Full Idea
There mere fact that Tibbles can survive the mutilation of losing a tail, whereas the sum of Tib and the tail cannot, is enough to distinguish them, even if no such mutilation ever occurs.
Gist of Idea
Tibbles isn't Tib-plus-tail, because Tibbles can survive its loss, but the sum can't
Source
Peter Simons (Parts [1987], 6.1)
Book Reference
Simons,Peter: 'Parts: a Study in Ontology' [OUP 1987], p.210
A Reaction
See Idea 12835 for details of the Tibbles example. Either we go for essentialism here, or the whole notion of identity collapses. But the essential features of a person are not just those whose loss would kill them.
Related Idea
Idea 12835 Does Tibbles remain the same cat when it loses its tail? [Simons]