What is William's affliction? How does it affect him with Dr. Sacks?

William has Korsakov's, which prevents him from remembering who he is or any facts about himself. When Dr. Sacks meets him, he identifies himself as four different people over the time they're together, first as a customer in William's old deli, then as an old friend, then as Hymie the butcher next door and finally as a doctor.