A delusion is an irrational false belief that someone holds so firmly that nobody can talk him or her out of it. Delusions are a feature of some major mental illnesses, such as schizophrenia. Narcissists also often try to make you believe things that are clearly false. This tactic is called gaslighting. Sometimes, narcissists even seem to believe the nonsense that they spout. Yet there is a difference between the delusions of someone with schizophrenia and the foolish nonsense that appears on a narcissist’s Twitter feed. Schizophrenics care about the truth, and narcissists do not.