Bridging Token Identity Theory and Supervenience Theory Through Psychological Construction

A psychologist’s task is to discover facts about the mind by measuring responses at the level of a person (e.g., reaction times, perceptions, eye or muscle movements, or bodily changes). A neuroscientist’s task is to make similar discoveries by measuring responses from neurons in a brain (e.g., electrical, magnetic, blood flow or chemical measures related neurons firing). Both psychologists and neuroscientists use ideas (in the form of concepts, categories, and constructs) to transform their measurements into something meaningful. The relation between any set of numbers (reflecting a property of the person, or the activation in a set of neurons, a circuit, or a network) and a psychological construct is a psychometric issue that is formalized as a “measurement model.” The relation is also a philosophical act. Scientists (both neuroscientists and psychologists) who make such inferences, but don’t explicitly declare their measurement models, are still doing philosophy, but they are doing it in stealth, enacting certain assumptions that are left unsaid. In “Mind the Gap,” Kievit and colleagues (this issue) take the admirable step of trying to unmask the measurement models that lurk within two well-defined traditions for linking the actions of neurons to the actions of people. They translate identity theory and supervenience theory into popular measurement models that exist in psychometric theory using the logic and language of structural equation modeling. By showing that both philosophical approaches can be represented as models that relate measurements to psychological constructs, Kievit et al. lay bare the fact that all measurement questions are also philosophical questions about how variation in numbers hint at or point to reality. They make the powerful point that translating philosophical assumptions into psychometric terms allows both identity theory and supervenience theory to be treated like hypotheses that can be empirically evaluated and compared in more or less a concrete way. The empirical example offered by Kievit et al. (linking intelligence to brain volume) is somewhat simplistic on both the neuroscience and psychological ends of the equation, and the nitty-gritty details of applying an explicit measurement approach to more complex data remains open, but this article represents a big step forward in negotiating the chasm between measures taken at the level of the brain and those taken at the level of the person. The overall approach is applauded, but a closer look at the details of how Kievit et al. operationalized identity and supervenience theory is in order. In science, as in philosophy, the devil is in the details. In the pages that follow, I highlight a few lurking demons that haunt the Kievit et al. approach. I don’t point out every idea that I take issue with in the article, just as I don’t congratulate every point of agreement. Instead, I focus in on a few key issues in formalizing identity and supervenience theory, with an eye to asking whether they are really all that different in measurement terms, as well as whether standard psychometric models can be used to operationalize each of them equally well. Like Kievit et al., I conclude that a supervenience theory might win the day, but I try to get more specific about a version of supervenience that would successfully bridges the gap between the brain and the mind.