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Single Idea 19209

[filed under theme 4. Formal Logic / D. Modal Logic ML / 1. Modal Logic ]

Full Idea

Logical consequence guarantees preservation of truth. The Converse Barcan, a theorem of Simple Quantified Modal Logic, says that an obvious truth implies an obvious falsehood. So SQML gets logical consequence wrong. So SQML is mistaken.

Gist of Idea

Simple Quantified Modal Logc doesn't work, because the Converse Barcan is a theorem

Source

Trenton Merricks (Propositions [2015], 2.V)

Book Ref

Merricks,Trenton: 'Propositions' [OUP 2015], p.65

A Reaction

I admire this. The Converse Barcan certainly strikes me as wrong (Idea 19208). Merricks grasps this nettle. Williamson grasps the other nettle. Most people duck the issue, I suspect. Merricks says later that domains are the problem.

Related Idea

Idea 19208 The Converse Barcan implies 'everything exists necessarily' is a consequence of 'necessarily, everything exists' [Merricks]

The 18 ideas with the same theme [general ideas about the nature of modal logic]:

9728 Modal Square 1: □P and ¬◊¬P are 'contraries' of □¬P and ¬◊P [Aristotle, by Fitting/Mendelsohn]
9729 Modal Square 2: ¬□¬P and ◊P are 'subcontraries' of ¬□P and ◊¬P [Aristotle, by Fitting/Mendelsohn]
9730 Modal Square 3: □P and ¬◊¬P are 'contradictories' of ¬□P and ◊¬P [Aristotle, by Fitting/Mendelsohn]
9731 Modal Square 4: □¬P and ¬◊P are 'contradictories' of ¬□¬P and ◊P [Aristotle, by Fitting/Mendelsohn]
9732 Modal Square 5: □P and ¬◊¬P are 'subalternatives' of ¬□¬P and ◊P [Aristotle, by Fitting/Mendelsohn]
9733 Modal Square 6: □¬P and ¬◊P are 'subalternatives' of ¬□P and ◊¬P [Aristotle, by Fitting/Mendelsohn]
5745 Quine says quantified modal logic creates nonsense, bad ontology, and false essentialism [Melia on Quine]
13591 Quantified modal logic collapses if essence is withdrawn [Quine]
10928 Maybe we can quantify modally if the objects are intensional, but it seems unlikely [Quine]
16951 It was realised that possible worlds covered all modal logics, if they had a structure [Dummett]
10163 Propositional modal logic has been proved to be complete [Kripke, by Feferman/Feferman]
10559 Kripke's modal semantics presupposes certain facts about possible worlds [Kripke, by Zalta]
16985 Possible worlds allowed the application of set-theoretic models to modal logic [Kripke]
15163 The interest of quantified modal logic is its metaphysical necessity and essentialism [Soames]
10206 Modal operators are usually treated as quantifiers [Shapiro]
9924 Modal logic gives an account of metalogical possibility, not metaphysical possibility [Burgess/Rosen]
5744 First-order predicate calculus is extensional logic, but quantified modal logic is intensional (hence dubious) [Melia]
19209 Simple Quantified Modal Logc doesn't work, because the Converse Barcan is a theorem [Merricks]