If I awake in a windowless room and hear the radio announce there is a 10 percent chance of rain, I might think to myself there is no need to carry an umbrella to work. But, if I open a door and peer outside and see a heavy downpour I would no longer think there is a 10 percent chance of rain, though before I opened the door I thought there was a 10 percent chance of rain. The way we so frequently and casually use probabilities today makes us believe that probability is describing the things we apply it to. Probability describes the observer, not the observed. His regained sight was not perfect, his eyes having suffered other disease and degeneration.
Sacks also provides references to other historical observations of similar cases.