Full Idea
In the time of a single world, the same individual can undergo a change of sex, but it is less clear that an individual of one sex could have been, from the outset, an individual of another.
Gist of Idea
An individual might change their sex in a world, but couldn't have differed in sex at origin
Source
Graeme Forbes (The Metaphysics of Modality [1985], 6.5)
Book Reference
Forbes,Graeme: 'The Metaphysics of Modality' [OUP 1985], p.148
A Reaction
I don't find this support for essentiality of origin very persuasive. I struggle with these ideas. Given my sex yesterday, then presumably I couldn't have had a different sex yesterday. Given that pigs can fly, pigs can fly. What am I missing?