Full Idea
If, in judging a heap as grains are removed, omniscient speakers all stop at the same point, it must does mark some sort of previously hidden boundary. ...If there is no hidden boundary, then different omniscient speakers would stop at different points.
Gist of Idea
If a heap has a real boundary, omniscient speakers would agree where it is
Source
Timothy Williamson (Vagueness [1994], 7.3)
Book Reference
Williamson,Timothy: 'Vagueness' [Routledge 1996], p.200
A Reaction
A very nice thought experiment, which obviously won't settle anything, but brings out nicely the view the vagueness is a sort of ignorance. God is never vague in the application of terms (though God might withhold the application if there is no boundary).