The easiest (and most common way) to discredit someone is to label them “delusional”. History is replete with such examples – you may even know someone. The implications can be devastating when applied in a medical context.
The new version of the psychiatrists’ diagnostic manual (DSM-5) has redefined “delusion”:
No longer are psychiatrists asked to decide whether the patient has “a false belief based on incorrect inference about external reality that is firmly sustained despite what almost everyone else believes and despite what constitutes incontrovertible and obvious proof or evidence to the contrary”. A wordy and unhelpful definition that has so many logical holes you could drive a herd of unicorns through it.
Instead, the new definition of delusions describes them as fixed beliefs that are unswayed by clear or reasonable contradictory evidence, which are held with great conviction and are likely to share the common themes of psychosis: paranoia, grandiosity, bodily changes and so on.