The target article is correct that some motivation constructs in psychology are questionable, and that something needs doing. The dubious construct problem is not unique to motivation. The rate at which constructs are being introduced in psychology generally is rising and the number of times each is deployed empirically simultaneously falling (Elson, Hussey, Alsalti, & Arslan, Reference Elson, Hussey, Alsalti and Arslan2023). In the case of motivation, the diagnostic parts of the target article and its constructive proposal both neglect the crucial consideration that all tendencies to behaviour compete for the same finite set of degrees of freedom, primarily of the body, which have alternative uses.

While the target article correctly notes that motivation is a “determinant of behaviour” but fails to take what we know about behaviour control and production sufficiently seriously. Most actions or behaviours involve some deployment of the body which has a finite number of joints and muscles. Some deployments are mutually exclusive: Nobody can stand and lie down at once. Some aren't: Many people can walk and chew gum at once. The finite number of joints and muscles, along with facts about the orientation of the body and relevant surfaces and media, set a finite number of available degrees of freedom. These deployments stand, furthermore, in heterogeneously structured relations of mutual exclusivity.

Sherrington (Reference Sherrington1906) introduced the notion of a final common path, referring to the last neural stage at which competition between incompatible deployments of combinations of muscles can be resolved. There is no definitive and total final common path for the whole body because the selection of possible movements that is available at any time is sensitive to factors including orientation, gravitation, inertia, and the arrangement and properties of nearby surfaces. As Gallistel (Reference Gallistel1980) has explained this variability implies that control of skeletal muscles must be expressed through a “lattice hierarchy” in which the level at or before which competition over deployment of degrees of freedom must be resolved is not fixed, but depends on the properties of candidate actions and the situation of the body (Spurrett, Reference Spurrett2021b). Since the available bodily means are scarce and have alternate uses, the problem of selecting between deployments of them is irreducibly amenable to an economic analysis in terms of efficiency of goal attainment (Spurrett, Reference Spurrett2021a).

There can be various types of explanation for the movements of bodies. Some refer to motivation whether basic (she was hungry) or higher level (she needs “competence”) while others don't and might involve reflexes or habits. While the problem of allocating scarce means with alternative uses is essentially economic in character, the solutions need not be, and theorists have contemplated both processes that are sensitive to returns and opportunity costs, and ones that aren't. Any genuine cause of embodied activity must compete for control of the required degrees of freedom with the causes of other possible deployments. And genuine competition must happen at or before the applicable places in the lattice hierarchy. So a test of any motivation construct is what it has to say about how the hypothesised factor joins this competition. For example, if a genuine motivation for “competence” is competing with fatigue over whether to get up from the couch to train, the two are in conflict over what the legs do next. So however “basic” or “higher-order” a motivational source might be it must interact in some way with any other process that could control the body.

These considerations provide methodologically significant constraints on satisfactory theorising about motivation. Worthwhile hypotheses about higher-order behaviour should have specific commitments regarding this interaction. The target article, however, makes no mention of bodies or competition of control for it and so misses an enormously valuable tool for evaluating motivation constructs. This lapse of diagnosis also applies to the prescription since the same constraints play no role in articulating or defending the offered remedies. This isn't a merely theoretical criticism because some research programmes have been taking this seriously for decades. In neuroeconomics, focusing on circuits constituting or being upstream of “final common paths” for body control led to significant discoveries about neural processes of valuation and selection (e.g., Platt & Glimcher, Reference Platt and Glimcher1999). Subsequent work has shown that the very same bottlenecks process rewards of widely varying modalities including money, food, drink, relief from pain, and social reputation, where rewards can be certain or risky, immediate or delayed, larger or smaller (Levy & Glimcher, Reference Levy and Glimcher2012 and Bartra, McGuire, & Kable, Reference Bartra, McGuire and Kable2013 are useful meta-analyses). These findings aren't simply read off the brain but depend on behavioural estimates of subjective value. The behavioural data that are required to interpret the neural data include determining how subjects trade-off rewards in various modalities, that is, how much money would be given up for how much drink, or relief from pain, and so on. These considerations point to a more demanding notion of what “unpacking the black box” should amount to and show that work meeting those criteria has already engaged some “higher-order” rewards like social reputation. Opening the black box does mean “specifying mental computation” but that demands more than occasionally saying “emergent property” without providing criteria or empirical content. What it requires is identifying neural circuits in ways sensitive to the economic character of the embodied selection problem and constrained by empirical discoveries in neuroeconomics. It also requires studying activity in those circuits armed with choice data that meaningfully characterises subjective values.

The point here isn't that neuroeconomics has definitively shown that all competition for action involves estimations of utility or expected subjective value. The regulative hypothesis that it does guides much neuroeconomic research and has not yet been refuted, but it remains an empirical claim. The point is that hypotheses about motivation, however “higher order” their postulates, are hypotheses about what make us do things or refrain from doing them. We do them with our bodies, and close attention to how bodies are controlled and to how their movements are selected simply isn't optional.