What exactly do brains do? The usual answer is that they form mental representations of the world on the far side of the skull. Brains, that is, create internal virtual worlds—their best or most useful simulations of the real external world, one which exists independently of any of them but within which they all reside.

The problem, however, is that fundamental physics denies the existence of this observer-independent world. From quantum physics in the early 20th century to the black hole firewall debate that rages today, physicists have found that we tangle ourselves in paradox and violate laws of physics when we attempt to compile multiple viewpoints into a single spacetime. The state of a physical system, we’ve learned, can only be defined relative to a given observer. (Here “observer” does not mean consciousness, but a physical system capable of acting as a measuring device—yet one that itself must enjoy only a relational existence.) Slices of spacetime accessed by different observers cannot be considered broken shards of a single, shared world, but rather as self-contained and incommensurable versions of reality, each a universe unto itself.

In other words, there’s no third-person view of the world. There is one world per observer, and no more than one at a time.

What happens, then, to the concept of representation? What is it that brains are doing if there is no observer-independent world out there for them to represent?

One possibility is that instead of representing the world, brains enact one.

The term “enactivism” was introduced by Francisco Varela and colleagues in the 1990s, but it’s taken on fresh significance in light of an emerging set of ideas at the forefront of cognitive science today, including embodied cognition, the Bayesian brain, active inference and the free energy principle—ideas that emphasize top-down, generative perception wherein observers actively shape the worlds they perceive. The old passive view of perception is giving way to an active one, just as the Newtonian observer was replaced by the participator of modern physics. Suddenly the words of physicist John Archibald Wheeler apply to cognitive science: “We used to think that the world exists out there, independent of us, we the observer safely hidden behind a one-foot thick slab of plate glass, not getting involved, only observing. However, we’ve concluded that that isn’t the way the world works. We have to smash the glass, reach in.”

According to enactivism, observer and world co-evolve, hoisting one another up by their bootstraps through reciprocal interaction. Perception and action are inextricably and cyclically linked: our perceptions guide our actions and our actions determine what we perceive. Their symmetry ensures an even more essential symmetry, that between observer and world. The observer’s actions form the world’s perceptions, and the world’s actions form the observer’s perceptions. Labels such as “observer” and “observed,” “inside” and “outside,” thus become profoundly interchangeable, removing any need to resort to mystical, magical or hopelessly vague talk about consciousness as something over and above the physical world.

What the enactivist perspective leaves us with, ontologically speaking, is an observer and world that exist relative to one another and not in any absolute way. The enacted world is not one that can be described from a third person perspective. It is observer-dependent, rendered in first person, no more than one at a time. Which, of course, is exactly the ontology prescribed by fundamental physics.

Does it really matter if cognitive science aligns with fundamental physics? For the everyday purposes of research and practice in neuroscience, representation works well enough, just as our belief in a single, shared physical world mostly suffices, whether we’re driving to work or launching rockets to the moon. It’s only when we push to the outermost edges of either discipline—when dealing with tiny distances or intense gravity in physics, or when asking about the fundamental nature of consciousness in cognitive science—that the cracks in the third person perspective begin to show and a more fundamental theory is needed.

Fundamental physics and cognitive science have long been embroiled in a perpetual game of chicken or egg: the brain arises from the physical world and yet everything we know of the physical world we know only through the brain. Each serves as foundation for the other. So long as we restrict our inquiry to one at a time, something critical is left unexplained. If we want to understand reality as a whole, we need to understand both sides of the coin and how they are fused together. Physics finds that the world is observer-dependent but remains silent on the nature of the observer. Cognitive science finds that the observer is an active participant but never questions the nature of the physical world. Enactivism might just be the concept we need to begin to piece the two sides together.