From Wired’s profile of British neuroscientist Karl Friston:

For the past decade or so, Friston has devoted much of his time and effort to developing an idea he calls the free energy principle… With this idea, Friston believes he has identified nothing less than the organizing principle of all life, and all intelligence as well. “If you are alive,” he sets out to answer, “what sorts of behaviors must you show?”

The article elaborates:

The second law of thermodynamics tells us that the universe tends toward entropy, toward dissolution; but living things fiercely resist it. We wake up every morning nearly the same person we were the day before, with clear separations between our cells and organs, and between us and the world without. How? Friston’s free energy principle says that all life, at every scale of organization—from single cells to the human brain, with its billions of neurons—is driven by the same universal imperative, which can be reduced to a mathematical function. To be alive, he says, is to act in ways that reduce the gulf between your expectations and your sensory inputs. Or, in Fristonian terms, it is to minimize free energy.

[Emphasis in the original.]

To the degree that I understand the idea (and the Wired piece acknowledges it’s “maddeningly difficult”), the free energy principle sounds fascinating, deep, and potentially useful. It could help explain the behavior (and therefore the design) of self-organizing systems.

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