Full Idea
The natural response to an unreasonable hypothesis of possibility for an object x, that in such a state of affairs it would not be x which satisfies the conditions, is evidence that we do possess concepts of thisness for individuals.
Gist of Idea
We believe in thisnesses, because we reject bizarre possibilities as not being about that individual
Source
Graeme Forbes (The Metaphysics of Modality [1985], 9.4)
Book Reference
Forbes,Graeme: 'The Metaphysics of Modality' [OUP 1985], p.235
A Reaction
We may have a 'concept' of thisness, but we needn't be committed to the 'existence' of a thisness. There is a fairly universal intuition that cessation of existence of an entity when it starts to change can be a very vague matter.