Some puzzles relating to the free-energy principle: comment on Friston

In Friston's recent article [1], the structure of an agent's world is taken to be represented by a ‘conditional density’, a probabilistic mapping from ‘causes’ to sensory stimulation. Friston argues that the brain can arrive at an approximation of this mapping by minimizing ‘free energy’, which is a function of sensory stimulation and brain states. A generative model of causal structure in the environment is then obtained, on which basis the agent is able to infer the ‘causes of sensory samples’ [1]. What is unclear is how this mechanism would function when sensory samples are ambiguous. In general, there are multiple interpretations for the causes of any sensory data, and these cannot be resolved on the basis of inspecting the data alone [2]. For any sense data, there will also generally be causes at multiple levels of description, with causes at one level of description being embedded in causes at higher levels. Sensory stimulation is the result not of distinct causes, but of causal structure. How would a mechanism that acts to infer causes measure up to the task of inferring causal structure? Friston asserts that almost ‘any adaptive change in the brain’ can be viewed as resulting from minimization of free energy [1]. On the face of it, no particular stand is taken on the emergence of the structures that mediate minimization. However, by looking at the definition of free energy [3], one finds a significant part being played by the variable ϑ. It is values of ϑ that encapsulate the representation of ‘environmental causes’ by the brain [3]. The range of ϑ then dictates the gross structural form of any representation acquired. With the framework providing no principle for deciding this range, the representation by the brain of the conditional density is inevitably a ‘slightly mysterious construct’ [4]. The expectation might be that ϑ will be fixed through instantiation of fortuitous ‘matches’ between internal and external structures. ‘Those systems that can match their internal structure to the causal structure of the environment will attain a tighter bound.’ [3]. However, there is a problem of circularity here: agents are posited to be able to form an internal structure matching the environment just in case they already have it. Neither it is clear whether this is intended to be the ‘mechanism’ for fixing ϑ. If there is no principle deciding this crucial designator of representational capacity, then one can only assume that it is fixed at random. It seems right for Friston to emphasize that the entropic basis of surprise reveals a deep connection between processes of knowledge, behavior and life. However, this idea has been in common currency for some time (e.g. Refs [5] and [6]) and it is unclear how introduction of the ‘free energy’ concept, specifically, adds any explanatory content. Free energy is taken to be a ‘good proxy’ for surprise: surely it is minimization of ‘surprise’ that is explanatorily salient. The inability of the present formulation to address the issue of structure emergence also poses difficulties with regard to the specification of ϑ ranges, resolution of sensory ambiguity and inference of causal structure