Full Idea
If a vague language is made precise, its expressions change in meaning, so an accurate semantic description of the precise language is inaccurate as a description of the vague one.
Gist of Idea
You can't give a precise description of a language which is intrinsically vague
Source
Timothy Williamson (Vagueness [1994], 5.1)
Book Reference
Williamson,Timothy: 'Vagueness' [Routledge 1996], p.142
A Reaction
Kind of obvious, really, but it clarifies the nature of any project (starting with Leibniz) to produce a wholly precise language. That is usually seen as a specialist language for science.