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A hallucination is a "perception without a stimulus" — in other words, seeing something that isn't there. It is distinct from delusion, or demonstrably false belief of pathological origin; however, the two are often caused by the same mental illnesses.
Hallucinations are often a symptom of certain mental illnesses, such as schizophrenia, certain forms of dementia, Parkinson's disease, and epilepsy. They can also be experienced by the ingestion of psychoactive substances, most notably LSD.
There are two common forms of hallucination unrelated to mental illness called hypnagogic and hypnopompic hallucinations, the former occurring before sleep and the latter during the process of waking up. Blurred vision and switching between alert and sleeping states can cause anyone to see strange things. These hallucinations can create false memories. Similarly, sleep paralysis can cause memorably frightening hallucinations. There are many other conditions of high mental or bodily stress during which a person may possibly experience hallucinations.