If Maher is correct then maybe this is a different class of delusions with perceptual ‘grounds’ and requiring different treatment from delusions that have been developed over time due to cognitive processes. There is now neuro-scientific evidence that this kind of ‘perceptual’ explanation applies to the Cotard and the Capgras delusion (Ramachandran and Blakeslee 1999). There is still, however, debate about whether perceptual anomalies alone are enough to cause these (and perhaps other) delusions or whether other cognitive anomalies are required as well. These are usually described as the one factor model (perceptual anomaly alone) (advocates include Gerrans 2002; Vosgerau and Newen 2007) and the two-factor model (perceptual anomaly plus cognitive anomaly) (advocates include Davies et al 2001) – and debate about some aspects of the neuroscience behind this is ongoing (between Max Coltheart and Phil Corlett) on this blog.
I cannot say here whether delusions with different characteristics and different aetiologies respond differently to different therapies (although I think this would be an interesting area for research) as we do not fully understand the phenomenology of the myriad of different kinds of delusions let alone the aetiology. However, whether delusions are formed through perceptual anomalies, cognitive anomalies or a combination of these and other factors (biological, psychological, environmental and/or affective) we do know that delusions are sometimes amenable to counter argument. There is evidence that delusions respond to CBT and research is continuing in this area (see Hutton and Taylor 2014; Williams et al. 2014).
Some delusions seem to have faith-like qualities and are perhaps un-falsifiable. If the evidence of my experience tells me that my husband is an imposter (Capgras delusion) then how is it possible to dis-prove this? Perhaps turning my attention to reality testing would simply highlight what a good imposter he is. If I looked at photographs and discussed shared memories with him, yet I knew he was not my husband, I would simply think that he had prepared incredibly well for the role.
It seems that some delusions may be impervious to counter argument perhaps because they are un-falsifiable because the anomalous experience includes the proof. Other delusions may be less firmly held and therefore open to revision through counter argument and other evidence. I propose that the characteristics – intensity of belief or degree of conviction and amenability to revision through counter argument and evidence may be measurable. Further, a delusion might be described as un-falsifiable which may mean that amenability to revision by ordinary cognitive methods is impossible. In addition, measuring intensity of belief and amenability to revision through counter argument might tell us something about different kinds of delusions.
There are many problems associated with the definition of delusion, which remain unexamined here – I hope to address some of these in my work and through future empirical research.