Full Idea
It was one of the assumptions of Hume's empiricism that all necessities are de dicto: i.e. they are artefacts of language.
Clarification
'De re' refers to things themselves; 'de dicto' refers to things under a description (Winston/the PM)
Gist of Idea
Hume assumes that necessity can only be de dicto, not de re
Source
Roger Scruton (Modern Philosophy:introduction and survey [1994], 13.5)
Book Reference
Scruton,Roger: 'Modern Philosophy: introduction and survey' [Sinclair-Stevenson 1994], p.168