Just a bit of concern here:
we're given 3 valid formulas on page 314 . THey are always true in all worlds in all models.
Then on page 218 we are given formulas which are not valid, but should hold for certain interpretations of BLOCK and DIAMOND
now the decision on whether these hold, seem very very subjective.
for instance: It ought to be true
it says that if things ought to be true, then it should be permitted. hence saying if BLOCK p --> DIAMOND p (ie. if all should be permitted, then we should have at least one)
Are these interpretations fixed, should we know them based on the table on p 318. Or are they just given to help us understand the reasoning behind them?
many thanks
Danie van Eeden
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