Full Idea
The existence of someone in another world who is a lot like me, but happier, is irrelevant to whether I - this very person - could have been happier, even if we call that other-worldly someone 'my counterpart'.
Gist of Idea
If my counterpart is happy, that is irrelevant to whether I 'could' have been happy
Source
Trenton Merricks (Objects and Persons [2003], §1.IV)
Book Reference
Merricks,Trenton: 'Objects and Persons' [OUP 2003], p.27
A Reaction
He says this is a familiar objection. I retain a lingering deterministic doubt about whether it ever makes to sense to say that I 'could' have been happy, given that I am not. It does seem to make sense to say that I was close to happiness, but missed it.