http://en.wikipedia.org/wiki/Recovery_from_blindness provides a succinct overview of Molyneux's Problem, a philosophical thought experiment that attempts to address this very issue.

This quote is especially interesting from a 13 year old whose sight was restored in 1728: "When he first saw, he was so far from making any judgment of distances, that he thought all object whatever touched his eyes (as he expressed it) as what he felt did his skin"