Full Idea
If the relative identity theorist denies transitivity (to deal with the Ship of Theseus, for example), this would make us suspect that relativised identity relations are not identity relations, since transitivity seems central to identity.
Clarification
Transitivity: a=b, b=c, so a=c
Gist of Idea
Relative identity may reject transitivity, but that suggests that it isn't about 'identity'
Source
Ryan Wasserman (Material Constitution [2009], 6)
Book Reference
'Stanford Online Encyclopaedia of Philosophy', ed/tr. Stanford University [plato.stanford.edu], p.17
A Reaction
The problem here, I think, focuses on the meaning of the word 'same'. One change of plank leaves you with the same ship, but that is not transitive. If 'identical' is too pure to give the meaning of 'the same' it's not much use in discussing the world.