Full Idea
If whatever thing in world w6 it is that resembles you more closely than anything else in w6 is nevertheless quite unlike you; nothing in w6 resembles you at all closely. If so, you have no counterpart in w6.
Gist of Idea
If the closest resembler to you is in fact quite unlike you, then you have no counterpart
Source
David Lewis (Counterpart theory and Quant. Modal Logic [1968], I)
Book Reference
Lewis,David: 'Philosophical Papers Vol.1' [OUP 1983], p.29
A Reaction
This is the nub, because the whole theory rests on deciding whether two things resemble sufficiently 'closely'. But then we need a criterion of closeness, so we must start talking about which properties matter. Essences loom.