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Full Idea
Two things can't be vaguely identical, because then a would have an indeterminacy which b lacks (namely, being perfectly identical to b), so by Leibniz's Law they can't be identical.
Gist of Idea
There can't be vague identity; a and b must differ, since a, unlike b, is only vaguely the same as b
Source
report of Gareth Evans (Can there be Vague Objects? [1978], 4.7) by PG - Db (ideas)
Book Ref
Hawley,Katherine: 'How Things Persist' [OUP 2004], p.118
A Reaction
[my summary of Katherine Hawley's summary (2001:118) of Evans] Hawley considers the argument to be valid. I have grave doubts about whether b's identity with b is the sort of property needed for an application of Liebniz's Law.
| 16129 | Evans argues (falsely!) that a contradiction follows from treating objects as vague [Evans, by Lowe] |
| 16459 | Is it coherent that reality is vague, identities can be vague, and objects can have fuzzy boundaries? [Evans] |
| 16460 | Evans assumes there can be vague identity statements, and that his proof cannot be right [Evans, by Lewis] |
| 16457 | There clearly are vague identity statements, and Evans's argument has a false conclusion [Evans, by Lewis] |
| 14484 | If a=b is indeterminate, then a=/=b, and so there cannot be indeterminate identity [Evans, by Thomasson] |
| 16224 | There can't be vague identity; a and b must differ, since a, unlike b, is only vaguely the same as b [Evans, by PG] |