If sameness of sperm is held essential to personal identity, we can further ask whether a counterpart sperm with some different molecules is the same sperm.

Let us now apply this thought to conscious subjects. Suppose that a given human individual had had origins different from those which he in fact had such that whether that difference affected who he was was not obvious to intuition. What would count as such a case might be a matter of controversy, but there must be one. Perhaps it is unclear whether, if there had been a counterpart to Jones’ body from the same egg but a different though genetically identical sperm from the same father, the perso