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Delusional beliefs are key symptoms of mental illness, and physicians over hundreds of years have attempted to understand and offer treatments for patients with such beliefs. In this chapter, the authors will explore the experience of delusional beliefs within the clinical context: i.e. with people who present to mental health services for help. The authors begin with definition of the descriptive psychopathology, prevalence and context in which delusional beliefs occur and their clinical relevance. Delusional beliefs have a core role in distress, depression and risk within psychosis. Real examples will be used to reflect on both the form delusional beliefs take and their personal content, concluding with treatment options and challenges.

Different empirical and theoretical traditions approach delusions differently. This chapter is about how cognitive neuroscience – the practice of studying the brain to draw conclusions about the mind – has been applied to the problem of belief and delusion. In particular, the focus is on a particular bridging theory, that of predictive coding. This theory holds that the brain contains a model of the world (and the self as an agent in that world). It uses that model to make predictions in order to adapt to the environment. Errors in those predictions can garner belief updating or be ignored, depending on how each prediction error response sustains adaptive fitness. The discussion will cover how delusions might arise and be maintained under the influence of aberrant prediction errors and what psychological and neural mechanisms of prediction error processing pertain to delusions, comparing and contrasting the theory with other prominent theories of delusions. The conclusion is that the single factor, prediction error account gives a parsimonious account of delusions that generates novel predictions about how best to treat delusions and incorporates numerous biological, clinical and phenomenological data regarding delusions.

The difficulty of distinguishing between delusions and non-pathological beliefs has taxed some of the greatest minds in psychiatry. This chapter argues that this question cannot be resolved without first having an understanding of what is involved in holding an ordinary belief. Although we should not assume that ordinary-language words such as ‘belief’ will correspond with a specific psychological mechanism or process, sufficient evidence is available from diverse areas of psychology to reach some conclusions about what happens when someone ‘believes’ something. Beliefs are propositions about the world that are generated dynamically, often during interactions with other people, and therefore depend on the human capacity for language. Although many beliefs are mundane, it is possible to identify a class of master interpretive systems that includes political ideologies and religious belief systems, which are highly resistant to challenge and capable of generating considerable emotion. These systems seem to depend not only on the ability to generate propositions about the world but also on implicit cognitive processes that are related to fundamental biological and social needs, for example the need to avoid contagion, the need to form close intimate relationships or the need to avoid out-groups. Delusions share many of the properties of master interpretive systems but differ because they are idiosyncratic. They may arise when individuals are very isolated or if they lack the cognitive tools to function effectively in groups. Further progress in understanding delusions is likely to be made if research is informed by findings from political psychology and the psychology of religion.

This chapter addresses the contribution that the delusion literature has made to the philosophy of belief. Three conclusions will be drawn: (1) a belief does not need to be epistemically rational to be used in the interpretation of behaviour; (2) a belief does not need to be epistemically rational to have significant psychological or epistemic benefits; (3) beliefs exhibiting the features of epistemic irrationality exemplified by delusions are not infrequent, and they are not an exception in a largely rational belief system. What we learn from the delusion literature is that there are complex relationships between rationality and interpretation, rationality and success, and rationality and knowledge.