more from Rod Girle

### Single Idea 7794

#### [catalogued under 4. Formal Logic / D. Modal Logic ML / 3. Modal Logic Systems / g. System S4]

Full Idea

In S4 there are fourteen modalities: no-operator; necessarily; possibly; necessarily-possibly; possibly-necessarily; necessarily-possibly-necessarily; and possibly-necessarily-possibly (each with its negation).

Clarification

S4 is one of the five main systems of modal logic

Gist of Idea

There are seven modalities in S4, each with its negation

Source

Rod Girle (Modal Logics and Philosophy [2000], 3.5)

Book Reference

Girle,Rod: 'Modal Logics and Philosophy' [Acumen 2000], p.46

A Reaction

This is said to be 'more complex' than S5, but also 'weaker'.