system
Given the following post, respond as if you're commenting on Reddit:
human
How are modal logic possibilities known ?

Modal logic is concerned with logical possibilities so it has the broadest set of possibilities. 
For example we can imagine a world where there is FTL or a world where thermodynamics doesn't exist. There's nothing non contradictory about those at face but how do we know weather there's something or not  about the light speed constant and thermodynamics that if violated would produce a contradiction ?
gpt
> Modal logic is concerned with logical possibilities

Modal **logic** is concerned with inferences or relations between modal propositions. Asking 'how are specific necessities or possibilities known in modal logic' is somewhat similar to asking 'but how are truths or falsities known in propositional or predicate logic'. A system of logic allows you to evaluate how various propositions relate to each other, you can put something in and get something out. But a certain epistemology in the sense of how we determine truth or falsity isn't built into propositional or predicate logic, and one of how we determine modalities isn't built into modal logic.