Haecceitistically

[…] I am contemplating the possibility of being poor Fred in a world just life this one. The Haecceitist will suggest that I have in mind a qualitative duplicate of this world where the nonqualitative determinants of cross-identification somehow link me with the qualitative counterpart of Fred. But this distorts my thought: I thought not just that I might have lived Fred's life, but that I might have been Fred living Fred's life. […] I suggest that the possibility I have in mind is not a world that is like ours qualitatively but differs from ours Haecceitistically.

[T]here are no clear actual cases of individual substances discernible only with respect to relational and haecceitistic properties. On the other hand, there do seem to be pairs of possible individuals which differ only with respect to such properties. Max Black has argued that there is a possible universe containing only two qualitatively indiscernible brass spheres. These spheres will have the same mass, shape, color; be made of the same kind of stuff; and so on. But they will differ with respect to spatial relational properties involving the haecceity of what they're made of, such as being made of this matter, as well as differing with respect to purely haecceitistic properties like being this and impurely haecceitistic properties like being this brass sphere. If, as seems plausible, there are possible universes of the sort Black envisions, then there are individuals that cannot be distinguished save relationally or haecceitistically.

Two individuals differ haecceitistically just in case they have different haecceities. Differing haecceitistically is consistent with sharing many properties and also with differing with respect to many properties. In contrast, two individuals differ merely haecceitistically just in case they share all the same properties but differ haecceitistically. Haecceitism entails that there is a possible world qualitatively identical to, but nevertheless distinct from, the actual world where my haecceity is swapped with yours.

[H]aecceitism leads to trouble in a way that quidditism does not. Suppose we allegedly have two indiscernible worlds that differ haecceitistically: at the place where individual A is in W₁, B is in W₂, and vice versa. We ask: in virtue of what does B rather than A occupy that place in W₂? Because the worlds are indiscernible, it seems we can have no informative answer. We can only say "It just does". That is, we need to invoke the transworld identity of B in W₁ with B in W₂.

Bill's and Tina's experiential properties differ haecceitistically in their object but are indiscriminable; Bill's and Inez's experiential properties differ quidditistically in their object but are indiscriminable; Bill's and Dean's experiential properties differ with respect to factivity, and haecceitistically and quidditistically in their object.