Full Idea
Your counterparts resemble you closely in content and context in important respects. They resemble you more closely than do the other things in their worlds. But they are not really you.
Gist of Idea
Counterparts are not the original thing, but resemble it more than other things do
Source
David Lewis (Counterpart theory and Quant. Modal Logic [1968], I)
Book Reference
Lewis,David: 'Philosophical Papers Vol.1' [OUP 1983], p.28
A Reaction
It is a dilemma. If my counterpart were exactly me, I couldn't contemplate possibly losing a leg, or my sanity. But if my counterpart isn't exactly me, then I don't have much interest in its fate. Only essences can save us here. Cf. me tomorrow.