Why Can We Only Think One Thing at a Time? Four Perspectives from Jellyfish to 86 Billion Neurons

> Based on the single-thread speculation in *The Unbearable Slowness of Being*, cross-validated from four independent perspectives: evolution, information theory, complex systems, and philosophy.

1. Evolutionary Perspective: Not a Flaw, but an Inheritance

Imagine a jellyfish: no brain, just a ring of nerves drifting in the sea like a transparent necklace. It senses chemical gradients, moves, chases food. In that primitive world, there is no need to 'consider multiple options simultaneously'—you are in one place, sense one gradient, make one action.

From this lineage, evolution did not 'add processor cores' but layered new structures on the original chassis: brainstem for breathing, cerebellum for motor coordination, limbic system for emotion, neocortex for abstract reasoning. Yet the deepest logic—one input, one action, in sequence—never disappeared. Eighty-six billion neurons do not mean 86 billion parallel trains of thought; the vast majority maintain the machinery of life: regulating blood sugar, parsing retinal signals, keeping balance.

[Evidence strength: medium]

The octopus is an interesting contrast. Its 500 million neurons are distributed across eight arms, each with semi-autonomous ganglia sensing and gripping independently—like eight parallel processors. Yet when it comes to octopus 'decisions'—where to flee, whether to attack—the central brain still decides. The arms' parallelism is local reflex; high-level integration remains serial. An octopus does not have eight streams of consciousness; it has one stream commanding eight semi-autonomous arms.

Seriality may not be a flaw but the necessary cost of a 'unified actor.' An organism that cannot compress perception into one coherent 'now' and converge decisions into one definite 'next step' is merely a collection of reflexes, not a 'self.'

[Evidence strength: medium]

2. Information-Theoretic Perspective: 10 bits/s Is Verification Bandwidth, Not Search Bandwidth

Sensory input bandwidth is enormous—roughly 10⁸ bits/s for the retina, 10⁵ bits/s for the cochlea. This flood of data does not vanish; it is simply never 'reported.'

Consciousness is not the brain's total output. It is a summary, a 'reportable layer.' Like a company where tens of thousands of employees work in parallel but the CEO reporting to shareholders can say only one thing at a time. The 10 bits/s figure measures what you can tell someone in the next second, not the brain's actual processing.

[Evidence strength: weak—depends on the assumed functional split between conscious and unconscious processing]

A computational insight applies here: P vs NP teaches that verifying a solution can be fast while finding one may require exhaustive search. Unconscious processes may perform parallel 'searching'—billions of neurons exploring possibilities. But consciousness is a 'verifier': it can hold only one hypothesis at a time, test its consistency, then accept or reject it.

If consciousness could simultaneously verify two contradictory hypotheses, action would be paralyzed—simultaneously 'believing' in turning left and right ends in原地 explosion. Seriality is not a search limitation; it is a requirement of decision-making.

[Evidence strength: speculative—mapping P/NP onto consciousness is metaphor, not theorem]

3. Complex-Systems Perspective: Seriality as the Product of Phase Transition

The brain is an ocean of 86 billion coupled oscillators, each firing on its own rhythm like countless small drums. Mostly this is chaos. But under certain stimuli, scattered rhythms begin to phase-lock—they synchronize. Like an audience spontaneously clapping in rhythm, independent units suddenly become one whole.

Dehaene's Global Workspace Theory holds that the brain contains a 'broadcast system' where selected information is globally broadcast. But this system can broadcast only one thing at a time—not for lack of hardware, but because two simultaneous broadcasts would drown each other out, like two people shouting into one microphone.

[Evidence strength: medium—GWT has experimental support, but the inference that 'broadcast must be serial' awaits verification]

There is an elegant phase-transition picture: below threshold, the system is decentralized chaos; above threshold, a coherent mode 'condenses' and suppresses competitors. Consciousness is not a light switch but a superconductor's transition—an 'order parameter' comes to dominate global dynamics. Seriality is a necessary property of that order parameter.

[Evidence strength: speculative—the phase-transition analogy is beautiful, but the neural 'order parameter' remains empirically unconfirmed]

4. Philosophical/Cognitive Perspective: Seriality Defines the 'Self'

William James's 'stream of consciousness' is not a metaphor but a phenomenological fact. What we experience is not 'multiple processes competing' but a continuous, indivisible 'now.' Even while the brain processes countless streams in parallel, experience is always linear—like a book that can be read only one line at a time.

Split-brain research offers stark evidence. After the corpus callosum is severed, the hemispheres process information independently. The left hemisphere sees 'spoon,' the right sees 'knife.' Asked, the left says 'spoon'; the right selects 'knife.' But when pressed on 'why the knife,' the left hemisphere confabulates: 'I saw a knife-shaped object.'

It does not know the right hemisphere's input, yet it must produce a unified narrative. This suggests the seriality of consciousness is not merely a feature of information processing but a basic requirement of the 'self.' The self is not an observer but a narrator—it must weave scattered neural events into a story of 'what I experienced.' If the story is incoherent, the self dissolves.

[Evidence strength: strong—split-brain research is classic and replicable]

Dissociative identity disorder seems to challenge this: one body, multiple 'selves.' But clinical observation shows these personalities appear alternately, never simultaneously. One emerges as another recedes. This is not parallel computing but time-division multiplexing—the same hardware running different programs, one at a time.

Schizophrenic experiences of 'thought insertion' are also telling: patients report 'someone else's thoughts entering my mind.' Note the wording—not 'I coexist peacefully with another thought,' but 'my single stream of consciousness has been violated.' Even in the most disordered states, the basic structure of subjective experience remains 'one line'—merely distorted and interrupted.

[Evidence strength: medium]

The Integrated Picture: Four Perspectives Converge

| Perspective | Core Claim | Evidence | |---|---|---| | Evolution | Seriality is an inheritance from primitive nervous systems | Medium | | Information theory | Seriality is the bottleneck of the conscious verifier | Speculative | | Complex systems | Seriality is an emergent property of global phase-locking | Speculative | | Philosophy/Cognition | Seriality defines the narrative 'self' | Strong |

The final picture looks like this: the brain's 86 billion neurons are indeed parallel, and the vast majority of their activity remains forever outside awareness. Consciousness is not their 'sum' but the product of a special relationship among them—a globally coherent pattern, a self-referential narrative.

Seriality is not a computational defect but a defining feature of 'one actor.' A system that must integrate massive parallel information, produce coherent behavior, and maintain a unified self-model must converge to serial at some level.

But there is a blind spot none of the four perspectives fully resolves: if seriality is 'necessary,' why must it be *this one*? Why not multiple consciousnesses taking turns at the helm? Information theory says 'verification must be serial' but does not explain why the verifier cannot time-share multiple tasks. Complex-systems theory says 'phase transitions produce seriality,' but different physical conditions could in principle allow different emergent modes.

The most honest answer: we don't know. As the authors of the 10 bits/s paper put it—naming is not understanding. Calling it an 'attention bottleneck' merely gives ignorance a name. Calling it a 'stream of consciousness' merely names another ignorance.

The jellyfish's simple ring and the human neocortex share one deep logic: one organism, in one world, one step at a time. Why it must be 'one,' we still do not know. But this 'not knowing' is itself the most valuable discovery—it tells us that beneath the clamor of 86 billion neurons hides a most basic fact that remains unexplained.

References

• Zheng, J., & Meister, M. (2024). The unbearable slowness of being: Why do we live at 10 bits/s? *Neuron*, 113(2), 192-204. https://doi.org/10.1016/j.neuron.2024.11.008
• Hasselmo, M. E., et al. (2002). SPEAR model. *Neural Computation*, 14(4), 793-817.
• Dehaene, S., et al. (2006). Global workspace theory of consciousness.
• Gazzaniga, M. S. (2000). The split-brain revisited. *Scientific American*.
• James, W. (1890). *The Principles of Psychology*.