Markov Blankets: The Boundaries That Make Things Things

Series: The Free Energy Principle | Part: 3 of 11

Where do you end and the world begins?

Point to your skin and you've already made assumptions. Skin cells slough off constantly. Bacteria in your gut outnumber your own cells. The oxygen in your blood came from outside. The thoughts in your head are shaped by language you didn't invent. So where, precisely, is the boundary that makes you a distinct thing?

Friston's answer involves borrowing a concept from statistics that most people have never heard of: the Markov blanket. It sounds technical—it is technical—but the idea is profound. A Markov blanket is what makes it possible to talk about "systems" at all. Without it, the universe is just one undifferentiated soup of interacting particles.

With it, you can define cells, organisms, brains, societies, ecosystems. Not as arbitrary divisions, but as natural boundaries that emerge from dynamics.

And once you see how Markov blankets work, you'll never look at selfhood the same way again.

What Is a Markov Blanket?

In statistics, a Markov blanket is a set of variables that "screens off" a target variable from everything else. If you know the Markov blanket, you know everything you need to predict the target—nothing outside the blanket adds information.

Imagine trying to predict tomorrow's weather. You could, in principle, measure every air molecule on Earth. Or you could just measure temperature, pressure, humidity, and wind patterns in your region. Those variables constitute a Markov blanket around your local weather. Everything else—what's happening in Australia, what happened last century—doesn't matter once you know the blanket.

For an organism, the Markov blanket is the set of states that mediate interaction between internal states (your physiology, your brain, your beliefs) and external states (the environment). The blanket has two sides:

Sensory states: How the external world influences the internal (input)
Active states: How the internal world influences the external (output)

Together, sensory and active states form the boundary. Everything inside affects the environment only through active states. Everything outside affects the organism only through sensory states.

This is what makes you a thing. You're not defined by your molecules—those change constantly. You're defined by the statistical boundary that separates internal dynamics from external dynamics while allowing controlled exchange across it.

Blankets All the Way Down (and Up)

Here's where it gets interesting. Markov blankets aren't just for organisms. They exist at every scale where something maintains itself as a coherent system.

A single cell has a Markov blanket: the membrane and associated proteins constitute sensory states (receptors detecting external chemistry) and active states (pumps modifying external chemistry). Inside: metabolic networks, genetic regulation. Outside: extracellular environment. The membrane separates them while mediating exchange.

Your body has a Markov blanket: sensory receptors (eyes, ears, skin) and motor systems (muscles, glands). Inside: neural dynamics, physiological regulation. Outside: physical and social environment. Your nervous system infers what's out there from sensory input and acts to change it through motor output.

A brain region has a Markov blanket: afferent and efferent connections with other regions. Inside: local neural dynamics. Outside: other brain regions. Each region infers what other regions are doing and influences them through outgoing connections.

A society might have a Markov blanket: communication channels and institutional actions. Inside: cultural beliefs, power structures. Outside: other societies, ecological conditions. Societies maintain themselves through inference (intelligence gathering) and action (policy, trade, war).

Markov blankets compose. Your cells have blankets. You (as an organism) are made of cells but also have your own blanket at a higher scale. Societies are made of organisms but have emergent blankets at civilization scale.

This is how hierarchy works: nested Markov blankets at different scales, each minimizing free energy relative to its own boundary.

How Blankets Enable Free Energy Minimization

Remember the core problem from Part 2: you need to stay in viable states, but you can only sense the world indirectly. The Markov blanket is why sensing is indirect.

You (internal states) don't directly interact with the environment (external states). You interact with the environment through the blanket. Sensory states tell you about the environment. Active states let you influence it. But the blanket itself creates a separation.

This separation is what makes modeling possible. Because internal and external states are conditionally independent given the blanket, you can build a generative model: a representation of what's "out there" causing your sensory inputs.

Without the blanket, there's no inside or outside. No self or world. No modeling. Just coupled dynamics.

With the blanket, you get:

• A boundary (what counts as "you")
• A modeling problem (what's causing my sensations?)
• A solution strategy (minimize free energy through inference and action)

The Markov blanket is what makes you an agent. It's the topological feature that allows goal-directed behavior to emerge from physics.

Blankets Are Functional, Not Physical

Here's a crucial point: Markov blankets aren't necessarily physical membranes. They're statistical structures.

Your skin is a physical boundary, sure. But your Markov blanket as an organism includes your sensory and motor systems—the functional interfaces that mediate exchange with the environment. Skin is part of it, but so are retinas, cochleas, proprioceptors, muscles, vocal cords.

This means identity is functional, not essential. You're not a specific collection of atoms. You're the pattern of inference and action that maintains a statistical boundary over time.

When you die, the atoms don't vanish. But the Markov blanket dissolves—the functional separation between internal and external states breaks down. Sensory states stop reliably carrying information about external causes. Active states stop systematically influencing the environment. The boundary vanishes, and with it, the system.

This is why free energy minimization is finite. The blanket is actively maintained through metabolic work. Stop the work, and the blanket dissipates. Maximum entropy isn't some distant future—it's the default you're constantly pushing against.

Self and World Co-Emerge

One of the most radical implications: you don't exist before the world, observing it from outside. You and the world co-define each other through the blanket.

The internal states (your model of the world) exist because there are external states to model. The external states (the "world" as it matters to you) exist because your sensory and active states carve out a relevant subset of the universe.

Before you existed, there was no "world"—just quantum fields, star formation, molecular chemistry. The "world" as a meaningful category emerged with the first Markov blanket: the first system that carved a boundary and modeled what lay beyond it.

This doesn't mean external reality doesn't exist independent of minds. It means what counts as "world" (as opposed to irrelevant background) is defined by the systems that model it. Trees don't interact with the stock market. Bacteria don't care about cosmic microwave background radiation. The "world" is the subset of reality that affects the system through its blanket.

Meaning, on this view, is relational. It emerges at boundaries where systems interact. And those boundaries are Markov blankets.

Where Blankets Blur: The Extended Mind

If Markov blankets are functional, not physical, then the boundaries can extend beyond the body.

Consider a blind person using a cane. Where does their sensory blanket end? At their fingertips? Or at the tip of the cane?

The answer from FEP: wherever sensory states reliably carry information about external states. If the cane is integrated into predictive models such that vibrations through it generate expectations about terrain, then the cane is part of the blanket. The boundary extends.

This connects to Andy Clark's extended mind thesis. Your phone, your notebook, your partner who remembers things for you—all of these can become part of the functional blanket if they're integrated into your inference and action cycles.

The blanket isn't fixed. It's dynamically defined by coupling. When you learn to use a tool, you're not just adding information—you're extending your Markov blanket to include the tool as part of your sensory-active interface with the world.

This is how expertise works. A musician's blanket includes the instrument. A surgeon's includes the scalpel. An athlete's includes the ball or the bike or the water. Skill is expanding and refining the blanket.

Multiple Blankets, Multiple Selves

Even within a single organism, there can be multiple overlapping Markov blankets.

Your visual system has a blanket: retinal inputs and eye movements. Your gut has a blanket: vagal afferents and autonomic efferents. Your immune system has a blanket: receptor-mediated detection and cytokine signaling.

These subsystems minimize free energy at their own scale, with their own models. They're not globally coordinated by some master homunculus. They're coupled through shared internal states (bloodstream, hormones, neural signals), but each maintains its own boundary.

This is why "you" are not unified. You're a coalition of subsystems with partially aligned objectives, communicating through shared internal states. Consciousness might be the high-level blanket that integrates across lower-level blankets, but it doesn't replace them.

When subsystems fall out of alignment—when the gut model predicts threat while the cortical model predicts safety—you get physiological conflict. Anxiety is often this kind of blanket misalignment: different systems minimizing free energy relative to incompatible priors.

Blankets as Existence Proof

Here's the deepest implication: if you have a Markov blanket, you exist.

Not as metaphysical substance, but as dynamical system. You're the boundary, maintained by inference and action. You're the pattern that persists by minimizing free energy across the blanket.

This means existence is not binary. It's a matter of degree. The more clearly defined the blanket—the more reliably sensory states carry information, the more systematically active states influence the environment—the more "real" the system.

A cell: strong blanket, clear existence.
An organism: strong blanket at a higher scale.
A species: weaker blanket (gene flow and selection pressures define boundaries, but they're diffuse).
A nation: even weaker (porous borders, contested identities, but still some functional separation).

Consciousness, on this view, might require not just a blanket but a certain kind of blanket: one that models itself as having a blanket. A meta-blanket. A boundary that knows it's a boundary.

But that's speculation. What's not speculation: wherever you find persistent structure far from equilibrium, you'll find something functionally equivalent to a Markov blanket. It's the topological prerequisite for free energy minimization.

The Boundary Is the Thing

When you ask "what am I?", the FEP answer is: you are the boundary.

Not the stuff inside. Not the stuff outside. The interface itself—the sensory and active states that separate while connecting internal and external dynamics.

You are not contained by your skin. You are constituted by the functional separation between what you model and what you act on. That separation is maintained, moment by moment, through prediction and action.

When the blanket dissolves, "you" vanish—not into nothing, but back into undifferentiated coupling with the environment. The atoms remain. The patterns disperse.

So take care of your blanket. It's all you are.

Further Reading

• Friston, K. J. (2013). "Life as we know it." Journal of the Royal Society Interface, 10(86), 20130475.
• Kirchhoff, M., Parr, T., Palacios, E., Friston, K., & Kiverstein, J. (2018). "The Markov blankets of life: autonomy, active inference and the free energy principle." Journal of the Royal Society Interface, 15(138).
• Hipólito, I., Ramstead, M. J., Convertino, L., Bhat, A., Friston, K., & Parr, T. (2021). "Markov blankets in the brain." Neuroscience & Biobehavioral Reviews, 125, 88-97.
• Palacios, E. R., Razi, A., Parr, T., Kirchhoff, M., & Friston, K. (2020). "On Markov blankets and hierarchical self-organisation." Journal of Theoretical Biology, 486, 110089.

This is Part 3 of the Free Energy Principle series, exploring how statistical boundaries define systems and enable selfhood.

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