### Sets and Possible Worlds

This semester I'm directing an independent study on modal logic with a couple of students with strong math background. Yesterday some questions about sets and possible worlds came up, so I wrote up some notes for my students on the subject. This blog post is adapted from those notes. Introduction The development of axiomatic set theory was launched by consideration of Russell's Paradox: let A be the set of all sets that do not contain themselves. Does A contain itself or not? (This was on Existential Comics just yesterday!) The collection of axioms mathematicians developed to avoid paradox has the...

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