Full Idea
It isn't clear which formulas of modal propositional logic are logical truths, ...especially for sentences that contain iterations of modal operators. Is □P→□□P a logical truth? It's hard to say.
Gist of Idea
It is hard to say which are the logical truths in modal logic, especially for iterated modal operators
Source
Theodore Sider (Logic for Philosophy [2010], 6.3)
Book Reference
Sider,Theodore: 'Logic for Philosophy' [OUP 2010], p.137
A Reaction
The result, of course, is that there are numerous 'systems' for modal logic, so that you can choose the one that gives you the logical truths you want. His example is valid in S4 and S5, but not in the others.
Related Idea
Idea 13711 System B introduces iterated modalities [Sider]