By @Ariel Cheng

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Meta

Much of this article is basically me re-explaining the content in Friston, 2019 and Millidge et al., 2021, but in a slower, restructured, more accessible way with added context.

• Notes (important! please read! click on the triangle to expand)
• Table of contents
• Acknowledgments </aside>

If you are reading this sentence, then you exist.

What information can we deduce from this fact? It turns out, quite a lot. We can deduce that the universe is governed by laws such that your existence was possible, which constrains the value of some fundamental physical constants. This is the anthropic principle.

The key moves here are to (1) make a really obvious observation – to notice that “hey, I exist!” – (2) to formalize that observation somehow, and (3) squeeze the formalization really hard to see what else you can deduce about the world based on it.

But why stop here? Why stop at observing that you exist? Bacteria also exist. So do fungi, and other humans, and cities, and plants… Maybe we can generalize what we did with the anthropic principle to all of these “things” somehow, i.e. we can (1) observe that these “things” exist, (2) formalize this observation, (3) deduce some cool stuff about how the world must be, given this observation.

This is the core idea of Karl Friston’s Free Energy Principle (FEP). The specific question we are asking here is:

If things exist, what must they do?

To answer this question, we need to formalize what exactly is meant by

• things exist,
• what must things do

First, what kinds of things are we even talking about? We are curious about the kind of thing that is self-organizing and maintains its “thingness” regardless of external perturbations, up to a point. We are not for the moment concerned with particles or snowflakes, even though they are things in some sense (although we can apply similar logic to them, since they are still things - that are a lot worse at maintaining their thingness.) We are concerned with bacteria, plants, humans, cultures…

By what must things do, we just means “what must the thing’s mechanics be”, where “mechanics” refers to how the states that comprise a thing “move” or change through space and time.

We can now rephrase the extremely vague question we had at the start into a less vague but still really vague question:

If self-organizing things that maintain their thingness exist, what must the mechanics of those things be?