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4. Formal Logic / D. Modal Logic ML / 4. Alethic Modal Logic

[inference from truths concerning necessity and possibility]

11 ideas
There are three different deductions for actual terms, necessary terms and possible terms [Aristotle]
Modal logic is not an extensional language [Parsons,C]
For modality Lewis rejected boxes and diamonds, preferring worlds, and an index for the actual one [Lewis, by Stalnaker]
Metaphysical (alethic) modal logic concerns simple necessity and possibility (not physical, epistemic..) [Salmon,N]
The modal logic of C.I.Lewis was only interpreted by Kripke and Hintikka in the 1960s [Jacquette]
The main modal logics disagree over three key formulae [Yablo]
Modality affects content, because P→◊P is valid, but ◊P→P isn't [Fitting/Mendelsohn]
S4: 'poss that poss that p' implies 'poss that p'; S5: 'poss that nec that p' implies 'nec that p' [Orenstein]
Possible worlds logics use true-in-a-world rather than true [Girle]
Modal logic has four basic modal negation equivalences [Girle]
Modal logics were studied in terms of axioms, but now possible worlds semantics is added [Girle]