more from Rod Girle

### Single Idea 7793

#### [catalogued under 4. Formal Logic / D. Modal Logic ML / 3. Modal Logic Systems / h. System S5]

Full Idea

The critical formula that distinguishes S5 from all others is: ◊p → □◊p.

Clarification

S5 is the best known system of modal logic

Gist of Idea

◊p → □◊p is the hallmark of S5

Source

Rod Girle (Modal Logics and Philosophy [2000], 3.3)

Book Reference

Girle,Rod: 'Modal Logics and Philosophy' [Acumen 2000], p.34

A Reaction

If it is possible that it is raining, then it is necessary that it is possible that it is raining. But if it is possible in this world, how can that possibility be necessary in all possible worlds?