Single Idea 13114

[catalogued under 4. Formal Logic / D. Modal Logic ML / 3. Modal Logic Systems / c. System D]

Full Idea

System D is usually thought of as Deontic Logic, concerning obligations and permissions. □P → P is not valid in D, since just because an action is obligatory, it does not follow that it is performed.

Gist of Idea

□P → P is not valid in D (Deontic Logic), since an obligatory action may be not performed

Source

M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 1.12.2 Ex)

Book Reference

Fitting,M/Mendelsohn,R: 'First-Order Modal Logic' [Synthese 1998], p.34

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