  Biehl et al (2020) present some interesting observations on an early
formulation of the free energy principle in (Friston, 2013). We use these
observations to scaffold a discussion of the technical arguments that
underwrite the free energy principle. This discussion focuses on solenoidal
coupling between various (subsets of) states in sparsely coupled systems that
possess a Markov blanket - and the distinction between exact and approximate
Bayesian inference, implied by the ensuing Bayesian mechanics.
