3 philosophical debates from the 20th century that neuroscience is reshaping

Neuroscience isn’t dissolving philosophy’s hardest problems — it’s forcing us to rethink where they live.

Credit: Yves Forestier / Getty Images / Resource Database / Unsplash / Public Domain / Public Domain Review / Wikimedia Commons / Sarah Soryal

By Rachel Barr

Philosophers and scientists have always kept close company. Look back far enough, and it’s hard to tell where one ends and the other begins.

Before we had instruments to measure reality, we had to reason our way into it, but that intellectual lineage is what eventually gave us the scientific method. As technology advanced and the scope for observation expanded, specializations splintered off from philosophy to reconstitute as the sciences.

Astronomy cleared the sky of deities and showed us a universe governed by gravity, not gods. Geography mapped a not-so-flat Earth, then geology dated it, stratifying earthly time in isotopes and sedimentary layers. Physics folded time into space, and with it, reimagined us not as beings apart from nature, but as a continuation of its energy and mass. We are not, as Pink Floyd suggested, “lost souls swimming in a fishbowl.” We are matter, muddling our way through life in relativistic motion.

Now, in the 21st century, science is tracing a map through the other great unknown: the mind.

Advances in biophotonics and neuroimaging have brought us closer than ever to a material picture of the mind, but the questions we’re now brushing up against aren’t melting away under empirical gaze. Instead, neuroscience has wandered back to philosophy’s front door, testing the limits of its most durable questions.

1. Free will

In the early 19th century, French physicist Pierre-Simon Laplace imagined the Universe as clockwork, each gear turning in obedience to natural law. He conceived of a demon who, knowing the position and momentum of every particle, could predict the future with perfect accuracy. This thought experiment crystallizes classical determinism: a world where there is no freedom, only inevitability.

Modern neuroscience can feel like Laplace’s demon in biological dress; if thoughts and actions arise from the physical machinery of the brain, are we anything more than cogs in the same cosmic clock?

Stanford neurobiologist Robert Sapolsky presses that case in Determined: A Science of Life Without Free Will. The deterministic nature of our neural universe, he writes, is a totalizing argument against free will. Every act is an inevitable output of prior conditions — from genes to stress to social context. Even the air in the room, he notes, subtly alters our behavior.

Some cases seem to justify his position. In Our Brains, Our Selves, Oxford neurologist Masud Husain tells the stories of patients who were dramatically reshaped by disease and injury. One such patient, David, developed profound apathy after a stroke damaged circuits that link the frontal lobes with the basal ganglia — structures heavily involved in motivation and action. He was awake, aware, and physically capable, yet the inner spark seemed gone. Neurologists call this syndrome abulia, the loss of will. Before treatment, others had to prompt him repeatedly to do even simple actions, but a simple dopamine-boosting medication restored David to his former ambitious self. Whether the drug restored free will is a philosophical question. What the case makes hard to deny is that whatever we call will — free or not — depends largely on the health of a few cubic centimetres of tissue and the concentration of a particular neurotransmitter.

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Legal systems, psychiatry, and ethics all operate on a sliding scale of agency. Courts distinguish between crimes committed under premeditation and those committed under psychosis, for example.

This graded scaffold of responsibility sits more comfortably with the compatibilist view that determinism and free will can coexist. Philosopher and cognitive scientist Daniel Dennett has argued that freedom and responsibility arise from acting according to your own motives. The obvious objection is never far behind: “But where do those motives come from? Who chose them?”

This line of reasoning recurs endlessly in free-will debates — a philosophical whack-a-mole of causes causing causes. There’s a metaphor for this impasse: turtles all the way down. The phrase comes from a folk tale in which a scientist explains that the Earth orbits the Sun. A woman in the audience objects, “That’s nonsense, young man. The world rests on the back of a giant turtle.” When asked what the turtle stands on, she replies, “It’s turtles all the way down.”

One way out of this infinite regress is to stop sprinting back to the beginning of time and instead pay attention to what actually happens in the here-and-now of a living brain.

The past shapes us, but shaping is not the same as puppeteering. Causality is the medium in which agency emerges; it is a precondition of free will. A creature that could not be influenced by its history or environment would also be a creature that could not learn, plan, or take advice. In a very literal sense, you need causes in order to become the kind of system that can weigh options at all. As Dennett puts it, “the past does not control you; it causes you, but it does not control you.” Even the studies Sapolsky cites to illustrate biological and contextual determinism rely on statistics. They deal in distributions and averages, not one-to-one inevitabilities. For all practical purposes, brains and behavior must be described probabilistically.

Brains are not like the simple physical systems that populate a physics textbook. It is not a swinging pendulum, an ideal gas, or a neat circuit with a fixed input-output table. It’s a vast, nonlinear, adaptive network. Billions of neurons — each with thousands of synapses — form feedback-rich loops that are constantly being reshaped. When those neurons interact, their collective behavior no longer resembles a simple chain of causes.

At any moment, different coalitions of neurons can temporarily synchronise, form a functional team to guide perception or action, and then dissolve again. Neuroscientists describe this as a metastable system; it doesn’t lock into one pattern and stay there. The brain’s activity wanders across a landscape of possible patterns. Some regions of that landscape are attractors, preferred configurations the system tends to fall into. Others are ridges or passes that allow transitions between those attractors.

All of this is, of course, shaped by genetics and experience, but it doesn’t behave like a simple line of dominoes. Within this probabilistic terrain, neural circuits don’t dictate a single inescapable fate so much as bias the odds. Given your current state — your mood, your level of fatigue, the cues in the room — some patterns of activity are more likely to ignite than others. Causality constrains the menu of possibilities, but it does not pre-write the exact sequence of states you will traverse. This is where Laplace’s demon starts to lose its nerve.

Since the underlying dynamics are nonlinear, small differences in timing or input can, in the right conditions, be amplified into very different outcomes. Dynamical systems theorists call this sensitive dependence on initial conditions. In the brain, that sensitivity shows up at the boundary between competing options, where tiny fluctuations — an extra spike here, a few milliseconds’ delay there — can bias which attractor wins out. That isn’t indeterminism magically giving birth to freedom; rather, it is sensitivity placed where control signals can matter. It’s a thoroughly material feature of the brain’s organization that leaves the door to something like free will open a crack.

A decision does not require some magical breaking of the causal chain. It is a reconfiguration of the system’s dynamics: a shift in which neural coalition comes to dominate, a redirection of probabilistic flow through a lawful network. In theory, any non-zero degree of agency could be sufficient to move the needle.

To call this free will may stretch the term, but it captures a naturalistic form of agency, the ability of a physical system to use its own internal organization and history to navigate its causal possibilities. Neural computation operates squarely within the laws of physics. No synaptic transmission outruns light; no action potential violates Maxwell’s equations. Yet the brain transforms these laws into degrees of freedom. Brains are neither pure dice nor pure clockwork; they sit somewhere in between. That in-between space may be where whatever is worth salvaging under the name “free will” actually lives.

2. The existentialist crisis of meaning

Existentialism emerged from the collapse of theological certainty. In the 18^th and 19^th centuries, God was dead, or dying, and humanity found itself cut loose from the moral scaffolding that had once anchored its world. In the vacuum that followed, early existentialist thinkers such as Søren Kierkegaard and later Friedrich Nietzsche rejected the idea that reason alone could replace what had been lost.

By the mid-20^th century, after two world wars and the horror of Auschwitz, reason itself had come to look like a false idol. Writing amid the ruins of postwar Paris, second-wave existentialists Jean-Paul Sartre and Simone de Beauvoir found that both religious and secular systems of moral governance had crumbled under the weight of human brutality. What remained was the individual, alone with the burden of choosing.

For a paper-knife, Sartre explains, “essence precedes existence.” Its maker conceives its purpose first; only then is the knife brought into being. Humans have no such luck, however. We give knives their purpose, but who gives purpose to us? According to Sartre and many other existentialists, we do. Since meaning is not given, it must therefore arise from how we live and act in the world.

Albert Camus, writing a few years later, found this project misguided. The very hunger for meaning was the problem, he argued. The mismatch between that yearning and the Universe’s indifference is what he called l’absurde. Any attempt to reconcile this impossible correspondence was, for Camus, “philosophical suicide.” According to his view, we must live for “the struggle itself,” in full awareness of its futility.

Today, existentialism has entered a third phase — a movement philosophers Owen Flanagan and Gregg Caruso call neuroexistentialism. If consciousness is “the hard problem” in mind science, then “the really hard problem,” writes Flanagan, is explaining how subjective significance can arise in a purely material brain. His answer — eudaimonistic naturalism — suggests meaning can be studied empirically, by examining what allows human beings to flourish.

I don’t necessarily disagree. However, looking at it from my perspective turns this question slightly on its axis. Meaning, I would argue, is not something we elect to create; it’s something that happens to us. To be conscious at all is to translate sensation into experience. The brain cannot help but impose coherence on the flux of sensory data — stitching cause to effect, moment to moment — because that is the mechanism by which it constructs and perceives reality.

At Northwestern University, researchers asked volunteers to write about the past or future, imagining themselves in the experience. Whether the scenes they pictured were joyful or sad didn’t seem to matter — the very act of temporal simulation increased their reported sense of meaning. And the greater detail they imagined into the experience, the stronger the effect, on average. This suggests meaning is dialogical, emerging when engaged in process. The very act of being alive to our experience, and to time’s unfolding, appears to feed some part of our existential hunger, which, don’t forget, is the brain’s fault in the first place.

The brain constructs our hunger for meaning, just as it conceives of meaning in the first place. Nobody else experiences meaning, aside from, perhaps, some other intelligent creatures whose existential despair remains private. Meaning has always been a brain-made construct. It was ours to begin with.

Seen this way, Camus’ absurd takes on a new texture. The Universe is unfeeling because, of course it is. It’s the very environment from which feeling emerged. It provided a world of sensations, and then organisms evolved to feel them — spawning abilities to help them navigate material reality and, crucially, survive inside it. For what other reason could we have awoken, were it not for evolutionary pressures that privileged the survival of reality-sensing organisms? Absurdity is simply the natural condition of consciousness awakened from unresponsive matter.

We are, whether we like it or not, phenomenological creatures. However we define it, meaning is ultimately a felt sense of coherence and value, not a fact about the world but a relation to it. This shifts the existential task. Meaning isn’t something to be manufactured ex nihilo, but a felt sense that arises when we feed the brain the kinds of patterns and environments it reliably metabolises into a sense of coherence. Which is not unlike the eudaimonistic naturalism Owen Flanagan suggested. I did tell you I don’t necessarily disagree.

3. The self

If you follow the trail of 20th-century philosophers chasing the self, what strikes you is how restlessly the thing keeps moving. Martin Heidegger moved it out of the skull and into the world. In his view, selfhood is expressed in what you do, what you care about, and how your life is organized under the awareness of mortality, or “being toward death.”

Maurice Merleau‑Ponty tightened the focus from world to flesh, describing selfhood as a lived body. For him, the self isn’t a story you tell, but a pre-reflective feeling of mineness braided into perception and movement.

Derek Parfit located it in something more abstract: in the continuing causal organization of mental life. He arrived at this conclusion via a thought experiment. Imagine your brain is divided and transplanted into two new bodies. Which one is you? Parfit argues that identity, as we imagine it, can’t do the job we want it to, because identity can’t branch. Psychological continuity, however, can — and that, he thinks, is what really grounds your concern about the future. You plan ahead, assuming that your future self will remember your past, carry your intentions forward, and feel the consequences of what you do now. After the transplant, your life continues in two streams, and what you care about is present in both. So perhaps, Parfit suggests, “you” can survive as two.

Above, we have three different answers to the same question: Where does the ‘I’ live? At the turn of the millennium, a new candidate was discovered.

When you let your attention drift inward — to your past, your future, your inner monologue — the default mode network (DMN) kicks into gear. Neurologist Marcus Raichle first noticed it when certain midline areas would hum to life when his volunteers were waiting idly in the scanner between trials. Mind-wandering was the first function linked to this network. Since then, the DMN has been implicated in autobiographical memory, rumination, and self-referential thinking — exactly the sort of heavy lifting you’d expect from a narrative self-system.

When that network is perturbed, the felt shape of the self can change. Under psychedelics, the DMN becomes less internally coherent. As its activity falls away, so too does the bounded, narrating self — a state referred to as ego dissolution. At the same time, sensation and emotion flood more freely into awareness. Experience can feel more immediate and emotionally saturated, as if the editorial voice has gone quiet and the world has rushed in.

Depersonalization looks like the bleak mirror image. Here, DMN hubs chatter away, but their links to salience and interoceptive networks are weakened. The part of the brain that keeps up a running commentary about “me” is still humming, sometimes even overactive, but its conversation partners in the body and emotional brain have gone quiet. Patients describe feeling like a spectator sealed behind glass. They know who they are, and they remember the events of their lives. What’s missing is the felt mineness of experience.

Depersonalization exposes the limits of what Parfit’s psychological continuity can explain. Continuity may be enough to ground the forward flow of memory, intention, and character, but it is not sufficient for the phenomenology of selfhood. In depersonalization, the continuer persists; the mineness does not.

Experience is not a faithful readout of external reality, or of internal viscera; it’s the brain’s best Bayesian explanation of viscerosensory inputs. You can’t see your pupils dilate; you often can’t place a visceral shift precisely in space or time. That forces the brain to lean heavily on estimation models, making guesses that help it to integrate interoceptive signals — like heart rate, breathing, and temperature — with sensory feedback from the external world.

In depersonalization, the system appears to down-weight those interoceptive signals, treating them as noisy and uninformative. The DMN keeps rehearsing the script of the self, but it’s no longer anchored to the visceral stream coming up from the body. From the inside, that feels like your life continuing in theory while someone else does the living.

The brain is ultimately an organ of regulation, which is why cognitive neuroscientist Anil Seth describes us as “beast machines.” We are biological control systems first, reflective narrators only later. Nervous systems arrive late in evolutionary history, appearing as specialized gadgets for helping bodies anticipate and avoid trouble. Perception and action evolved in service of keeping the body alive, and so our experiences are never really disembodied.

Merleau‑Ponty was onto something, it seems.

Still, Seth relocates the self once more. He’s proposed that conscious selfhood arises from the brain’s role as a prediction-driven control system for the body. Feeling like a self, he argues, is the brain’s best effort to wrangle body, narrative, and world into a coherent stance. Selfhood lives in the connective tissue that binds story to sensation.

Psychedelics and depersonalization are instructive precisely because they pry those agreements apart. One loosens the narrator while flooding the body; the other preserves the narrator while muting ownership.

The primary project of a living system is not to understand the world; it is to avoid dying in it. Perception, action, memory, and even our hunger for meaning are elaborations of that basic constraint. The brain is not made of celestial material. It is tissue and salt water, warmed, fed, and continuously informed by the rest of the organism.

A brain removed from a body is not a mind; it is a rapidly failing organ.

Inside the organism that feeds it, the brain becomes a dynamical system that can model its own future, argue with itself about responsibility, suffer the absence of meaning, and feel like someone rather than something. We perceive the world and ourselves because of, not in spite of, being “beast machines.”

Editor’s note: This article was updated on 1/5/26 to correct a description of how Kierkegaard and Nietzsche thought about reason as a replacement for religious sources of meaning.

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