Full Idea
The theory that things have 'haecceities' must be sharply distinguished from the theory referred to as 'haecceitism', which says there may be differences in transworld identities that do not supervene on qualitative differences.
Clarification
One might translate 'haecceity' [pr 'hex-ey-ety'] as 'thisness'
Gist of Idea
The theory of 'haecceitism' does not need commitment to individual haecceities
Source
Penelope Mackie (How Things Might Have Been [2006], 2.2 n7)
Book Reference
Mackie,Penelope: 'How Things Might Have Been' [OUP 2006], p.21
A Reaction
She says later [p,43 n] that it is possible to be a haecceitist without believing in individual haecceities, if (say) the transworld identities had no basis at all. Note that if 'thisness' is 'haecceity', then 'whatness' is 'quiddity'.