Full Idea
Observational knowledge of any particular fact, e.g. that this is green, presupposes that one knows general facts of the form 'X is a reliable symptom of Y'.
Gist of Idea
Observations like 'this is green' presuppose truths about what is a reliable symptom of what
Source
Wilfrid Sellars (Does Emp.Knowledge have Foundation? [1956], p.123)
Book Reference
'Epistemology - An Anthology', ed/tr. Sosa,E. /Kim,J. [Blackwell 2000], p.123
A Reaction
This is a nicely observed version of the regress problem with justification. I would guess that foundationalists would simply deny that this further knowledge is required; 'this is green' arises out of the experience, but it is not an inference.