Full Idea
Quine says that no good sense can be made of quantification into opaque contexts.
Clarification
'Opaque contexts' usually involve uncertainty of reference
Gist of Idea
No sense can be made of quantification into opaque contexts
Source
report of Willard Quine (works [1961]) by Bob Hale - Abstract Objects Ch.2
Book Reference
Hale,Bob: 'Abstract Objects' [Blackwell 1987], p.17
A Reaction
This is because poor old Quine was trapped in a world of language, and had lost touch with reality. I can quantify over the things you are thinking about, as long as you are thinking about things that can be quantified over.