Psychologists and cognitive neuroscientists have argued that subjects with monothematic delusions have anomalous experiences in which the delusions are rooted. However few take anomalous experience to be the only clinically relevant factor. This is the one-factor approach. The orthodoxy, fashioned from the interdisciplinary work of cognitive neuroscientists and philosophers, takes there to be a second clinical factor such as a cognitive deficit, bias, or performance error primarily to explain why not all people with anomalous experiences have delusions. This is the two-factor approach. Despite the popularity of two-factor theories, they have struggled to provide a convincing characterisation of the second factor and its surprising monothematic operation.
Our leading hypothesis is that we need not go searching for a second factor. Rather, the explanation for different responses to anomalous experiences derives from individual differences between subjects in the normal range, not some distinctive cognitive failing symptomatic of delusions.We argue that the way in which people with delusions reason, from their highly anomalous experience to their very bizarre belief, is normal. To say that the reasoning is normal is not to say that it is rational. Healthy subjects reason in all sorts of irrational ways - it is normal to be irrational! Just as conspiracy theorists, horoscope advocates, and some religious believers have some strange belief-forming practices, so too do people with delusions who face strange experiences, which their resulting delusions help to explain. We do not need to posit special kinds of irrationality to explain delusional belief.
We will investigate how our work on delusional belief interacts with philosophical work on perceptual experience and the nature of belief. Some philosophers in the former area have argued that hallucinatory experiences lack phenomenal character (which is to say, there's nothing it feels like to undergo them). We think that philosophers attracted to this view of hallucinations cannot accept a theory of delusion that appeals to them for, at least, an important class of monothematic delusions (those with positive hallucinatory content as opposed to experience of absence). As for the nature of belief, it is commonplace to think that belief is connected to truth. This is demonstrated by the lack of control we have over our beliefs. Although we can imagine things we do not take to be true - which the world of novels, artworks, and movies make use of - we cannot believe things we do not take to be true. Facts like these have led some philosophers to claim that all beliefs aim at truth. We will explore whether delusional beliefs are linked to truth in this way. If they are not, then perhaps we need to reorient our theory of belief, and if they are so truth-aimed, why do they get things so wrong?