### Single Idea 7791

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

Full Idea

C.I.Lewis constructed five axiomatic systems of modal logic, and named them S1 to S5. It turns out that the simplest of the logics based on possible worlds is the same as Lewis's S5.

Gist of Idea

The simplest of the logics based on possible worlds is Lewis's S5

Source

report of C.I. Lewis (works [1935]) by Rod Girle - Modal Logics and Philosophy 2.1

Book Reference

Girle,Rod: 'Modal Logics and Philosophy' [Acumen 2000], p.13

A Reaction

Nathan Salmon ('Reference and Essence' 2nd ed) claims (on p.xvii) that "the correct modal logic is weaker than S5 and weaker even than S4". Which is the greater virtue, simplicity or weakness?