Summary
The free energy principle is the proposal that brains, and perhaps all living systems, work by keeping surprise to a minimum. It is the neuroscientist Karl Friston’s, building on Hermann von Helmholtz’s idea of unconscious inference; his 2010 review presents a principle that “accounts for action, perception and learning”. On this view the brain is an ‘inference engine’, forever comparing what it predicts with what the senses report.
‘Surprisal’ is the measure of how unexpected an input is. A system can lower it two ways: by revising its predictions, or by acting until the world matches them. Friston says it “cannot be falsified”: it’s a principle, not a testable hypothesis.
Rationale
The theory behind ‘priors’, a term Part 1 names. It suggests why dispositions persist: we don’t only revise expectations to fit the world, we arrange the world to fit them. We describe that loop.