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4. Formal Logic / D. Modal Logic ML / 3. Modal Logic Systems / c. System D

[version with guaranteed access to some world]

6 ideas
D is valid on every serial frame, but not where there are dead ends [Cresswell]
□P → P is not valid in D (Deontic Logic), since an obligatory action may be not performed [Fitting/Mendelsohn]
The system D has the 'serial' conditon imposed on its accessibility relation [Fitting/Mendelsohn]
Intuitively, deontic accessibility seems not to be reflexive, but to be serial [Sider]
In D we add that 'what is necessary is possible'; then tautologies are possible, and contradictions not necessary [Sider]
Deontic modalities are 'ought-to-be', for sentences, and 'ought-to-do' for predicates [Vetter]