Identity
This video presents the same text shown beside it, spoken and on screen. It adds nothing the text does not say.
State
a = b asserts that a and b name the same individual; identicals are intersubstitutable everywhere.
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Twain = Clemens. Whatever's true of Twain is true of Clemens — same man, two names.
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Identity is a relation between things, not names. The names differ; the individual doesn't.