more from Stephen Yablo

### Single Idea 8859

#### [catalogued under 4. Formal Logic / D. Modal Logic ML / 4. Alethic Modal Logic]

Full Idea

Lewis's different systems of modal logic differed about such formulae as □P implies □□P; ◊□P implies □P; and ◊S implies □◊S

Clarification

'Nec' (usually a square) and 'poss' (a diamond) are necessarily and possibly

Gist of Idea

The main modal logics disagree over three key formulae

Source

Stephen Yablo (Apriority and Existence [2000], §06)

Book Reference

'New Essays on the A Priori', ed/tr. Boghossian,P /Peacocke,C [OUP 2000], p.203

A Reaction

Yablo's point is that the various version don't seem to make much difference to our practices in logic, mathematics and science. The problem, says Yablo, is deciding exactly what you mean by 'necessarily' and 'possibly'.