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6 Natural Kinds in Folk Psychology and in Psychiatry Dominic Murphy In this chapter, using delusions as my chief example, I will argue that our attribution of mental illness relies on a conception of psychiatric kinds as fundamentally psychological categories with distinctive if unknown causal signatures—characteristic ways of producing similar effects across different people. This aligns psychiatric thought, as well as the folk psychology of psychiatry, with well-established practices of scientific discovery and explanation that assume that mental illnesses are natural kinds. I shall follow other recent writers in assuming that the relevant conception of natural kind for psychiatry is Richard Boyd’s. However, the ways in which folk psychology groups patients together may not match the ways in which nature does it. Our folk thought may be a poor guide to natural kinds in psychiatry. Natural Kinds Let me begin by sketching an account of the representation and explanation of psychiatric conditions. Psychiatric diagnoses should be seen as referring to idealizations: exemplars (Murphy 2006) or ideal types (Ghaemi 2003). These are representations of disorders that abstract from the details of their realization in patients. We may think of exemplars as representing the ideal, textbook patient with a particular condition, even though such an ideal patient may never in fact enter the clinic. We group symptoms together according to their tendency to occur together in nature and unfold in particular ways, and we assume that causal connections exist both synchronically between symptoms in the group and diachronically between the cluster of symptoms itself and some earlier processes, genetic and developmental, that cause the symptoms to appear. The causal connections between components of the exemplar can sometimes be formulated with great precision, investigated experimentally and tested with 106 Murphy great rigor. But often they cannot, since we are in the dark about the nature of the causal connections. When they are applied to individual cases, however, much of the precision is lost, since the disorder afflicts different people in different ways. Perhaps not all symptoms are present, or not in their typical form. Patients may also suffer from other conditions at the same time, and those other conditions may interfere with the predicted expression of the diagnosis. It does not follow that the diagnosis is incorrect . The expression of the condition might vary greatly, but that might depend on regularities in the failure of normal function within a human being in a given social or natural environment. A diagnosis then, is useful insofar as it captures a genuine phenomenon and at the same time directs our attention to cases that can be further specified. Armed with an exemplar, we go on to construct a model to inform our understanding of an individual’s condition: in that sense, explanation in psychiatry, as in medicine more generally, is indirect. We first try to understand the explanatory relations that hold among parts of an idealization, for no one is exactly like the exemplar. The bet is that real patients will be similar to the exemplar in enough respects so that the explanation of the exemplar carries over to the patient. We assume that within the individual there are phenomena and causal relations that are relevantly similar to those worked out for the exemplar, but we cannot expect very precise predictions. The exemplar is thus a partial representation of the class of people who receive the diagnosis. It is silent about most of their properties, including most of those that are in the provinces of medicine, psychology, and neuroscience . What it represents is a disorder that is shared by a population of subjects; their possession of the disorder explains a number of things about them. One way to express this idea is to argue that a mental disorder is a natural kind. Because we assume that membership in a natural kind depends on a common basis that holds the kind together, we have a ready solution to the problem of why patients who meet the diagnosis share a set of symptoms, viz. that they are all undergoing the same processes and the symptoms are effects of those processes. Betting on a shared causal process in this way also gives us a basis for inquiry; we can attempt to tie the manifestations of the disorder to underlying mechanisms. What we are looking for in these cases is what I will call a causal signature—telltale manifestations of the disorder that suggest that in every case we are dealing with the same causal processes, or...

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