A delusion is a deeply held belief by a patient about a fact that is not true, even when the patient has strong evidence of the truth. This is often a symptom of some type of brain damage, but is common in many types of mental illness as well. It differs from a hallucination in that it happens when the patient is otherwise lucid and that patient can often determine that hallucinations are not true. It differs from a psychotic break in that the patient can rationally discuss the delusion. Delusions fall into four general types: