Full Idea
If I only have counterparts in possible worlds who are not identical to me, statements about what I could have done will seem irrelevant to me, because they will be about someone else.
Gist of Idea
Why should statements about what my 'counterpart' could have done interest me?
Source
comment on David Lewis (On the Plurality of Worlds [1986]) by Thomas Mautner - Penguin Dictionary of Philosophy p.115
Book Reference
Mautner,Thomas: 'Dictionary of Philosophy' [Penguin 1997], p.115
A Reaction
We might rephrase the statement as "I could have been the person who did x". Presumably my counterpart is not just any stranger, but someone I could have been. "I could have been a brick" - now that seems irrelevant to me!