Free energy principle

Karl Friston, 2005 onwards

Karl Friston's theory that any self-maintaining system, from a cell to a brain, must act so as to minimise the surprise of its sensations, by updating its model of the world (perception) and changing the world (action, or active inference).

The free energy principle (FEP) is a theoretical framework developed by the neuroscientist Karl Friston from the mid-2000s. It aims to explain perception, action, learning and the very existence of living systems from a single principle.

The idea

To go on existing, an organism must stay within a limited range of states: a fish must stay in water, a body at the right temperature. In statistical terms, it must avoid surprising states. It cannot measure surprise directly, but it can minimise a quantity that bounds it, variational free energy, which roughly measures the mismatch between what its internal model predicts and what it senses.

There are two ways to reduce that mismatch:

  • Perception: change the model to fit the senses (closely related to predictive coding and the “Bayesian brain”).
  • Action: change the world, or one’s position in it, so that the senses fit the model. This is active inference.

The boundary between a system and its environment is formalised as a Markov blanket, the set of sensory and active states through which inside and outside influence each other.

Cybernetic roots

The FEP is, in effect, a modern mathematical version of core cybernetic ideas: Walter Cannon’s homeostasis, W. Ross Ashby’s essential variables, and Conant and Ashby’s Good Regulator Theorem (a system that regulates its world must model it). It also shares its central move with William T. Powers’s perceptual control theory: organisms act to control what they sense.

Debates

Supporters see the FEP as a unifying theory of life and mind. Critics question whether it is testable, or whether it is better understood as a mathematical framework for building models. It is applied in computational psychiatry, robotics and AI.

Further reading

  • Karl Friston, “The Free-Energy Principle: A Unified Brain Theory?”, Nature Reviews Neuroscience (2010)
  • Thomas Parr, Giovanni Pezzulo and Karl Friston, Active Inference (2022)

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