Full Idea
Evans tries to derive a contradiction from the supposition that a given identity statement is of indeterminate truth-value. (As it happens, I consider that this argument is flawed)
Gist of Idea
Evans argues (falsely!) that a contradiction follows from treating objects as vague
Source
report of Gareth Evans (Can there be Vague Objects? [1978]) by E.J. Lowe - The Possibility of Metaphysics 1.3
Book Reference
Lowe,E.J.: 'The Possibility of Metaphysics' [OUP 2001], p.11
A Reaction
A priori, I wouldn't expect to be able to settle the question of whether there are any vague objects simply by following some logical derivation. Empirical examination, and conceptual analysis (or stipulation) have to be involved.