What Makes a Belief Delusional?

In philosophy, psychiatry, and cognitive science, definitions of clinical delusions are not based on the mechanisms responsible for the formation of delusions, since there is no consensus yet on what causes delusions. Some of the defining features of delusions are epistemic and focus on whether delusions are true, justified, or rational, as in the definition of delusions as fixed beliefs that are badly supported by evidence. Other defining features of delusions are psychological and focus on whether delusions are harmful, as in the definition of delusions as beliefs that disrupt good functioning. Even if the epistemic features go some way towards capturing what otherwise different instances of clinical delusions have in common, they do not succeed in distinguishing delusions as a clinical phenomenon from everyday irrational beliefs. Focusing on the psychological features is a more promising way to mark the difference between clinical and non-clinical irrational beliefs, but there is wide variability in the extent to which delusions are psychologically harmful, and some everyday irrational beliefs can affect functioning in similarly negative ways. In this chapter we consider three types of belief that share similar epistemic features and exhibit variation with respect to how psychologically harmful they are: (1) delusions of thought insertion, (2) alien abduction beliefs, and (3) self-enhancing beliefs. In the light of the similarities and differences among these cases, we highlight the difficulty in providing an answer to what makes an irrational belief delusional.