In assessing the explanatoriness of a functional theory like FEP it is useful, as Harkness proposes, to consider it as a mechanism sketch. Sketchiness, however, comes in degrees, and it is hard to think of any extant scientific account that is not sketchy in some respects—no matter how abundantly mechanistic it is. There doesn’t seem to be any principled point at which a sketchy functional account passes over into being a non-sketchy mechanistic account. Rather, an account may become less and less sketchy as the full functional role and its realisers are increasingly revealed. This would be one respect in which the explanation in question would expand: more types and ranges of evidence would be explained, accompanied by a richer understanding of the functional workings of the mechanism.
The idea here is that mechanistic explanation comes in degrees, which makes it hard to say clearly when something is a mechanism sketch. Speaking of organs, consider again the case of the heart. Harvey is often said to have provided the first full account of pulmonary circulation, and it might be true that his account is less sketchy than that of his precursors, such as the much earlier Ibn al-Nafis. Yet even Harvey had areas of ignorance about the heart, and had to deduce some parts of his theory from his hypothesis about the overall function of the heart. Indeed, he readily acknowledges the difficulty of his project:
When I first gave my mind to vivisections, as a means of discovering the motions and uses of the heart, and sought to discover these from actual inspection, and not from the writings of others, I found the task so truly arduous, so full of difficulties, that I was almost tempted to think, with Fracastorius, that the motion of the heart was only to be comprehended by God. (Harvey 1889, p. 20)
A key question then is how sketchy FEP is—is it more like Harvey’s rather comprehensive sketch of the heart, or is it like that of al-Nafis? (If it is not completely misguided, like Galen’s claim that there are invisible channels between the ventricles.) Harkness suggests that part of the attraction of FEP is that it comes with more empirical specification than mere Bayesian theory. It is true that much of the literature on FEP tries to map mathematical detail onto aspects of neurobiology. However, the mathematical detail of FEP itself is devoid of particular empirical fact—it is purely functionalist. (We might even say FEP is more fundamental than the Bayesian brain hypothesis, since the latter seems to be derivable from the former.)
However, this austerity with respect to specification of particular types of fact does not make FEP inherently sketchy. The starting point for FEP is the trivial but contingent fact that the world is a changing place and yet organisms exist—that is, that they can maintain themselves in a limited set of fluctuating states. This very quickly leads to the idea that organisms must be recapitulating (modelling) the structure of the world, and that they must be approximating Bayesian inference in their attempt to figure out what their expected states are.
This starting point for FEP gives us a lot of structure to look for in the brains of particular creatures. It calls for hierarchical structures the levels of which can encode sufficient statistics (means and variances) of probability distributions, pass these as messages throughout the system, and engage in explaining away and updating distributions over various time-scales. This has a much more mechanistic flavour than a more pure appeal to Bayes’ rule, which leaves many more questions about the inferential mechanistics of the brain unanswered. (Part of the difference here is that FEP suggests that the brain implements approximate Bayesian inference, described in terms of variational Bayes.)
It is reasonable, then, to say that, even when stripped of extraneous neurobiological scaffolding, FEP is not inherently sketchy. It might not have the wealth of particular fact that would make it analogous to Harvey’s theory of the heart. But it gives a surprisingly very rich description of the functional role implemented by the brain of living organisms.