Full Idea
There is a difference between having just an indexical concept which one can apply to a perceptual characteristic (just saying 'this is thus'), and having a thicker perceptual concept of that characteristic.
Gist of Idea
Perception may involve thin indexical concepts, or thicker perceptual concepts
Source
Ernest Sosa (Beyond internal Foundations to external Virtues [2003], 7.2)
Book Reference
Bonjour,L/Sosa,E: 'Epistemic Justification' [Blackwells 2003], p.124
A Reaction
Both of these, of course, would precede any categorial concepts that enabled one to identify the characteristic or the object. This is a ladder foundationalists must climb if they are to reach the cellar of basic beliefs.