Full Idea
Categories, like 'being', or 'individuality', are already mingled into every proposition, even when it has a completely sensible content, such as "this leaf is green".
Gist of Idea
Even simple propositions about sensations are filled with categories
Source
Georg W.F.Hegel (Logic (Encyclopedia I) [1817], §246 Add), quoted by Stephen Houlgate - Hegel p.95
Book Reference
'A Companion to Continental Philosophy', ed/tr. Critchley,S/Schroeder,W [Blackwell 1999], p.95
A Reaction
This is the source of the idea that observation is theory-laden (which tracks back to Kant). Not Duhem, who gets the credit among analytic philosophers. Quine obviously never read Hegel. But the idea is overrated.