-tolerant accessibility relations.
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relevant logic(Contrasted with classical monotonic logic where X ⊢ A implies X,Y ⊢ A for any Y)
A logical system that restricts premise combination so that all premises must bear some relevance to the conclusion; adding irrelevant material does not preserve entailment
¬ (negation symbol)(used in formal logic notation)
The ¬ symbol means 'not'—so ¬Q means 'not Q.'
□ (box symbol)(logical notation in the formula)
In provability logic, this symbol means 'it is provable that' or 'it can be proven that'—it's shorthand for a whole phrase.
◇ (diamond symbol)(modal logic (logic of possibility and necessity))
In logic, this symbol means 'it is possible that' the thing following it could happen or be true.