Obviously, if someone is spending their days being chased by demons or wandering around aimlessly having conversations with their invisible friends or some such thing they need to see a psychologist and need medical help ASAP. However, one can hallucinate without drugs or mental illness. What about an experience where someone has a hallucination of their name being called or they see something weird out of the corner of their eye? Something like that probably happens to a lot of people on occasion even if they have no mental disorders at all. However, if someone without a "condition" can hallucinate then what causes hallucinations? Simply a misfire of the brain that happens more often when under the severe stress that a lot of mental illnesses can cause (hallucinations in the mentally healthy and sober are generally triggered by stress or isolation). Sometimes though, hallucinations in "sane" people can take much more elaborate forms than one might expect yet the biology behind such experiences seems like an incredible mystery. Also, not to go on a metaphysical tangent, but there are certain things such as auras which are visible to some people but not others. Why would this be and it wouldn't make sense to label them a hallucinatory or illusory experience if probably a few percent of the population sees the same thing.

[kaliope]

interesting thoughts. I hallucinate when I am highly anxious. I have often for the longest time heard my name called out but then I am DID and often heard voices talking to me. I pretty much chalk up all my hallucinations to mental illness.

[freefallin]

I start to hallucinate when I'm really tired, especially if I close my eyes but sometimes without closing my eyes. Based on the presentation and what people have told me, I think it's a type of nocturnal seizure. I don't know for sure, though.

[Sometimes psychotic]

Hypnagogic Hallucinations | Doctor | Patient.co.uk

Could be hypnogogic of hypnopompic hallucinations which are considerd entirely normal...although I know people with bipolar who get brief hallucinations after missing a few nights sleep...

Generally hallucinations are thought to be caused by either too much dopamine or too little glutamate signalling in your brain....there is no iron clad evidence of this though and thus the actual answer is we don't know...

[HazelGirl]

We don't know for sure, but I bet at least sometimes it's wishful thinking (in sane people). Like people who are going through grief have "hallucinations" of seeing those they love who have died, only to find out they were mistaken.