A Philosopher Asked What It’s Like to Be a Bat. A Brainless Slime Mold Gave a Weirder Answer.

In 1974, the philosopher Thomas Nagel posed a seemingly simple question in his famous essay, “What is it like to be a bat?” His point was not about imagining ourselves with wings and sonar, but about the profound mystery of consciousness itself. Nagel argued that for any conscious organism, there is something it is like to be that organism—a private, first-person reality, a “subjective character of experience” that objective science struggles to explain. It’s the difference between a scientific instrument detecting a 440 Hz sound wave and you hearing the note ‘A’. We can describe a bat’s brain and its echolocation, but we can never truly know what it feels like for the bat to be a bat.

What happens to this philosophical puzzle when we encounter an organism far more alien than a bat? A single-celled, brainless slime mold called Physarum polycephalum displays behaviors so shockingly intelligent that it forces us to question our most basic assumptions about cognition, memory, and decision-making. This strange, pulsating blob doesn’t solve Nagel’s mystery. Instead, it makes it deeper, weirder, and far more fascinating.

A Single Cell Can Make Decisions Like an Urban Planner

The brainless Physarum is a surprisingly sophisticated decision-maker. When faced with a choice between high-quality food in a dangerously bright environment and safer, lower-quality food in the dark, it demonstrates a clear risk-reward calculation: it will gamble on the dangerous, high-value option, but only if the nutritional reward is at least five times greater than the safe one.

Its computational abilities, however, go far beyond simple trade-offs. In a landmark experiment, researchers placed food sources on an agar plate in a pattern that mimicked the major stations of the Tokyo railway system. The slime mold, starting from one “station,” spread its network to connect them all. Over time, it pruned back inefficient connections, leaving behind a design of remarkable sophistication. The resulting network had comparable efficiency, fault tolerance, and cost to the actual Tokyo Railway infrastructure, a feat of engineering that took humans decades to achieve.

Most stunningly, Physarum can even exhibit “irrational” decision-making. When presented with two equally attractive options, its preference can be swayed by the introduction of a third, inferior “decoy” option—a cognitive bias once thought to require complex neural mechanisms. This creature falls for the same cognitive traps that marketing experts use on humans. But while we feel conflicted or tricked when making such a choice, is there any reason to believe the slime mold feels anything at all?

“This remarkable process of cellular computation implies that cellular materials can show a primitive intelligence.”

It “Outsources” Its Brain, Using Slime as an External Memory

Physarum navigates its world using a clever form of “externalized spatial memory.” As it moves, it leaves behind a trail of extracellular slime (ECS). It then uses this trail as a map of explored territory, actively avoiding its own slime to prevent wasting energy by re-exploring areas it has already depleted. This simple mechanism allows it to efficiently find the shortest path through complex mazes and solve challenges like escaping a U-shaped trap to reach a food source.

This biological function is a real-world example of the philosophical concept of Extended Cognition, where the mind isn’t confined to the skull but offloads cognitive processes onto the environment. For the slime mold, the slime trail literally becomes part of its cognitive system. The slime is part of its “mind,” a physical record of “what it knows.” But this raises a profound question: is there a difference for the slime mold between this external chemical data and an internal, subjective memory like our own?

“Slime mold uses an externalized spatial ‘memory’ to navigate in complex environments.”

It Can Learn a Lesson—and Then “Teach” It Through Fusion

The slime mold is capable of habituation, the simplest form of learning. When repeatedly exposed to a harmless but repellent substance, like quinine or caffeine, it gradually learns to ignore it and will cross a barrier coated in the substance to reach food. While this form of learning is proven, more complex forms like associative learning (the kind of Pavlovian conditioning seen in dogs) remain unproven and controversial in brainless organisms.

The most counter-intuitive discovery, however, is how it shares this knowledge. When a “trained” slime mold that has habituated to a substance is physically fused with a “naive” one that has not, the newly merged organism collectively retains the learned behavior. The naive parts quickly learn to tolerate the repellent.

This suggests the learning mechanism is a form of “circulating memory”—a chemical or cellular trace that can be physically transferred. Knowledge, for this organism, is a transferrable substance, a chemical passed from one to another. This reduces “learning” to a physical exchange, making the gap between its “knowledge” and our conscious understanding feel vast and mysterious.

This Blob Is Forcing Science to Rethink What “Cognition” Even Means

Traditionally, cognitive science has taken a “top-down” approach, starting with the complexities of human intelligence and working down the evolutionary tree. This often leads to a brain-centric view, where cognition is seen as a property exclusive to organisms with nervous systems.

Organisms like Physarum have inspired a radical “bottom-up” approach known as basal cognition. This framework starts with the simplest forms of life to identify fundamental principles. It reframes cognition not as a special property of brains, but as a more fundamental property of life itself—a set of adaptive information-processing tools for survival. Proponents of basal cognition remind us that neural networks evolved from far more ancient signalling pathways; neurons mainly optimised existing mechanisms for speed. The slime mold’s abilities are a powerful argument that we must redefine cognition away from something that happens in a brain to something that living systems do to stay alive.

“The belief that cognition requires a nervous system remains pervasive, but faces continued and growing challenge.”

The Slime Mold Doesn’t Answer the Bat Question—It Makes It Deeper

Let’s return to Thomas Nagel’s bat. His question was never about intelligence or problem-solving; it was about subjective, first-person consciousness.

The slime mold provides a stunning argument that highly complex, functional cognition can exist in the complete absence of consciousness as we understand it. It processes information, learns from experience, and makes sophisticated decisions. We are witnessing intelligent behavior without any reason to assume there is a subjective experience that accompanies it.

This forces us to see a profound separation between the machinery of intelligence and the inner light of awareness. The slime mold can “know” where it has been, “decide” which food is riskier, and “learn” to ignore a threat. But does this “knowing” have anything in common with our experience of knowing something? Or is it all just a breathtakingly complex biochemical algorithm, running in the dark, completely devoid of an inner world? The slime mold shows us what intelligence can look like without a mind, which makes the nature of our own minds all the more mysterious.

As Nagel himself framed the ultimate puzzle:

“…fundamentally an organism has conscious mental states if and only if there is something that it is to be that organism—something it is like for the organism.”

The Intelligent Universe

Our journey has taken us from the philosophical problem of a bat’s inner life to the tangible, bizarre, and intelligent behaviors of a brainless slime mold. Physarum polycephalum doesn’t diminish the wonder of our own minds. Instead, it expands our vision of where intelligence can be found in the universe, forcing us to see it not as a rare spark in a few complex brains, but as a widespread and creative force inherent to life itself.

The slime mold reveals that intelligence is woven into the fabric of life itself. But it leaves us stranded before the ultimate mystery: Is the rich, private theater of our own consciousness—the very “what it’s like to be us”—simply an elaborate evolutionary trick, or is there a flicker of that inner light even in the humblest of cells pulsating in the dark?

References

1. Nagel, T. (1974). What is it like to be a bat? The Philosophical Review, 83(4), 435–450.

2. Reid, C. R. (2023). Thoughts from the forest floor: a review of cognition in the slime mould Physarum polycephalum. Animal Cognition, 26, 1783–1797.

3. Nakagaki, T., Yamada, H., & Tóth, A. (2000). Maze-solving by an amoeboid organism. Nature, 407(6803), 470–470.

4. Reid, C. R., Latty, T., Dussutour, A., & Beekman, M. (2012). Slime mold uses an externalized spatial “memory” to navigate in complex environments. Proceedings of the National Academy of Sciences, 109(43), 17490–17494.

5. Boisseau, R. P., Vogel, D., & Dussutour, A. (2016). Habituation in non-neural organisms: evidence from slime moulds. Proceedings of the Royal Society B: Biological Sciences, 283(1829), 20160446.