What is vision?
We tend to think, in general, that the vision (visual perception) is a passive process; visible light comes into our eyes and makes images on the retina. These are then transmitted via optic nerves and represented in the visual areas of the cerebral cortex. Once you open your eyes, a precise impression of the surrounding world is given to you. The visual system (eyes, optic nerves, visual areas) functions like a photograph or a mirror. It is believed to reflect the present world as it is.
But it does not seem the way things are. Oliver Sacks, a neurologist, describes the case of his patient Virgil, who restored the vision by surgery after 45 years of blindness:
Virgil told me later that in this first moment he had no idea what he was seeing. There was light, there was movement, there was color, all mixed up, all meaningless, a blur. Then out of the blur came a voice that said, "Well?" Then, and only then, he said, did he finally realize that this chaos of light and shadow was a face --- and , indeed, the face of his surgeon.
[Sacks, O. (1995). An Anthropologist on Mars. p.114]
There is no problem with patient's eyes, retina, or optical nerves. He is not blind anymore after the surgery, but his visual world is totally chaotic. To put it accurately, he is seeing but he is not able to get the meaning of what he sees. There is light, movement and colors but they are all mixed up as 'a blur'. He is receiving the visible light perfectly but his visual world is perfectly meaningless.
How should we think about this case?