Mood-congruent delusion: Any delusion with content consistent with either a depressive or manic state, e.g., a depressed person believes that news anchors on television highly disapprove of them, or a person in a manic state might believe they are a powerful deity. Mood-neutral delusion: A delusion that does not relate to the patients emotional state; for example, a belief that an extra limb is growing out of the back of ones head is neutral to either depression or mania. Themes
In addition to these categories, delusions often manifest according to a consistent theme. Although delusions can have any theme, certain themes are more common. Some of the more common delusion themes are:

Delusion of control: False belief that another person, group of people, or external force controls ones general thoughts, feelings, impulses, or behaviors. Cotard delusion: False belief that one does not exist or that one has died. Some cases also include the belief that one is immortal or that one has lost their internal organs, blood, or other body parts. Delusional jealousy: False belief that a spouse or lover is having an affair, with no proof to back up the claim. Delusion of guilt or sin (or delusion of self-accusation): Ungrounded feeling of remorse or guilt of delusional intensity. Delusion of mind being read: False belief that other people can know ones thoughts. Delusion of thought insertion: Belief that another thinks through the mind of the person. Delusion of reference: False belief that insignificant remarks, events, or objects in ones environment have personal meaning or significance.
This use of fiction to decrease the malleability of a delusion was employed in a joint project by science-fiction author Philip Jose Farmer and Yale psychiatrist A. James Giannini. They wrote the novel Red Orcs Rage, which, recursively, deals with delusional adolescents who are treated with a form of projective therapy. In this novels fictional setting other novels written by Farmer are discussed and the characters are symbolically integrated into the delusions of fictional patients. This particular novel was then applied to real-life clinical settings.Another difficulty with the diagnosis of delusions is that almost all of these features can be found in "normal" beliefs. Many religious beliefs hold exactly the same features, yet are not universally considered delusional. For instance, if a person was holding a true belief then they will of course persist with it. This can cause the disorder to be misdiagnosed by psychiatrists. These factors have led the psychiatrist Anthony David to note that "there is no acceptable (rather than accepted) definition of a delusion." In practice, psychiatrists tend to diagnose a belief as delusional if it is either patently bizarre, causing significant distress, or excessively pre-occupying the patient, especially if the person is subsequently unswayed in belief by counter-evidence or reasonable arguments. Joseph Pierre, M.D. states that one factor that helps differentiate delusions from other kinds of beliefs is that anomalous subjective experiences are often used to justify delusional beliefs.