There are two buildings on the high street of the town where I grew up. The first is a GP surgery -- a low brick rectangle with frosted glass and a waiting room that smells of floor polish and old Reader's Digest magazines. The second is a counselling centre, four doors down, in a converted Victorian house with a different receptionist, a different phone number, a different waiting list, and a different budget line in the local health authority's accounts. If you walk into the first building and say that you cannot sleep, that your chest hurts, that you have lost two stone in four months, a doctor will examine your body. Blood pressure. Heart rate. Palpation. Referral to a cardiologist, perhaps, or a gastroenterologist, or an endocrinologist -- each of whom occupies yet another building, with yet another waiting list. If you walk into the second building and say that you cannot sleep, that your chest hurts, that everything feels pointless and you have not left the house in three weeks, a therapist will examine your mind. Cognitive patterns. Emotional history. Relational dynamics. Referral, perhaps, to a psychiatrist -- who occupies yet another building. Same animal. Same chest. Same sleeplessness. Two doors. Two systems. Two languages. Two budgets. One organism that does not know it has been cut in half.
This is the split. It is so deeply embedded in the architecture of Western healthcare that most of us experience it as natural -- as obvious as the difference between a broken leg and a broken heart. The body goes to one building. The mind goes to another. The division feels intuitive because we have been trained, for nearly four centuries, to believe that these are genuinely separate domains -- that what happens in the skull is one kind of thing and what happens beneath it is another. The training has a name, a date, and a single author. In 1641, in his Meditations on First Philosophy, Rene Descartes proposed that reality consists of two fundamentally distinct substances: res cogitans, the thinking substance, and res extensa, the extended substance. Mind and matter. Soul and body. The thinking thing and the thing that takes up space. Descartes located their interaction in the pineal gland -- a small endocrine organ in the centre of the brain that he chose because it was the only midline structure he could find that was not duplicated on both sides. The soul, he reasoned, must interface with the body through a single, unified point. The pineal gland was his best candidate.
Almost nobody accepted the pineal gland theory, even in Descartes' lifetime. Princess Elisabeth of Bohemia wrote to him directly, pointing out that if the soul is immaterial, it cannot push on matter, and no pineal gland solves this. Thomas Willis, Baruch Spinoza, Gottfried Leibniz, and Immanuel Kant all rejected the mechanism. But here is the difficulty: they rejected the mechanism while absorbing the architecture. The two-substance model -- mind here, body there -- survived the death of its proposed interface. It survived because it was useful. Descartes, by declaring the body a machine, freed it from the authority of the Church. Physicians could study anatomy, perform dissections, develop surgery, without trespassing on the soul's territory. The deal was implicit but powerful: you can have the body, as long as we keep the mind. Medicine took the deal. It has been operating under its terms ever since. And we -- all of us, every time we walk into the body building or the mind building without questioning why there are two -- are living inside the deal that medicine made four centuries ago.
The consequences are visible in every hospital, every clinic, every national health system on the planet. Psychiatric services exist separately from physical care. Training programmes produce doctors who specialise in the body and a different class of professional -- psychiatrists, psychologists, counsellors, psychotherapists, each with their own accreditation and their own theoretical framework -- who specialise in the mind. The funding is separate. In the United Kingdom, mental health accounts for approximately twenty percent of the total burden of disease but receives roughly nine percent of the NHS budget. The Royal College of Psychiatrists reported in 2025 that mental health's share of NHS spending was declining -- from 8.87 percent in 2022-23 to a projected 8.71 percent in 2025-26 -- a shortfall that translates, in practice, to approximately three hundred million pounds that the mind building will not receive. The body building gets a different allocation, administered through different mechanisms, assessed by different metrics. One animal. Two balance sheets. Does that seem rational? Twenty percent of the disease burden, nine percent of the budget, and the share is shrinking. What would we say about a zoo that spent nine percent of its resources on a problem that caused twenty percent of its animals' suffering?
The division is not merely administrative. It shapes what the organism is told about itself. When a person presents with symptoms that do not resolve through physical investigation -- fatigue that no blood test explains, pain that no scan reveals, digestive disturbance that no endoscopy can locate -- the system reaches for a phrase so common it has become a cultural artefact: "It's all in your head." The neurologist Suzanne O'Sullivan has written extensively about the consequences of this phrase. What it means, translated into zoological terms, is: we have examined the body and found nothing wrong, therefore the problem belongs to the other building. The organism is told that its suffering is real but not physical -- or, worse, that it is not real at all. The Cartesian split becomes, at the clinical interface, a diagnostic shrug. If the body building cannot find it, and the mind building has a six-month queue, the animal falls through the gap between two institutions that were never designed to communicate with each other. Has this happened to you? Have you sat in a consulting room and been told that everything looks fine, that the tests came back normal, while your body continued to insist that something was wrong? If so, you have been caught in the gap. The gap is four centuries wide.
In 1994, the neuroscientist Antonio Damasio published Descartes' Error: Emotion, Reason, and the Human Brain, and the title was not metaphorical. Damasio's research programme, conducted over two decades at the University of Iowa, demonstrated through meticulous clinical work with brain-injured patients that the Cartesian separation of emotion from reason is not merely philosophically questionable. It is neurologically false.
Damasio's key evidence came from patients with damage to the ventromedial prefrontal cortex -- the region of the brain where emotional processing and rational decision-making converge. These patients retained their intelligence. They could score normally on IQ tests, solve logic problems, articulate the pros and cons of any given decision. But they could not decide. Faced with a choice as simple as when to schedule their next appointment, they would deliberate for half an hour, weighing options with perfect rationality and no resolution. The reason, Damasio demonstrated, was that the emotional signals that normally bias decision-making -- what he called "somatic markers," bodily feelings associated with the predicted outcomes of different choices -- had been severed from the reasoning process. Without the body's input, the mind could analyse but not choose. Reason without emotion was not pure logic. It was paralysis.
The somatic marker hypothesis, as it came to be known, proposed that emotions are not irrational intrusions into cognitive life. They are cognitive processes -- arising from body states, informed by physiological signals, and essential to the very reasoning that Descartes claimed was the mind's exclusive territory. When you feel a "gut instinct" about a decision, that phrase is not a metaphor. It is a description of a physiological process in which visceral signals -- heart rate changes, skin conductance shifts, gastrointestinal activity -- are integrated with cortical processing to generate the feeling-state that biases your choice. The gut is not offering a poetic alternative to reason. It is participating in reason. The two substances are one substance. Descartes' error was not a minor philosophical miscalculation. It was a misidentification of the organism. And we have been building our hospitals, our clinics, our entire healthcare architecture on that misidentification ever since.
The evidence has only accumulated since. The gut -- the gastrointestinal tract, specifically its enteric nervous system and its resident microbiome -- produces approximately ninety to ninety-five percent of the body's serotonin. This figure appears in nearly every review of gut-brain communication published since Gershon's The Second Brain in 1998, and it is worth pausing on, because serotonin is the molecule most frequently associated with depression, the molecule that selective serotonin reuptake inhibitors -- SSRIs, the most widely prescribed class of psychiatric medication in the world -- are designed to modulate. Ninety-five percent of it is manufactured not in the brain but in the gut, primarily by enterochromaffin cells in the intestinal lining, with significant contributions from the gut microbiome itself. A 2015 study at the California Institute of Technology, led by Elaine Hsiao, demonstrated that specific gut bacteria -- predominantly Turicibacter sanguinis and members of the Clostridia -- directly stimulate enterochromaffin cells to produce serotonin. When these bacteria were absent in germ-free mice, more than fifty percent of gut serotonin was missing.
Now. The serotonin produced in the gut does not cross the blood-brain barrier directly, and this is the point at which the Cartesian instinct reasserts itself: the gut's serotonin is "peripheral," handling motility and secretion, while the brain's serotonin is "central," handling mood. Two departments. Two functions. But the organism does not respect the departmental boundary. The vagus nerve -- the longest cranial nerve in the body, running from the brainstem to the abdomen -- carries signals bidirectionally between the gut and the brain. Vagal afferent fibres relay information about the gut's biochemical environment directly to the nucleus tractus solitarius in the brainstem, which in turn modulates the monoaminergic systems -- including serotonergic systems -- that regulate mood, anxiety, and emotional processing. The gut does not need to send serotonin across the blood-brain barrier. It sends information about its serotonergic state, and that information alters brain function. The inflammatory state of the gut, the composition of the microbiome, the metabolites produced by bacterial fermentation -- all of these are communicated, continuously, to the brain through the vagus nerve. The organism is one system. The departments are an invention. Our departments. Not the organism's.
In 2013, Michael Berk and colleagues published a paper in BMC Medicine with a title that, had it appeared two decades earlier, would have been dismissed as fringe: "So depression is an inflammatory disease, but where does the inflammation come from?" The paper's central argument, supported by an extensive review of the literature, was that major depression is associated with a chronic, low-grade inflammatory response -- elevated pro-inflammatory cytokines such as IL-1-beta, IL-6, and tumour necrosis factor alpha -- and that this inflammation is not a side effect of depression but a candidate causal mechanism. Depression, in this framework, is not a disease of the mind that happens to affect the body. It is a disease of the body -- specifically, a dysregulation of the immune system -- that manifests as what we have been trained to call a mental illness.
The evidence is substantial and continues to grow. Patients with depression consistently show elevated C-reactive protein, a marker of systemic inflammation. Administration of pro-inflammatory cytokines to healthy volunteers produces depressive symptoms -- fatigue, social withdrawal, anhedonia, cognitive slowing -- within hours. Patients receiving interferon-alpha therapy for hepatitis C, which dramatically increases inflammatory cytokine levels, develop clinical depression at rates of thirty to fifty percent. The mechanism is not obscure: peripheral cytokines access the central nervous system through multiple pathways -- the vagus nerve, circumventricular organs, active transport across the blood-brain barrier -- and once in the brain, they activate microglia, reduce the availability of serotonin and dopamine precursors through upregulation of the indoleamine 2,3-dioxygenase pathway, and suppress brain-derived neurotrophic factor, which is essential for neuroplasticity and the maintenance of hippocampal neurons. The chemistry does not know about Descartes. The cytokine does not pause at the boundary between body and mind to check which department it belongs to. Why would it? The boundary does not exist in the organism. It only exists in our buildings.
Where does the inflammation come from? Berk's paper identifies a convergence of sources that will be familiar to any reader who has followed the preceding thirteen chapters of this book. Chronic psychological stress activates the hypothalamic-pituitary-adrenal axis and the sympathetic nervous system, both of which stimulate pro-inflammatory cytokine production. Poor diet -- specifically, the processed, high-sugar, low-fibre diet described in Chapter 2 -- disrupts the gut microbiome, increases intestinal permeability, and allows bacterial endotoxins to enter the bloodstream, triggering systemic inflammation. Sleep deprivation -- the sixty-to-one-hundred-and-fifty-minute nightly deficit described in Chapter 2 -- elevates inflammatory markers independently of other risk factors. Physical inactivity -- the sedentary pattern described in Chapter 2 -- is associated with elevated C-reactive protein and IL-6. Social isolation -- the disconnection described in Chapter 4 -- independently predicts inflammatory biomarker elevation. Every dimension of the enclosure failure described in the preceding chapters converges on a single biological mechanism: chronic low-grade inflammation. And that inflammation, through the pathways Berk and others have mapped, produces the cluster of symptoms that the system classifies as "mental illness" and refers to the other building.
Can you see what is happening here? The enclosure produces the inflammation. The inflammation produces the depression. The depression is classified as a mental illness. The mental illness is sent to the mind building. The mind building does not assess the enclosure. The circle closes. And the organism -- our organism, the one we are all carrying through this civilisation -- stays inflamed, stays depressed, and stays classified as having a problem in its head rather than a problem in its world.
The "mental health crisis," in this light, is substantially a misnomer. It is a body crisis -- a gut crisis, a sleep crisis, a movement crisis, a connection crisis, a diet crisis -- that presents as sadness, anxiety, fatigue, and despair because those are the subjective experiences produced when the organism's inflammatory burden exceeds its regulatory capacity. Calling it "mental" is not descriptively wrong -- the suffering is experienced mentally, in consciousness, as emotional pain. But the label directs treatment toward the mind building when the organism needs assessment of the whole animal. It is as though a zookeeper, observing a gorilla that had stopped eating, stopped socialising, and sat motionless in the corner of its enclosure, classified the problem as "gorilla psychology" and referred the animal to a gorilla therapist, rather than testing the food, checking the social dynamics, assessing the sleep patterns, and examining the enclosure.
The mind building, meanwhile, is full.
In the United Kingdom, NHS Talking Therapies data for 2024-25 show that while approximately ninety-one percent of patients are seen within six weeks -- meeting the programme's target -- this figure conceals the reality facing anyone whose condition is more complex than mild-to-moderate anxiety or depression. For specialist mental health services, the picture is different. Rethink Mental Illness reported in 2025 that 1.6 million people with mental illness are waiting for care and treatment, and these individuals are excluded from the headline plans to bring down NHS waiting lists, which focus primarily on physical health. The waiting is not incidental. It is structural: there are not enough therapists, not enough psychiatrists, not enough funded hours to meet the demand that the system's own diagnostic categories generate.
For children, the queue is longer. NHS England data for 2022-23 show that the average wait for children and young people accessing mental health services was 108 days. But this average obscures a distribution that would, in any other welfare context, constitute an emergency: 6,300 children had been waiting for over two years, with an average wait among that cohort exceeding three years. Three years. A quarter of a childhood. Let that land. Six thousand three hundred children, waiting for three years, while the organism continues to develop around its untreated distress. The Children's Commissioner for England reported that over a quarter of a million children are waiting for mental health support at any given time, and it is estimated that only twenty-five percent of children who need care actually receive it. The organism is told to wait because the building is full. But the organism is not producing a "mental" problem that requires a "mental" building. It is producing an organism-level distress response -- to bullying, to family breakdown, to poverty, to a food system that inflames its gut, to a school system that confines its body, to a social media environment that parasitises its attention -- and the only institutional response available is a referral to a waiting list for a service that will address the distress through conversation, in a room, once a week, for six sessions.
I do not mean to diminish therapy. Therapy works. The evidence base for cognitive behavioural therapy, for dialectical behaviour therapy, for trauma-focused approaches is robust and replicable. What I mean to say is that therapy is being asked to do something it was never designed to do: compensate for an enclosure that is systematically producing distress. A therapist working with a child who is sleep-deprived, socially isolated, physically inactive, eating processed food, spending seven hours a day on a screen, and living in a household under financial stress is not treating a psychological condition. They are treating an environmental condition with a psychological tool. It is like prescribing painkillers for a broken leg without setting the bone. The painkillers work. They reduce suffering. But the leg is still broken, and the organism is still in pain, and the painkiller budget is growing because the leg-breaking mechanism has not been addressed. And here is the question that should haunt every health service in the developed world: why are we spending more and more on painkillers while the machine that breaks the legs keeps running?
George Engel saw this in 1977. His paper in Science -- "The Need for a New Medical Model: A Challenge for Biomedicine" -- proposed what he called the biopsychosocial model: a framework that insisted on treating biological, psychological, and social factors as integrated and interdependent, rather than as separate domains requiring separate professional guilds. Engel's model was widely praised, widely cited, and almost universally ignored in practice. The institutional architecture -- the two buildings, the two budgets, the two training programmes -- proved more durable than the intellectual argument against it. Fifty years later, the biopsychosocial model is taught in medical schools and absent from medical systems. The GP's surgery and the counselling centre remain four doors apart. We teach our doctors that the organism is one thing and then send them to work in a system built on the assumption that it is two.
In 2012, Robin Carhart-Harris, then a researcher at Imperial College London, placed a volunteer in an fMRI scanner and administered psilocybin -- the psychoactive compound found in over two hundred species of mushroom. What he observed in the scanner would, over the following decade, do more to expose the absurdity of the mind-body split than any philosophical argument had managed in four centuries.
The psilocybin reduced blood flow and functional connectivity in the default mode network -- a set of brain regions, including the medial prefrontal cortex and the posterior cingulate cortex, that are most active when the mind is engaged in self-referential thought: rumination, autobiographical memory, projection into the future, the narrative voice that says "I" and constructs a continuous story about who you are and what your life means. In depression, the default mode network is hyperactive -- locked into repetitive loops of self-critical rumination. Psilocybin, at sufficient doses, quieted it. The scanner showed what the volunteers reported: a dissolution of the rigid boundaries of the self, a sense of connection to something larger, and -- critically -- lasting changes in psychological functioning that persisted weeks and months after a single session.
Matthew Johnson and Roland Griffiths at Johns Hopkins University extended this work across multiple clinical trials. In a 2011 study, MacLean, Johnson, and Griffiths administered a single high dose of psilocybin to fifty-one volunteers under controlled conditions and measured personality using the NEO Personality Inventory at screening, one to two months after the session, and again at approximately fourteen months. In participants who reported a "mystical experience" during the session -- assessed using a validated questionnaire -- the personality trait of Openness increased significantly and remained elevated at the fourteen-month follow-up. This finding is striking because personality traits in adults are generally considered stable after age thirty. A single pharmacological intervention produced a measurable, lasting change in a dimension of personality that encompasses aesthetic appreciation, imagination, intellectual curiosity, and broad-mindedness.
The intervention is a chemical. It is a molecule -- 4-phosphoryloxy-N,N-dimethyltryptamine -- that binds to serotonin 2A receptors in the cortex and produces, through a cascade of neurochemical events, an experience that participants consistently describe in the language of meaning, transcendence, and spiritual significance. The drug does not know about Descartes. It is a physical substance that produces what the two-building system would classify as a psychological outcome -- except that it does so through a mechanism that is entirely physical, in a manner that the two-building system has no institutional framework to accommodate. It is not a body treatment -- no surgeon administered it, no organ was repaired. It is not a mind treatment -- no cognitive restructuring occurred, no behavioural homework was assigned. It is an organism treatment. It addresses the whole animal, because the whole animal is what responds. What building does the psilocybin go to? The body building, because it is a chemical? The mind building, because it produces changes in personality and mood? The question is absurd. And its absurdity reveals the absurdity of the buildings.
The psychedelic research does not prove that psilocybin is the answer to the mental health crisis. The trials are small, the regulatory landscape is complex, and the mechanisms are not fully understood. What the research does prove -- incontrovertibly, on a scanner, in peer-reviewed journals from the most conservative institutions in science -- is that the boundary between "physical" and "mental" treatment does not exist at the level of the organism. The boundary exists only at the level of the institution. The organism does not have a mental health problem and a separate physical health problem. It has a problem. The institution has two buildings.
Here is the moment where the zoological lens requires us to look for the good impulse -- the legitimate need that the broken system was originally designed to meet. Because the mind-body split, for all its damage, did not arise from stupidity or malice. It arose from a genuine insight that was operationalised badly.
The insight was specialisation. The human body is extraordinarily complex -- thirty-seven trillion cells, seventy-eight organs, twelve interconnected physiological systems. No single practitioner can master all of it. Heart surgery requires a heart surgeon, not a generalist shaman who also treats anxiety and delivers babies. The development of medical specialisation -- cardiology, neurology, endocrinology, orthopaedics, psychiatry -- produced genuine advances in treatment that would have been impossible under a generalist model. Survival rates improved. Diagnostic precision increased. Surgical techniques became viable that would have killed the patient a generation earlier. The impulse to develop expertise was correct. And the arc is the same one we have traced through every chapter: a good impulse, a correct response, an extension past its working range.
The error was not specialisation itself. The error was organising specialisations around Descartes' two substances rather than around the organism. The body was divided not merely into systems -- which makes sense -- but into two ontological categories: the physical and the mental. And those categories were institutionalised into separate buildings, separate budgets, separate professions, and separate cultures. The cardiologist does not ask about the patient's sense of meaning. The psychiatrist does not order an inflammatory marker panel. The gap between them is not a failure of individual practice -- many clinicians work across it with great skill and dedication. The gap is structural. It is built into the architecture. It is built into our architecture, and we walk through it every time we choose a door.
A zoo veterinarian does not operate this way. Zoo welfare assessment -- as codified in the Five Domains model described in Chapter 1 -- integrates physical health, nutrition, environment, behavioural interactions, and mental state into a single evaluative framework. The behaviour IS the body. A gorilla that has stopped eating is not referred to a gorilla psychologist for six sessions of cognitive behavioural therapy while the dietician operates independently in another building. The veterinarian assesses the whole animal: What is it eating? How is it sleeping? What are its social dynamics? Has the enclosure changed? Is there a new individual in the group? Is there an inflammatory process? The assessment is organism-level because the organism is one thing. The zoo does not have two buildings because the zoo never adopted the Cartesian deal. It never agreed to surrender the animal's mind to one department and its body to another. It kept the animal whole because the animal is whole. Why do we do for gorillas what we refuse to do for ourselves?
If the organism is one thing -- if the gut and the brain are in continuous dialogue, if inflammation mediates between the food and the mood, if sleep deprivation produces both cardiovascular disease and depression through overlapping mechanisms -- then the response to the organism's distress cannot be two buildings. It must be one assessment.
What would that assessment look like? It would look, in many respects, like what a competent zoo veterinarian already does. The practitioner would sit with the organism and ask, not "What is your mental health like?" and "What is your physical health like?" as though these are separate questions, but: "How is the animal?" And the answer would be assembled from the full set of environmental and biological inputs.
Diet. What is the organism eating? Is the microbiome receiving the fibre and diversity it requires, or is it being fed the processed, emulsified, mineral-depleted output described in Chapter 2? Is there evidence of gut inflammation -- bloating, irregularity, food sensitivity -- that might be driving systemic cytokine elevation?
Sleep. Is the organism sleeping seven to nine hours in alignment with its circadian biology? Is the light environment supporting melatonin production? Is the organism waking with the artificial alarm that severs it from its final sleep cycle each morning?
Movement. Is the organism moving? Not exercising -- moving. Walking, climbing, carrying, crouching, reaching, in varied environments on varied terrain. A 2024 systematic review and network meta-analysis published in the BMJ, encompassing 218 randomised controlled trials and over fourteen thousand participants, found that exercise was comparable in effect to both psychological therapies and antidepressant medication for the treatment of depression -- with the additional benefit of improving cardiovascular, metabolic, and musculoskeletal health simultaneously. The body treatment is the mind treatment. The organism does not know the difference. Why should our institutions pretend there is one?
Connection. Does the organism have regular physical proximity to other organisms it trusts? Not digital proximity -- physical. Skin contact. Shared space. Eye contact at less than two metres. The social isolation that Chapter 4 described and Chapter 11 elaborated is not merely emotionally painful. It is physiologically inflammatory, through well-documented mechanisms involving cortisol dysregulation and sympathetic nervous system activation.
Purpose. Does the organism's daily activity connect to something it experiences as meaningful? The Monk dimension, established in Chapter 5, is not a luxury -- it is a biological requirement for an animal whose expensive brain generates narratives and then requires those narratives to cohere.
Environment. Where does the organism spend its time? Under what light? Breathing what air? In contact with what ground? These questions, which will occupy the entirety of the next chapter, are not separate from the "mental health" assessment. They are the assessment. They are, in fact, the only assessment that makes sense once you stop pretending the organism is two things and start acknowledging it is one.
The NHS, to its credit, has begun to recognise this convergence, though it frames the recognition in characteristically institutional language. "Social prescribing" -- the practice of referring patients not to a therapist or a pharmacist but to a community gardening group, a walking club, an art class, a volunteering programme -- was rolled out nationally in England's primary care system beginning in 2019, and by 2023, over 1.1 million patients had received social prescribing referrals, exceeding NHS targets by twenty-seven to fifty-two percent. The evidence base is growing: nature-based social prescribing shows measurable improvements in mood, psychological well-being, and reduction in loneliness. Community gardening, in particular, shows promise -- and here is a detail that connects to the inflammatory hypothesis through a path so elegant it seems designed: Christopher Lowry at the University of Colorado has demonstrated that Mycobacterium vaccae, a soil bacterium encountered through direct contact with earth, activates serotonergic neurons in the dorsal raphe nucleus and increases serotonin in the prefrontal cortex. In mice, pre-exposure to M. vaccae produces stress resilience -- a reduced PTSD-like response to subsequent stressors. The organism in the garden is not merely "relaxing." It is receiving a biochemical input through its skin and respiratory system that modulates the same serotonergic pathways that SSRIs target pharmacologically. The community gardening prescription and the antidepressant prescription are not alternative approaches to the same problem. They are, at the level of the neurotransmitter, the same approach -- delivered through different routes, classified by different buildings.
Dirt and Prozac. Working on the same pathway. Through the same receptors. On the same organism. If that does not make the absurdity of the two-building system vivid, nothing will.
I should say where I am in this.
I have two sons. The elder was, at the age of seven, a child who cried frequently, slept poorly, and found school overwhelming. My wife and I did what the system told us to do: we took him to the GP. The GP referred him to the child and adolescent mental health service. The waiting list was eleven months. During those eleven months, we did what frightened parents do -- we read, we searched, we tried to understand what was happening. What we found, gradually and without any guidance from the system that was supposed to help, was that our son was sleeping in a room with a screen that emitted blue-spectrum light until an hour before bed. He was eating school lunches that contained emulsifiers, artificial sweeteners, and approximately four grams of fibre per day. He was sitting in a classroom for six hours, with two fifteen-minute breaks in a concrete playground. His social world had been disrupted by a classroom reorganisation that separated him from his two closest friends. He was, in every zoological sense, an organism in an enclosure that was failing to meet his basic needs across multiple dimensions simultaneously.
We changed the light. We changed the food. We pulled him out of the after-school programme and let him run in the woods behind our flat in Leiden for two hours every afternoon. We arranged playdates with the two friends. Within three weeks, the crying had stopped. Within six weeks, the sleep had normalised. By the time the CAMHS appointment arrived, eleven months later, we had nothing to report. The clinician was kind and thorough and asked us a series of questions about his emotional state, none of which addressed his diet, his sleep environment, his light exposure, his physical activity levels, or his social ecology. She was working within the mind building. She had no remit, no training, and no time to assess the organism.
I do not tell this story to suggest that therapy is unnecessary, or that all childhood distress can be resolved by turning off screens and adding fibre. Some conditions are severe, neurological, and require precisely the specialist intervention that the mind building provides. I tell it because it illustrates, at the scale of one family, the structural failure that the mind-body split produces. The system's response to a distressed child was an eleven-month wait for a cognitive assessment. No one, at any point in the institutional pathway, asked: what is this animal eating, how is it sleeping, how much is it moving, and who is it bonding with? Not because these questions are unknown -- the research linking diet, sleep, exercise, and social connection to childhood mental health is robust and growing. But because the institutional architecture has no mechanism for asking them. The body building does not assess behaviour. The mind building does not assess the gut. The organism falls through the gap, and the gap is four hundred years old, and it was made by a French philosopher who thought the soul lived in a gland. Our children fall through this gap. Yours may have. Mine did.
There is one more piece of the story that I think deserves attention, because it reveals the absurdity of the split at the molecular level -- not through theory, but through the behaviour of a single compound.
The placebo effect has been growing stronger. This is not a metaphorical statement. A meta-analysis of clinical trials for antidepressant medications, tracking placebo response rates over decades, shows that the proportion of improvement attributable to the sugar pill has been increasing over time. The phenomenon is robust enough to have created a serious problem for pharmaceutical companies: drugs that would have beaten placebo thirty years ago now fail to, not because the drugs have become weaker but because the placebo has become stronger. Think about what that means. The belief that one is being treated is, year by year, becoming a more potent intervention. The sugar pill is catching up to the chemistry.
The mechanism, as best we understand it, involves expectation, ritual, and the therapeutic relationship -- the act of taking a pill, the interaction with a clinician, the belief that treatment is occurring -- triggering endogenous neurochemical cascades that produce measurable physiological changes. Endogenous opioid release. Dopaminergic activation. Altered activity in the prefrontal cortex and anterior cingulate. The belief produces the chemistry. The mind produces the body change. Or rather -- and this is the point -- the organism produces a unified response to a social and environmental signal, and the response has both experiential and biochemical dimensions, because those are not two dimensions. They are one dimension, observed from two angles by two departments that were never designed to look at the same thing.
The placebo effect is the organism's daily refutation of Descartes. Every sugar pill that reduces pain, every sham surgery that improves function, every therapeutic interaction that alters inflammatory markers is the body-mind saying: I am one thing. I have always been one thing. Your buildings are your problem, not mine.
The split is the most fundamental structural failure in the human enclosure because it distorts the system's ability to diagnose everything else. If you cannot see the organism as one thing, you cannot identify organism-level problems. You will see a "mental health crisis" when you should see an inflammation crisis. You will see "anxiety" when you should see a gut microbiome that has been stripped of its regulatory bacteria by a diet the body building never assessed. You will prescribe a talking therapy for a problem that originates in a food supply, in a light environment, in a built world that keeps the animal sitting still, indoors, under fluorescent light, eating substances that its evolutionary biology has never encountered. We have been doing this for decades. We are doing it now. We will continue doing it until we stop treating the organism as two things and start treating it as the one thing it has always been.
The split, like every system in this book, was a good idea that scaled badly. Specialisation was the right impulse. The Cartesian framework was the wrong scaffold. The organism does not have a mind and a body. It has a life. And that life is lived, overwhelmingly, in one place -- a place that the species evolved to inhabit and has, for reasons the next chapter will examine, almost entirely abandoned.
The species that evolved under the sky now spends ninety percent of its time under a roof. What has that done to us? What has it cost?
The split between mind and body is mirrored by a split between indoors and outdoors -- and the consequences for the animal are, if anything, more severe. Chapter 15 examines what happens when a species evolved for the full sensory spectrum of the natural world seals itself inside buildings of its own design.