William and Alex, John McCarthy made a very clear distinction in an article with the title "Modality, Si. Modal logic, No!" Modality involves reasoning about possibility and necessity. In short, it involves *metalevel* reasoning about the laws, rules, requirements, etc, that determine what is necessary or possible, permitted, etc. That is the foundation for modality. But modal logic is a very simple and inadequate set of axioms for reasoning about an immensely complicated subject. Please read the following references: Five questions about epistemic logic: http://jfsowa.com/pubs/5qelogic.pdf Worlds, models, and descriptions: http://jfsowa.com/pubs/worlds.pdf The 5q article is short, but it has a long list of references to the many complicated issues that must be addressed. The worlds article goes into more detail about how to do the reasoning. John
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