The DSM definition of delusion has been questioned on every substantial point that it makes. It is controversial whether delusions must be false, beliefs, based on inference, based on *incorrect* inference, be held despite *incontrovertible* evidence, be contrary to the persons culture / sub-culture, and so forth. The DSM definition isn't so much a definition (necessary and sufficient conditions) as a fairly substantial theory that throws up lots of fairly unrelated characteristics (as does the DSM definition of mental disorder).

The only way clinicians can access delusions is by way of what the client says and does. They can't access their beliefs directly, they can only access their beliefs indirectly by way of language. Clinicians are given lots of examples of delusional utterances and they probably learn to recognise / diagnose delusions on the basis of how similar a patients utterance is to some of the prototypical exemplars that they learned in medical school.

The most prevalent form of deluisons (typically found within the context of psychosis) is paranoid deluisons. The main affect is fear / anger and the person comes to say that they are being persecuted by some person, group of persons, agency etc.

Delusions of reference are fairly common as well. In delusions of reference people come to say that some phenomena that most of us wouldn't see much significance in is really a special message for the person. The classic case of this is the man who saw some table tops and concluded that the world was coming to an end. It is fairly frequently experienced in the form of television or radio containing special messages for the person.

There are lots of different classifications of delusions. I came up with a list of about 18 one day but some of them are more specific to acquired neurological damage rather than being found in the context of mental disorder. Part of the problem with stating how many different kinds of delusion there are include how fine-grained you want to classify delusions.

People with the Capgras delusion state that someone who is close to them (a husband, wife, child, etc) has been replaced by some kind of impostor (a robot, alien, clone, being from the planet zog). Some people make similar claims about objects (one man claimed that many items in his house had been replaced by identical duplicates of slightly inferior quality) and one woman stated that her canary had been replaced by duplicates. There are fairly good models of the Capgras delusion that have been developed. According to these models people experience the Capgras delusion in response to acquired Cerebral injury interfearing with the usual emotional (SGR) response to familiar faces. This explanation works well for people who believe that persons have been replaced but if this is accepted then it would seem that the object (and possibly the members of other species varieties) would be different kinds of delusion needing different explanations.

Bentall in his book 'Madness Explained: Psychosis and Human Nature' maintains that there is no hard and fast distinction between a person who has a delusion and a person who has not. There are many examples of phenomena that seem part way between. The distinction between 'over-valued ideas' and delusions. The belief that one is fat in the context of starvation in eating disorder. The belief that one is a 3 year old boy in a 26 year old female with DID. The belief that one is literally drinking the blood of Jesus Christ held by members of the congregation down the road...

There is controversy over whether psychiatric medication is the best treatment for delusion. Most psychiatrists prescribe it, of course, but some of them say that while the medication works to sedate people such that they are less likely to report on their delusions delusions typically pass over a bit of time. I guess the main thing is to make sure that the person isn't likely to (or able to) harm themself or someone else and also to make sure they don't become uncontrollably distressed etc.

People with paranoid personality disorder typically attribute malevolent intent to others. Heck, I know a whole bunch of people who have never been in contact with psychiatric services who attribute malevolent intent to others (or some special subgroups of them). The boundary between normality, paranoid personality disorder (ingrained traits) and episodes of paranoid deluison are blurry...