Classification and external resources
A delusion is commonly defined as a fixed false belief and is used in everyday language to describe a belief that is either false, fanciful or derived from deception. The International Statistical Classification of Diseases and Related Health Problems (most commonly known by the abbreviation ICD) provides codes to classify Diseases The International Statistical Classification of Diseases and Related Health Problems 10th Revision ( ICD -10) is a coding of diseases and signs symptoms abnormal findings The International Statistical Classification of Diseases and Related Health Problems (most commonly known by the abbreviation ICD) provides codes to classify Diseases The following is a list of codes for International Statistical Classification of Diseases and Related Health Problems. See False for other meanings of false FALSE is an esoteric programming language designed by Wouter van Oortmerssen in Belief is the psychological state in which an individual holds a Proposition or Premise to be true Deception (also called beguilement or subterfuge) is the act of convincing another to believe Information that is not true or not the whole truth as in In psychiatry, the definition is necessarily more precise and implies that the belief is pathological (the result of an illness or illness process). Psychiatry is a medical specialty which exists to study, prevent, and treat Mental disorders in Humans Psychiatric Psychopathology is a term which refers to either the study of Mental illness or mental distress or the manifestation of behaviours and experiences which may be indicative Illness (sometimes referred to as ill-health or ail) can be defined as a state of poor Health. As a pathology it is distinct from a belief based on false or incomplete information or certain effects of perception which would more properly be termed an apperception or illusion. In Psychology and the Cognitive sciences perception is the process of attaining awareness or understanding of sensory Information. Apperception (Latin ad + percipere, to perceive has the following meanings In Epistemology, it is "the introspective or reflective An illusion is a distortion of the senses revealing how the Brain normally organizes and interprets sensory stimulation
Delusions typically occur in the context of neurological or mental illness, although they are not tied to any particular disease and have been found to occur in the context of many pathological states (both physical and mental). Mental disorder or mental illness is a psychological or behavioral pattern that occurs in an individual and is thought to cause distress or disability that is not expected as However, they are of particular diagnostic importance in psychotic disorders and particularly in schizophrenia and mania in episodes of bipolar disorder. Psychosis (from the Greek ψυχή "psyche" for mind or soul and -οσις "-osis" for abnormal condition with adjective psychotic Schizophrenia ( from the Greek roots schizein (σχίζειν "to split" and phrēn Mania (from Greek μανία and that from μαίνομαι - mainomai, "to rage to be furious" is a severe medical condition
Although non-specific concepts of madness have been around for several thousand years, the psychiatrist and philosopher Karl Jaspers was the first to define the three main criteria for a belief to be considered delusional in his book General Psychopathology. Karl Theodor Jaspers ( February 23, 1883 – February 26, 1969) was a German Psychiatrist and Philosopher who These criteria are:
These criteria still continue in modern psychiatric diagnosis. In the most recent Diagnostic and Statistical Manual of Mental Disorders, a delusion is defined as:
There is some controversy over this definition, as 'despite what almost everybody else believes' implies that a person who believes something most others do not is a candidate for delusional thought.
The modern definition and Jaspers' original criteria have been criticised, as counter-examples can be shown for every defining feature.
Studies on psychiatric patients have shown that delusions can be seen to vary in intensity and conviction over time which suggests that certainty and incorrigibility are not necessary components of a delusional belief. 
Delusions do not necessarily have to be false or 'incorrect inferences about external reality'.  Some religious or spiritual beliefs by their nature may not be falsifiable, and hence cannot be described as false or incorrect, no matter whether the person holding these beliefs was diagnosed as delusional or not. 
In other situations the delusion may turn out to be true belief.  For example, delusional jealousy, where a person believes that their partner is being unfaithful (and may even follow them into the bathroom believing them to be seeing their lover even during the briefest of partings) may result in the faithful partner being driven to infidelity by the constant and unreasonable strain put on them by their delusional spouse. Delusional jealousy, Morbid jealousy, or Othello syndrome is a psychiatric disorder in which a person holds a Delusional belief that their spouse In this case the delusion does not cease to be a delusion because the content later turns out to be true.
In other cases, the delusion may be assumed to be false by a doctor or psychiatrist assessing the belief, because it seems to be unlikely, bizarre or held with excessive conviction. Psychiatrists rarely have the time or resources to check the validity of a person’s claims leading to some true beliefs to be erroneously classified as delusional.  This is known as the Martha Mitchell effect, after the wife of the attorney general who alleged that illegal activity was taking place in the White House. The Martha Mitchell effect is the process by which a psychiatrist psychologist or other mental health clinician mistakes the patient's perception of real events as Delusional The United States Attorney General is the head of the United States Department of Justice (see) concerned with legal affairs and is the chief law enforcement See also Executive Office of the President of the United States The White House, formerly known as the Executive Mansion, is the Official residence At the time her claims were thought to be signs of mental illness, and only after the Watergate scandal broke was she proved right (and hence sane). The Watergate scandals were a series of Political scandals during the presidency of Richard Nixon that resulted in the Indictment of several of Nixon's
Similar factors have led to criticisms of Jaspers' definition of true delusions as being ultimately 'un-understandable'. Critics (such as R. D. Laing) have argued that this leads to the diagnosis of delusions being based on the subjective understanding of a particular psychiatrist, who may not have access to all the information which might make a belief otherwise interpretable. Ronald David Laing ( 7 October 1927 – 23 August 1989 was a Scottish Psychiatrist who wrote extensively on Mental illness Subjectivity refers to a subject's perspective particularly feelings beliefs and desires
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. Similarly, Thomas Kuhn argued in The Structure of Scientific Revolutions that scientists can hold strong beliefs in scientific theories despite considerable apparent discrepancies with experimental evidence. Thomas Samuel Kuhn (surname ˈkuːn July 18, 1922  &ndash June 17, 1996) was an American intellectual who wrote extensively The Structure of Scientific Revolutions ( 1962) by Thomas Kuhn, is an analysis of the History of science. 
These factors have led the psychiatrist Anthony David to note that "there is no acceptable (rather than accepted) definition of a delusion. Anthony David is Professor of Cognitive neuropsychiatry at the Institute of Psychiatry, part of King's College London. " In practice psychiatrists tend to diagnose a belief as delusional if it is either patently bizarre, causing significant distress, or excessively pre-occupies the patient, especially if the person is subsequently unswayed in belief by counter-evidence or reasonable arguments.