Ideas from 'Counterparts and Identity' by Robert C. Stalnaker [1987], by Theme Structure

[found in 'Ways a World Might Be' by Stalnaker,Robert C. [OUP 2003,0-19-925149-5]].

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4. Formal Logic / D. Modal Logic ML / 7. Barcan Formula
To say there could have been people who don't exist, but deny those possible things, rejects Barcan [Rumfitt]
10. Modality / E. Possible worlds / 3. Transworld Objects / c. Counterparts
Unlike Lewis, I defend an actualist version of counterpart theory
If possible worlds really differ, I can't be in more than one at a time
If counterparts exist strictly in one world only, this seems to be extreme invariant essentialism
19. Language / C. Assigning Meanings / 8. Possible Worlds Semantics
Extensional semantics has individuals and sets; modal semantics has intensions, functions of world to extension