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It seems to me that Bertrand Russell was somewhat hostile to the idea that modality plays a role in logic.
I guess it was natural to be skeptical since at the time (1927) one didn't have a good semantics for modal operators, and so modal logic may not have seemed like proper mathematics.
That came only in the late 1950s with Kripke: possible worlds x, y and their accessibility relation xRy.
Required, but never shown