To make this kind of objection somewhat more tangible, we may contrast a number of different situations:
(1) Walking into an unfamiliar room, an agent discerns a long, low table against the wall. There is a compact disk player on it, but the agent cannot readily identify this thing, which looks to the subject like a black box.
(2) Walking into the same room, another agent sees the object on the table and conjectures, "Must be a stereo amplifier," wrongly identifying the box as an instance of a familiar kind of thing.
(3) Walking into the same room, a third agent, who is familiar with CD players, immediately recognizes the device on the table, thinking, "That's a CD player."
(4) Walking into the same room, a fourth agent, who is familiar with this CD player's history, recognizes the device on the table and thinks "That John's old CD player, which must have just come back from the shop. It's still got the long, ugly scratch little Tommy made on it."
The first two agents' failures to see that the black box is a CD player is not a result of some purely perceptual deficiency, for the perceptual conditions in these different cases are, ex hypothesi, identical. More generally, the differences between (1) seeing x, (2) mistaking x as q, (3) seeing that x is a p, and (4) seeing that x is some particular p depend on the different agents' cognitive processing and background concepts. These differences are not entailed by the perceptual input alone.