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Ideas for 'Parmenides', 'The Thought: a Logical Enquiry' and 'Scientific Essentialism'

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4. Formal Logic / D. Modal Logic ML / 3. Modal Logic Systems / h. System S5
Real possibility and necessity has the logic of S5, which links equivalence classes of worlds of the same kind [Ellis]
     Full Idea: The logic of real possibilities and necessities is just S5. This is because the accessibility relation for real possibilities links possible worlds of the same natural kind, which is an equivalence class.
     From: Brian Ellis (Scientific Essentialism [2001], 7.06)
     A reaction: Most people, except Nathan Salmon, agree with this. With full accessibility, you seem to take epistemological problems out of the system, and just focus on reality.