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Maybe this will help:

"Let us return, then, to the positive characterization of seeingn. If this state of affairs has no belief content, what content does it have ? [...] S seesn D = D is visually differentiated from its immediate environment by S.

The phrase "visually differentiated" is meant to suggest several things, the most important of which is that S's differentiation of D is constituted by D's looking some way to S, and moreover, looking different than its immediate environment."

F. Dretske (1969), Seeing and Knowing, Londres : Routledge &Kegan Paul. pp. 19-20