We are philosophers working on various topics that intersect with delusions. Colin Klein works on the philosophy of neuroscience and the application of interventionist accounts of causation to this area, and has also discussed the relation between psychopathologies like somatoparaphenia and his theory of pain. Stephen Gadsby works on distorted body representations and false body size beliefs in anorexia nervosa. And Peter Clutton has defended the doxastic status of delusions—offering a cognitive phenomenological account of delusions (forthcoming)—and explored the status of delusions on the harmful dysfunction account.
Any discussion of delusions needs some criteria by which patients are grouped together as having the same delusion. In our paper, ‘Taxonomising Delusions: content or aetiology?’, we compare content-based and aetiological taxonomies of delusions, arguing in favour of the latter.
Most authors taxonomise delusions by the content of the delusional belief: Capgras patients believe that their spouse has been replaced by an imposter, Cotard’s delusion involves the belief that one is dead, and so on. Taxonomising by belief content has intuitive appeal. Content is often what brings patients to clinical attention in the first place, and may be all a researcher has to work with. Additionally, content appears to offer a theory-neutral starting point, in that it does not presuppose any particular theoretical explanation of the underlying causes of delusions.
We argue that these intuitive advantages are not as strong as they seem, and that an aetiological taxonomy is to be preferred: that is, we think that patients ought to be grouped by the causes of their delusions, rather than by what they believe. An aetiological taxonomy has the advantage of supporting the kinds of empirical generalisations we want from scientific taxonomies. Good taxonomies identify similarities among group members that manifest across a variety of distinct circumstances. In studying the cognitive processes that lead to delusions (and thereby learning about the normal processes of cognition, as cognitive neuropsychiatrists aim to do), we want discoveries about one patient to guide our thinking about other patients we encounter with the same problem. Taxonomising delusions by their underlying cognitive causes allows for just such projectability.
Further, we argue that content-based taxonomies cannot be truly theory-neutral. For example, why do we group together those who believe their spouses have been replaced by an imposter? Is it the personal relation aspect that is important (i.e spouse)? If so, do we extend that to other close loved ones? What about pets, plants, objects, or oneself in the mirror? In so far as content-based approaches have answers to these questions, we think it is only to the extent that they smuggle in aetiological thinking: one might suspect, for example, that face perception plays a role in some of these delusions, and thus group spouses, friends, and maybe pets, but exclude objects and plants. Aetiology is thus smuggled in the back door.
The aetiological approach we advocate might recommend the lumping and splitting of many of the traditional content-based groupings. We give evidence, for example, that somatoparaphrenia is causally heterogenous: it can arise from paralysis, breakdowns in the sense of bodily ownership (itself a heterogenous phenomenon), and depersonalisation. Each of these causal antecedents ought to be distinguished for theoretical purposes: aside from consequences of the shared belief, there is no reason to believe that discoveries about a paralysed patient would generalise to a depersonalised one. If so, then there is no somatoparaphrenia per se, only the somatoparaphrenias. Conversely, we argue that some content-based groupings are arbitrarily narrow, and so may miss important generalisations.
The consequences of this view are further fleshed out in terms of an interventionist account of causation. We explore how the taxonomic process might proceed, emphasising that it is likely to be an ongoing, iterative process. We should expect this process of refinement to provide significantly more useful categories of delusions, even if they do not correspond neatly to the beliefs that seem to distinguish the patients who have them.