A delusion is commonly defined as a false belief, and is used in everyday language to describe a belief that is either false, fanciful or derived from deception. In psychiatry, the definition is necessarily more precise and implies that the belief is pathological (the result of an illness or illness process).

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). However, they are of particular diagnostic importance in psychotic disorders and particularly in schizophrenia.

Psychiatric definition

The psychiatrist and philosopher Karl Jaspers first defined the three main criteria for a belief to be considered delusional in his book General Psychopathology. These criteria are

  1. certainty (held with absolute conviction)
  2. incorrigibility (not changeable by compelling counterargument or proof to the contrary)
  3. impossibility or falsity of content (implausible, bizarre or patently untrue)

These criteria still live on in modern psychiatric diagnosis. In the most recent Diagnostic and Statistical Manual of Mental Disorders, a delusion is defined as:

A false belief based on incorrect inference about external reality that is firmly sustained despite what almost everybody else believes and despite what constitutes incontrovertible and obvious proof or evidence to the contrary. The belief is not one ordinarily accepted by other members of the person's culture or subculture (e.g. it is not an article of religious faith).

Primary and Secondary Delusions

Jaspers originally made a distinction between primary and secondary delusions.

According to Jaspers, primary delusions (sometimes called true delusions) are distinguished by a transformation of meaning, so that the world, or aspects of it, are interpreted in a radically different way by the delusional person. To others, this interpretation is 'un-understandable' in terms of the normal mental causality, mood, environmental influences and other psychological or psychopathological factors.

Read more about Primary Delusion from Jaspers' point of view.