The Idealized Mind (2025) suggests that the free energy principle (FEP) in theoretical neuroscience unifies all the different arguments covered in the book. The FEP is a GUT in more than one way.
Friston’s commentary opens with the physicists’ spherical cow. In The Idealized Mind, chapter 9 seeks to establish that the possibility of explaining the brain and cognition by appeal to generative models and probabilistic inference requires idealizing the mind and brain. The key question Friston asks is: are there any spherical cows lurking in the FEP interpretation of the brain and self-organization? From Box 1 in Friston’s commentary, we can see that the FEP implies that the internal paths of least action minimise variational free energy (see Mann et al (2022) for a philosophically friendly user-guide to the FEP). What is crucial is what follows from this; namely, “that it looks as if internal dynamics are inferring external causes of sensory data, under certain conditions.” Is this interpretation of neural dynamics appropriate? In answering this question, Friston appeals to the renormalization group (RG). RG allows one to recover the same description of a system over multiple scales – just as the FEP allows (Kirchhoff et al. 2018). By aligning RG to the FEP, Friston argues that “at a suitable scale neuronal dynamics can be described exactly as an inference … process under a generative model.” This is correct. However, the FEP is still left with its own spherical cows. Friston’s example is: “For an astronomer, the movement of heavenly bodies (e.g., the moon) can be described exactly – with no approximation – by treating it as a spherical body.” Precisely, exact description can be achieved only by placing the heavenly bodies in a theoretical vacuum (see also Kirchhoff et al. 2025).
This supports the conclusion I draw about the FEP in the book. The FEP, it turns out, is akin to Galileo’s law of equal heights. Galileo asked us to imagine a U-shaped cavity, imagine that we put a ball on the edge of one side, and imagine that we let the ball roll down into the cavity. What is the trajectory of the ball? Galileo argued that the ball would have to reach the same height on the other side irrespective of the shape of the cavity. This is Galileo’s law of equal heights. As we know all too well, the ball’s track is not perfectly smooth and the ball faces air resistance, which in an actual experiment would not provide the answer suggested by Galileo. Yet, Galileo’s answer did not depend on actual conditions. It depended on idealized conditions. Galileo’s point was: the law of equal heights is valid in an idealized model. So, it is with the FEP. This is no discredit to the FEP. It’s in the fine company of Galileo Galilei.
References
Kirchhoff, Michael. 2025. The Idealized Mind: From Model-based Science to Cognitive Science. Cambridge, MA: The MIT Press.
Kirchhoff, Michael., Julian Kiverstein, and Ian Robertson. 2025. “The literalist fallacy and the free energy principle: Model-building, scientific realism, and instrumentalism.” The British Journal for the Philosophy of Science, Doi: 10.1086/720861
Mann, Stephen., Ross Pain., and Michael D. Kirchhoff. 2022. “Free energy: a user’s guide.” Biology and Philosophy, 37: 1-35.
Kirchhoff, Michael., Thomas Parr, Ensor Palacios, Karl Friston, and Julian Kiverstein. 2018. “The Markov blankets of life: autonomy, active inference and the free energy principle.” Journal of the Royal Society Interface 15: 20170792.