Full Idea
S5 collapses iterated modalities (so ◊□P → □P, and ◊◊P → ◊P).
Gist of Idea
S5 collapses iterated modalities (◊□P→□P, and ◊◊P→◊P)
Source
R Keefe / P Smith (Intro: Theories of Vagueness [1997], §5)
Book Reference
'Vagueness: a Reader', ed/tr. Keefe,R /Smith,P [MIT 1999], p.55
A Reaction
It is obvious why this might be controversial, and there seems to be a general preference for S4. There may be confusions of epistemic and ontic (and even semantic?) possibilities within a single string of modalities.
Related Idea
Idea 14690 In the S5 account, nested modalities may be unseen, but they are still there [Salmon,N]