Synopsis

The rest of the thesis proceeds as follows. Chapter 2 considers the variety of epistemic, instrumental, and semiotic roles that diagnoses normally serve for clinicians, patients, and society. The overall aim of the chapter is to show that the explanatory role of a diagnosis is of particular importance, because it provides justificatory support for many of its other roles. I back this up with some evidence from sociological research on medically

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unexplained syndromes, which suggests that diagnoses that fail to explain also often fail to reliably inform predictions, effectively guide therapeutic interventions, elicit support from social services, and provide hope for patients. I then explore some of the arguments made by prominent critics of psychiatry, which suggest that these concerns may also apply to psychiatric diagnoses.

Chapter 3 examines in more detail how diagnoses in medicine normally explain patients’ symptoms. The general aim is to explicate the nature of the explanatory relation in a paradigmatic example of diagnostic explanation in medicine, which can serve as a point of comparison for my later discussion of diagnosis in psychiatry. This proceeds through consideration of models of explanation in the philosophy of science and their adequacy when applied to the medical context of diagnosis. I begin by considering Carl Hempel’s (1965a) covering law account of scientific explanation and showing why it does not adequately capture the way in which a diagnosis explains a patient’s symptoms. Rather, the nature of diagnostic explanation in medicine is best captured by a causal model of explanation. I endorse the proposal by Margherita Benzi (2011) that many medical diagnoses, though by no means all, are causal explanations based on particulars. That is to say, they explain by indicating the actual causes of the symptoms in individual cases, rather than by subsuming them under general causal regularities. However, in addition to making a simple causal claim of the form “C causes E”, I argue that the diagnostic explanation also relies on some mechanistic knowledge of how C produces E to make the causal connection intelligible. Drawing on the work of Kenneth Schaffner (1986) and Jeremy Simon (2008), I suggest that this knowledge of mechanisms is supplied by the theoretical framework in which the clinician operates.

Chapter 4 addresses the conceptual problem described in §1.1.2. The descriptive definitions of psychiatric diagnoses in the DSM suggest that they refer to clusters of symptoms. Given that causes are distinct from their effects, this might seem to suggest

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that diagnoses in psychiatry cannot serve as causal explanations of patients’ symptoms in the ways that many medical diagnoses do as described in Chapter 3. In this chapter, I argue that this is not necessarily so. The argument proceeds through examination of the semantics of diagnostic terms with appeal to theories of reference in the philosophy of language. I begin by considering Jennifer Radden’s (2002) distinction between descriptive and causal conceptions of diagnostic terms, and the view suggested by Carl Hempel (1965b) and Paul Thagard (1999) that the historical development of a diagnostic term involves a progressive change from the former to the latter. A problem with this is that it implies radical incommensurability between older and newer conceptions of a diagnostic term (Feyerabend, 1962; Kuhn, 1962; Fleck, [1935] 1981). This is untenable, because it contradicts the intuition that scientific discoveries do not merely involve changes in the meanings of disease terms, but actually do increase our understanding of the respective diseases. I then look at how the causal theory of reference developed by Saul Kripke ([1972] 1980) and Hilary Putnam (1975a) can offer a more reasonable account of the meanings of diagnostic terms that avoids the implication of radical incommensurability. In spite of its strengths, a problem with a pure causal theory of reference is that it relegates the symptoms of psychiatric disorders to mere contingent features of the diagnoses, which contradicts the fact that such symptoms are often necessary conditions for applying the diagnoses according to DSM-5. To resolve the problem, I draw on the conceptual framework of two-dimensional semantics, as developed by Robert Stalnaker (1978), David Chalmers (1996), and Frank Jackson (1998). Such a framework permits a semantic pluralism that accommodates the actuality of diagnostic terms being defined through their symptoms, yet being used to refer to the putative causes of these symptoms.

Chapter 5 moves on to the ontological problem described in §1.1.3. Although the solution to the conceptual problem presented in Chapter 4 shows that symptom-based

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descriptive definitions do not necessarily preclude psychiatric diagnoses from alluding to the causes of these symptoms, whether categorical diagnoses in psychiatry actually provide satisfactory causal explanations of individual patients’ symptoms is also dependent on whether we have enough scientific understanding of these causes and, more fundamentally, on whether the diagnostic categories are respectively associated with distinctive causal profiles that are sufficiently invariant across cases. In this chapter, I review the current findings from empirical research into psychiatric aetiology for some disorders, paying special attention to the example of major depressive disorder. I use this example to illustrate the problems of causal heterogeneity and complexity that are associated with most psychiatric diagnoses. These problems suggest that psychiatric disorders cannot be conceptualised in simple essentialistic terms. In other words, the diagnostic categories in psychiatry do not correspond to distinct and invariant causative pathologies, but are associated with variable ranges of possible causal pathways, each involving complex interactions between diverse biological, psychological, and social factors. I review some recent attempts to conceptualise psychiatric disorders as homeostatic property clusters (Borsboom, 2008; Beebee and Sabbarton-Leary, 2010; Kendler et al., 2011; Tsou, 2013), an idea introduced by the philosopher of biology Richard Boyd (1999) to analyse kinds that are constituted by clusters of unnecessary and insufficient properties that are connected via contingent causal relations. I then present some problems for homeostatic property cluster accounts of psychiatric disorders. Finally, I consider whether the above considerations also apply to common psychiatric diagnoses other than major depressive disorder, including schizophrenia, bipolar disorder, generalised anxiety disorder, the dementias, panic disorder, obsessive- compulsive disorder, and some of the personality disorders.

Chapter 6 examines the implications of the problems discussed in Chapter 5 for the explanatory functions of psychiatric diagnoses. To address the problems of causal

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heterogeneity and complexity in psychiatry, theorists have suggested the respective strategies of idealisation (Murphy, 2006) and theoretical pluralism (Kendler, 2008; Mitchell, 2008) in disease explanation. With respect to diagnostic explanation, though, such heterogeneity makes them fall short of the paradigmatic case in medicine, described in Chapter 3, where a diagnosis picks out a specific cause of the patient’s symptoms. Nonetheless, I argue that some psychiatric diagnoses, though by no means all, can still supply different sorts of clinically relevant causal information. In particular, I suggest that some psychiatric diagnoses provide negative information to exclude certain medical disorders as causes of the patients’ symptoms, some provide probabilistic or disjunctive information about the range of possible causal processes that could be contributing to the patients’ symptoms, and some provide causal information about the relations between the symptoms themselves. I also discuss the limitations of these sorts of causal explanatory information and suggest some psychiatric diagnoses to which they do not apply.

Chapter 7 explores the normative and methodological implications for clinical psychiatric practice of the above issues concerning diagnostic explanation. As noted in Chapter 6, categorical diagnoses in psychiatry fall short of the paradigmatic explanatory diagnosis in medicine, although some may provide more modest sorts of causal

explanatory information. I consider three strategies for modifying and improving the discourse and practices regarding diagnoses in psychiatry. The first strategy is to amend the ways in which diagnoses are communicated in psychiatric discourse. The problems of causal heterogeneity and complexity suggest that psychiatric diagnoses are often

misleadingly essentialised, which Nick Haslam (2014) argues can encourage harmful stigma. I propose that this warrants modification of our language in psychiatry, so that psychiatric diagnoses and whatever explanatory information they might supply are conveyed more accurately to people. The second strategy involves revising diagnostic

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classification so that the categories correspond to more distinctive and stable causal structures (Poland et al., 1994; Bentall, 2003; Murphy, 2006; Tsou, 2015). While this is an epistemically respectable project, I argue that there are serious challenges that make it unlikely for a successful aetiologically-based classification to be implemented in the near future. The third strategy, which I endorse, is to supplement the categorical diagnosis with an individualised formulation (World Psychiatric Association, 2003). I show how a categorical diagnosis and an individualised formulation can complement each other to arrive at a more satisfactory causal explanation of the patient’s symptoms in the particular case. The upshot is that despite being causally heterogeneous, a psychiatric diagnosis can still serve an important role in the development of a causal explanation. However, again, the quality of the explanation remains limited by our incomplete scientific understanding of the mechanisms through which different causal factors interact, as well as by our ability to match certain causal factors to particular patients.

Chapter 8 is the conclusion of the thesis. Here, I recapitulate my main points and summarise my answer to the main research question. I also tentatively reflect on some of the further questions raised by my investigation that would be interesting to address in future research.

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