Full Idea
Since singular terms can contain predicates, and since vague predicates are common, vague singular terms are common. For 'the tallest man that Sally knows' there are lots of men for whom it is unclear whether Sally knows them.
Gist of Idea
Singular terms can be vague, because they can contain predicates, which can be vague
Source
Peter van Inwagen (Material Beings [1990], 17)
Book Reference
Inwagen,Peter van: 'Material Beings' [Cornell 1995], p.219