Jaspers describes four types of primary delusion:

1. delusional intuition - where delusions arrive 'out of the blue', without external cause.
2. delusional perception - where a normal percept is interpreted with delusional meaning. For example, a person sees a red car and knows that this means the person's food is being poisoned by the police.
3. delusional atmosphere - where the world seems subtly altered, uncanny, portentous or sinister. This resolves into a delusion, usually in a revelatory fashion, which seems to explain the unusual feeling of anticipation.
4. delusional memory - where a delusional belief is based upon the recall of memory or false memory for a past experience. For example, a man recalls seeing a woman laughing at the bus stop several weeks ago and now realises that this person was laughing because the man has animals living inside him.

Secondary delusions (sometimes called delusion-like ideas) are considered to be, at least in principle, understandable in the context of a person's life history, personality, mood state or presence of other psychopathology. For example, a person becomes depressed, suffers very low mood and self-esteem, and subsequently believes he or she is responsible for some terrible crime which he or she did not commit.

Diagnostic issues:
However, the modern definition and Jaspers's 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 belief1.

Delusions do not necessarily have to be false or 'incorrect inferences about external reality'2. Some religious or spiritual beliefs (such as 'I believe in the existence of God') including those diagnosed as delusional, by their nature may not be falsifiable, and hence cannot be described as false or incorrect3.

In other situations the delusion may turn out to be true belief4. For example, delusional jealousy, where a person believes that his partner is being unfaithful (and may even follow them into the bathroom believing her to be seeing her 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 her by her delusional 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 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 delusional5. 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. 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).

Similar factors have led to criticisms of Jaspers's 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.

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 considered delusional. Similarly, as Thomas Kuhn demonstrated in The Structure of Scientific Revolutions (his groundbreaking book on the history and sociology of science), scientists can hold strong fixed beliefs in scientific theories despite considerable counter evidence for their validity6.

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-occupies the patient, especially if the person is subsequently unswayed in belief by counter-evidence or reasonable arguments.

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