James Turner

In this article, we show that historical naturalistic theories of biological function can meet central desiderata, while ahistorical alternatives cannot. To this end, we develop a comprehensive taxonomy of naturalistic theories of function and malfunctioning and argue that all possible ahistorical theories are unable to meet the distinction challenge (i.e. explaining why different traits have different functions) and the futile functioning challenge (i.e. allowing functions and malfunctions even when the fulfilment of the function serves no further purpose). In contrast, the selected effects theory and other historical theories can easily meet these two challenges. We, therefore, have good reason to leave ahistorical theories behind.

In this paper, we explore whether a living being can have a medical disorder purely in virtue of its current structure, or whether its historical origins are also relevant. We do so by presenting two dialogues based on the Swampman thought experiment. These dialogues bring out two key points. First, although evolutionary history plays a central role in theoretical accounts of disorder, its relevance is less obvious in everyday medical practice. Second, there may be an asymmetry between somatic and mental disorders: somatic disorders are typically identified by biological dysfunction, while many paradigmatic mental disorders appear to involve violations of rationality. Nevertheless, we suggest that dysfunctions—and thereby historical considerations—still play a role in distinguishing normal from pathological irrationality. Rather than introducing a new theory, this paper tests existing accounts of function and disorder by applying them to an extreme but instructive case.

In this thesis, I offer a novel, overarching account of the cognitive architecture, evolution, and disorders of the capacity for low mood. First, I offer novel arguments for the propitiousness theory, according to which the proper function of the low mood system (LMS) is to limit resource expenditure in unpropitious circumstances. Following this, I develop an original account of the intentional content of low mood, according to which low mood has the following indicative-imperative content: Good events are, on average, less likely to occur than bad events & Limit [the subject’s] resource expenditure! Next, I argue that the LMS takes input from numerous sensory, interoceptive, and affective systems, but that higher cognition can only effect low mood indirectly—namely, by triggering low-mood-inducing emotions. Finally, I argue that (a) disorders are necessarily constituted by dysfunctional systems (systems that fail to carry out their proper function), (b) most cases of depression (severe, long-lasting low moods) are activations of properly functioning LMSs, and therefore (c) most cases of depression are not in fact disorders.

Does low mood have intentional content? If so, what is it? Philosophers have tried to answer both questions by appealing to low mood’s phenomenal character. However, appeals to phenomenology have not settled this debate. Thus, I take a different approach: I tackle both questions by examining low mood’s complex functional role in cognition. I argue that if we take this role into account, we have excellent reason to believe that low mood a) has content, and b) has the following indicative-imperative content: Good events are, on average, less likely to occur than bad events & Limit [the subject’s] resource expenditure!

There are three main accounts of the proper function of the low mood system (LMS): the social risk theory, the disease theory, and the propitiousness theory. Adjudicating between these accounts has proven difficult, as there is little agreement in the literature about what a theory of the LMS’s proper function is supposed to explain. In this article, drawing upon influential work on the evolution of other affective systems, such as the disgust system and the fear system, I argue that a theory of the proper function of the low mood system should: (i) account for the reliable, distal causes and effects of the system’s activation, and (ii) explain how having a system that performed such a function increased fitness in ancestral environments. On this basis, I show that the proper function of the low mood system is to limit resource expenditure in relatively unpropitious circumstances, exactly as hypothesised by the propitiousness theory.