Karl Friston’s free energy principle might be the most all-encompassing idea since Charles Darwin’s theory of natural selection. But to understand it, you need to peer inside the mind of Friston himself. wired 
The free energy principle explains how biological systems maintain their order by restricting themselves to a limited number of states which entail beliefs about hidden states in their environment. Originally introduced by Karl Friston as an explanation for embodied perception in neuroscience, where it is also known as active inference. wikipedia
The objective is to maximise model evidence or minimise surprise. This generally involves an intractable marginalisation over hidden states, so surprise is replaced with an upper variational free energy bound. However, this means that internal states must also minimise free energy, because free energy is a function of sensory and internal states.
The free-energy principle: a rough
guide to the brain? pdf
Generalized free energy and active inference. pdf
Active Inference: A Process Theory. pdf
These inference systems are Bayesian networks of Markov Blankets.
We ponder this principle in the context of Understanding Complexity.
Can an ambitious theory unify biology, neuroscience and psychology? conversation
in the Visual Cortex