Full Idea
The indefinite description in 'A man will meet you' is naturally treated as quantificational, but an occurrence in predicative position, in 'Jones is not a philosopher', doesn't have a natural quantificational counterpart.
Gist of Idea
Indefinite descriptions are quantificational in subject position, but not in predicate position
Source
Scott Soames (Philosophy of Language [2010], 1.23)
Book Reference
Soames,Scott: 'Philosophy of Language' [Princeton 2010], p.24