I have a friend who hallucinates. Over 40 years ago he was very, very thirsty and a babbling brook was within his reach yet he would probably die if he approached the water. Today when he bends over a water cooler he sees, smells and hears in excruciating details the scene. It's not just a flashback, it's real for the moment. He sweats and shakes and must set down. Is he nuts? No, he is a successful businessman, father and husband.
Oliver Sacks, a professor of neurology at NYU, knows blind people who 'see' and deaf people who 'hear'. Most times my friend and these people who hear and see things keep quiet about it, otherwise they might be put on meds and God knows what other 'treatments' they would be subjected to. Professor Sacks recently completed a book entitled Hallucinations. You can hear him discuss his views here.