Face blindness, affects 2.5% of the population. Those afflicted cannot recognise faces, even ones they have seen before and know well. They must learn to rely on other cues such as gait, spectacles and manner of dress. Neurologist and writer Oliver Sacks is himself face blind and has been living with ocular cancer. In his latest book, which also tells his own story, he relates a case where a man goes to a physician and says he can't recognise people, and so his life has become 'a round of apology and offence'. "The matter must be aired," Sacks says.