Kengo Miyazono‘s concise and densely-packed book Delusions and Beliefs: A Philosophical Inquiry is an original and important contribution to the growing philosophical literature on the nature of clinical delusions. To outsiders, this may justifiably beg the question: why would philosophers be interested in delusion? To insiders, this comes as no surprise since delusion poses a multitude of conceptual puzzles. Are delusions beliefs? Are they irrational? Are they pathological? Moreover, delusions also pose a multitude of explanatory puzzles. What is the direction of causation in the formation of delusions? Are they responses to anomalous experiences or a product of top-down disturbances? And how many etiological factors are needed to explain their formation and maintenance? These latter puzzles belong to the domain of cognitive science and there is an increasing number of philosophers willing to be informed by and contribute to the scientific state of the art, proposing explanatory models and avenues for future research. Miyazono numbers among these and his book tackles, directly or indirectly, all the puzzles I’ve just mentioned to produce a coherent account of delusion.

The book’s three main chapters each deal with a family of questions concerning delusion. Chapter 2 concerns what Miyazono calls the nature question. Chapter 3 the pathology question. And chapter 4 the etiology question. Miyazono claims that these questions, although the object of much independent discussion, have not thus far received a unified treatment. To provide one is the book’s main goal. Miyazono’s thesis is that delusions are malfunctioning beliefs.

They belong to the category of belief and, hence, doxasticism is correct (which is my answer to the nature question). However, unlike non-pathological irrational beliefs, they fail to perform some functions of belief (which is the crucial part of my answer to the pathology question). More precisely, delusions directly or indirectly involve some malfunctioning cognitive mechanisms. And an empiricist account of the delusion formation process (which answers the etiology question) makes the malfunctional belief hypothesis empirically credible. (2019, p. 4)

The first and third claims above have been extensively defended in the literature. What makes Miyazono’s account particularly original is the fact that it rests upon and is an application of teleo-functionalism, on which he expounds briefly in chapter 1. Rather than individuating cognitive attitudes by the causal roles they play (or are disposed to play), teleo-functionalists argue that we should individuate them by the functions they have (where a function is defined as the effects, consequences or performances for which something has been selected for in evolutionary history). Then, just like a human heart can be diseased or malformed, Miyazono maintains, so can human beliefs. The category of belief, just like the category of heart, should thus be defined in terms of belief-like (or doxastic) functions. However, although all and only the members of the category of belief purportedly have doxastic functions, not all of them actually perform these functions. Delusions, then, belong to the domain of the ones that don’t: they are defective beliefs of a distinct kind.      

Miyazono is certainly not alone in thinking that delusions are beliefs. Since the American Association of Psychiatry’s DSM began to include a glossary of technical terms, delusion has been defined in doxastic terms. Recently, experimental philosophers have claimed that even folk psychology views delusions as stereotypical beliefs. So what is the problem? Mainly that delusions often exhibit characteristics that are decidedly not belief-like. For instance, Eugen Bleuler stated that his delusional patients frequently failed to act according to their delusions: ‘None of our generals has ever attempted to act in accordance with his imaginary rank and station’ (Dementia Praecox or Group of Schizophrenias). 

Notice the word ‘imaginary’ there? The intuition that delusions may be imaginary states has prompted many to argue against doxasticism (some even suggesting that delusions are a hybrid of belief and imagination, a sui generis cognitive attitude). Miyazono’s teleo-functionalism allows him to offer a new defense of doxasticism. The failure of some delusions to play belief-like causal roles—which has motivated most attacks on doxasticism — is not enough to demote them from belief status if we think of them as states that are failing to perform certain functions. Miyazono also forcefully rebuts various attempts to paint delusion as intermediate or hybrid states in the literature.

Once again, Miyazono holds the majority view in thinking that delusions are pathological. However, he notices that extant explanations of delusion’s pathological character tend to be detached from considerations about what makes a condition pathological in general. Most have viewed delusions as pathological either because of their strange content, their resistance to folk psychological explanations or the impaired responsibility-grounding capacities that accompany them. Miyazono offers objections to all these proposals and applies Jerome Wakefield’s famous harmful dysfunction analysis of disorder, which is a very good fit for his prior teleo-functionalism: delusional beliefs have a significant negative impact on well-being and certain psychological mechanisms, directly or indirectly related to them, fail to perform the functions for which they were selected. So, while Miyazono agrees with Lisa Bortolotti that delusions are continuous with non-delusional beliefs with respect to rationality, he maintains that they are discontinuous with respect to their pathological character.

Finally, Miyazono defends an original version of two-factor empiricism, also the majority view concerning the formation and maintenance of delusions. Empiricists maintain that delusions are formed in response to anomalous perceptual or sensory experiences so that they’re mainly bottom-up, rather than top-down disturbances. Single-factor accounts make delusional patients out to be broadly instrumentally rational, attempting to explain delusions as normal responses to abnormal experiences. Two-factor accounts are based on the conviction that we need to postulate a second factor to explain why delusions are not only acquired, but maintained in the face of extraordinary implausibility, strong counterevidence, and the testimony of one’s peers. Many discussions have centered around the second factor, whether it’s a cognitive bias or a cognitive deficit, and if so, of what kind. More recently, the prediction error theory of delusion has been painted as an alternative. Miyazono offers a hybrid account, defending that the insights of prediction error theory can be combined with those which postulate cognitive biases as a second factor.

Even though Miyazono’s views on the belief status, the pathological character, and the formation of delusions can be seen as conservative, his careful defense of this conjunction of views is certainly not simply a restatement of old arguments but a fresh and up to date perspective on much-discussed problems. His book is impeccable in its clarity, consistency, and structure. For a student who is just dipping her toes in the philosophical literature on delusion, this would provide an excellent, opinionated introduction to its main questions. For specialists who already follow the discussions in this field, it is mandatory reading. Moreover, I’m sure the book will be of great interest not only to students and researchers in the philosophy of psychiatry and cognitive psychiatry but also to clinicians of a theoretical bent.

@ 2019 José Eduardo Porcher

José Eduardo Porcher is a Research Fellow at the Rutgers Center for the Philosophy of Religion working primarily in the philosophy of psychiatry and philosophical psychology. His latest paper, “Double Bookkeeping and Doxasticism about Delusion”, has been published in the most recent issue of Philosophy, Psychiatry & Psychology.