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Single Idea 9731

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

Full Idea

Modal Square of Opposition 4: 'It is necessary that not P' and 'It is not possible that P' are the contradictories (different truth values) of 'It is not necessary that not P' and 'It is possible that P'.

Gist of Idea

Modal Square 4: □P and ◊P are 'contradictories' of □P and ◊P

Source

report of Aristotle (On Interpretation [c.330 BCE], Ch.12d) by M Fitting/R Mendelsohn - First-Order Modal Logic 1.4

Book Reference

Fitting,M/Mendelsohn,R: 'First-Order Modal Logic' [Synthese 1998], p.7