Somebody Home

Consciousness is the one thing each of us knows directly and that science still cannot explain. What can be measured, where it goes in sleep, anaesthesia and coma, and what the first head-to-head test of two theories found.

A recent headline in New Scientist said that the understanding that even bees have feelings is changing our view of consciousness.[1] My first reaction was that it sounded far-fetched. My second was that I could not say what would count as evidence either way, because I could not say clearly what consciousness is, even in the one case I know from the inside.

This series tries to sort that out in three steps. This first essay is about people: what the word means, what can be measured, and how far the theories have got. The second is about animals: bees, octopuses, fish and crabs, and what the experiments on them can and cannot show. The third is about machines: language models that talk about their feelings, and the researchers and companies now taking the question seriously. A short overview ties them together. The essays describe the evidence and the positions; they do not try to settle the question, because nobody can yet.

On this blog, consciousness has come up before only in passing: in a short 2024 note on Anil Seth’s idea that the brain hallucinates your conscious reality, and in a section of Up to Us, the essay on free will, which asked whether consciousness is the real problem behind it. Here it is the subject.

The word

Three things the word means

“Conscious” is used in at least three ways, and many arguments go wrong by sliding between them.

The first is the level of consciousness: whether someone is awake and responsive at all. A doctor checking a patient after a head injury, or an anaesthetist watching a monitor, is asking about this. The second is the content: what someone is conscious of at a given moment, the red of a traffic light rather than the hum of the engine. Psychologists study this by showing people things at the edge of perception and asking what they saw. The third is the one philosophers mean when they speak of experience. Thomas Nagel put it in 1974 as the claim that there is “something it is like” to be a conscious creature,[2] and asked what it is like to be a bat: whatever it is, a complete account of a bat’s sonar would not tell us.

David Chalmers drew the line that has organised the field since. In 1995 he separated the “easy problems”, which are explaining how the brain discriminates, integrates information, reports and controls behaviour, from the hard one: “The really hard problem of consciousness is the problem of experience.”[3] The easy problems are easy only in the sense that we know what an answer would look like, a mechanism; Chalmers guessed they could take “a century or two of difficult empirical work”. The hard problem is why any of that mechanism is accompanied by experience at all, why the lights are on.

Much of the science of consciousness is about the first two meanings, and it has made real progress there. Whether that progress also chips away at the third is the main point of disagreement, and this essay comes back to it at the end.

Where it goes

Awake is not the same as aware

Every night, consciousness goes away and comes back. In deep sleep early in the night, most people woken and asked report little or nothing; in dreaming sleep they report vivid experiences, while their body is largely paralysed and they do not respond to the room. Being awake and being aware usually go together, but they come apart, and the ways they come apart are the field’s best natural experiments.

Steven Laureys summarised the clinical picture with two dials: wakefulness, which shows in open eyes and cycles of sleep and waking, and awareness of oneself and one’s surroundings.[4] Most states sit on the diagonal. Some do not.

Fig. 1 — Wakefulness and awareness in different states, schematic: the positions are qualitative, not measured. In green, the states off the dashed diagonal where most states sit. After Laureys 2005.

After a severe brain injury, a patient may come out of coma into what used to be called the vegetative state, now often called unresponsive wakefulness: eyes open, sleeping and waking, breathing alone, but showing no sign of awareness, no response to commands, no purposeful movement. Laureys described it as “wakefulness without awareness”. Some patients recover into a minimally conscious state, with flickering, inconsistent signs: following a moving object with their eyes, or responding to a command on some occasions but not others. At the other end, locked-in syndrome is the opposite mismatch: a patient fully aware but paralysed except, often, for movements of the eyes.

General anaesthesia is the controlled version of the same mystery. It is given millions of times a year, and it works, but there is still no complete account of how the drugs switch off experience. It also sometimes fails. The rate depends on how one looks. In a study at seven American hospitals, researchers interviewed 19,575 patients after surgery, using a structured questionnaire, and found 25 who had been aware during the operation and remembered it, 0.13%, or one or two per thousand.[5] The British national audit of 2014 counted the cases that patients reported on their own and found about one in 19,600 anaesthetics; one in 8,200 when muscle-paralysing drugs were used, one in 135,900 without them, and one in 670 for caesarean sections.[6] The two numbers differ by a factor of twenty-five, mostly because a person who is asked will mention things a person left alone will not. Either way, the most frightening cases are those where the drugs that stop movement worked and the drugs that stop experience did not: awake, unable to move, unable to say so.

The patients

Somebody home

That mismatch, awareness with no way to show it, turns out not to be confined to the operating theatre. In 2006 Adrian Owen and colleagues put a woman diagnosed as vegetative into a brain scanner and asked her to imagine playing tennis, and then to imagine walking around her home. Her brain lit up in the areas that light up in healthy volunteers doing the same, “in a manner indistinguishable from that of healthy volunteers”.[7] She could not move a finger on command, but she could, it seemed, follow an instruction with her thoughts.

A larger study in 2010, of 54 patients in Cambridge and Liège, found five who could modulate their brain activity in the same way. Two of them showed no voluntary behaviour at all at the bedside. One used the technique to answer yes or no to questions in the scanner, though no form of communication could ever be set up outside it.[8] The same paper noted that bedside diagnosis of these disorders goes wrong about 40% of the time.

In 2024 a consortium at six centres tested 353 adults with disorders of consciousness, using functional MRI, EEG or both.[9] Among the 241 who showed no response to commands at the bedside, 60, one in four, performed the mental tasks. The researchers call this cognitive motor dissociation: the mind follows the instruction, the body does not. The comparison group makes the point sharper. Among the 112 patients who could respond to commands at the bedside, only 43, 38%, passed the same tests in the scanner. So the tests miss most people who are certainly conscious, which means the true number of hidden-aware patients among the unresponsive is probably higher than one in four, not lower.

Fig. 2 — Patients whose brain activity followed spoken instructions in a scanner or on EEG. Sources: Monti et al. 2010; Bodien et al. 2024.

These are some of the most consequential findings in the whole subject, because decisions about treatment, pain relief and the withdrawal of life support are made on the basis of whether a patient is thought to be aware. They are also a lesson in method. The tests work by asking for a response, only an internal one. A patient who is aware but cannot understand speech, or cannot hold an instruction in mind for thirty seconds, will fail them. Absence of evidence here is weak evidence of absence, and that lesson will matter again for animals and machines.

Measuring it

A measure that does not need an answer

Can consciousness be detected without asking the patient to do anything? Marcello Massimini, Giulio Tononi and colleagues built a measure from a theoretical idea: that a conscious brain is one whose activity is at once integrated, spread across interacting areas, and differentiated, rich in information. They knock on the cortex with a magnetic pulse through the skull, record the echo with EEG, and compress the recorded pattern the way a computer compresses a file. A brain that answers with a complex, widespread pattern that does not compress well scores high; one that answers with a local blip, or with a big uniform wave, scores low. They called it the perturbational complexity index, PCI.[10]

Its validation is the cleverest part. In 2016 the team measured PCI in 150 people whose state could be checked by report: healthy volunteers awake, dreaming, in deep sleep and under different anaesthetics, and brain-injured patients who could communicate. One cut-off separated the conscious from the unconscious conditions with 100% sensitivity and specificity. Applied to patients who could not communicate, it classed 36 of 38 minimally conscious patients as conscious, and 9 of 43 vegetative patients too.[11] Those nine are, on this measure, people whose capacity for experience survived without any way to show it.

PCI does not solve the hard problem; it is calibrated on people who can say whether they were conscious, and it measures a capacity, not the content or quality of an experience. But it shows that the first meaning of the word, the level, can be measured from outside, independent of behaviour. The trouble begins when one tries to take such a test where nobody can report at all. A group of researchers asked in 2024 what tests for consciousness could be trusted in newborns, patients, animals, organoids grown from stem cells and AI systems, and concluded that most proposed tests “are of limited use” and that for many of the populations where they matter most there are none.[12]

The contrast

Seeing without knowing it

The content of consciousness, the second meaning, has been studied mostly by contrast: keep the input the same and compare the brain when it is seen and when it is not. A word flashed for a few hundredths of a second, followed by a pattern, is often not seen; flashed slightly longer, it is. Two images shown one to each eye alternate in awareness, though neither changes. The question is what differs in the brain.

Damage gives the starkest contrasts. In 1974 Lawrence Weiskrantz and colleagues described a man who had lost part of his visual cortex and was blind in part of his field of view.[13] Asked to guess the position or orientation of things shown there, he did far better than chance, while insisting he saw nothing. Weiskrantz called it blindsight. In people whose two brain hemispheres had been surgically separated to treat epilepsy, Roger Sperry and Michael Gazzaniga found that each half could know things the other did not: an object shown only to the right hemisphere could be picked out by the left hand, while the speaking left hemisphere said it had seen nothing.[14] A great deal of what a brain does, then, it does without the person being aware of it. The search for the neural correlates of consciousness is the search for what is added when awareness is there.

It has been slower than its founders hoped. In 1998 the neuroscientist Christof Koch bet the philosopher Chalmers that by 2023 the mechanism by which neurons produce consciousness would be found. On 23 June 2023, at the annual meeting of the Association for the Scientific Study of Consciousness in New York, both agreed it had not been, and declared Chalmers the winner.[15]

The theories

Four families of theory

There is no shortage of theories. A catalogue published in 2024 sorted them into ten broad categories, from materialist ones through quantum theories and panpsychism to dualism and idealism, each with many subcategories.[16] In neuroscience, Anil Seth and Tim Bayne reviewed four prominent families.[17]

Global workspace theories, associated with Bernard Baars and Stanislas Dehaene, say that a piece of information becomes conscious when it is broadcast across the brain, made available at once to memory, planning, speech and action. Dehaene and Changeux describe the signature as an “ignition” of a large network in prefrontal and parietal cortex, linked by long-range neurons.[18] On this view consciousness is a kind of access: what is in the workspace is what the whole system can use.

Higher-order theories, developed by David Rosenthal and Hakwan Lau among others, say a state is conscious when the brain represents itself as being in that state, a kind of inner monitoring, and place much of the work in prefrontal cortex.[19] Seeing red is not enough; something in the brain must, in effect, register that it is seeing red.

Integrated information theory, Tononi’s, starts the other way round: from the features of experience itself, and asks what a physical system must be like to have them. Its answer is a maximum of intrinsic cause-and-effect power, measured by a quantity called phi.[20] It is the most ambitious of the theories, because it claims to say not only which brain areas matter but which physical systems of any kind are conscious, and how much.

Recurrent and predictive theories form the fourth family. Victor Lamme argued that conscious seeing requires feedback, signals looping back from higher to lower visual areas, and that a single forward sweep of processing stays unconscious.[21][17] Predictive processing treats the brain as a machine for guessing the causes of its inputs and correcting the guesses; on Seth’s version, experience is the brain’s running prediction of the world and of the body, anchored in the regulation of a living organism.[22]

How do researchers themselves rate them? A survey of 166 people at the field’s main conference in 2018 and 2019 asked which theories seemed promising.[23] None had a clear majority behind it. Predictive processing came first, which the authors noted was striking, since at the time it was not yet an elaborated theory of consciousness. Two-thirds of the respondents, 67%, thought machines could have consciousness, now or in the future, and most thought consciousness comes in degrees across the animal kingdom.

Fig. 3 — Share of 166 consciousness researchers rating each theory promising, 2018–19. In green, the two theories tested head to head. Source: Francken et al. 2022.

Each theory has its favourite evidence and its awkward cases. The deeper problem is that they were rarely tested against each other. Each school ran the experiments its theory predicted would succeed.

The test

The first head-to-head test

A consortium called Cogitate set out to change that with an “adversarial collaboration”. The proponents of global workspace theory and of integrated information theory, including Dehaene, Tononi and Koch, agreed in advance with neutral teams on an experiment, on what each theory predicted, and on which outcomes would count against each. The design was registered publicly before the data came in.

The full results were published in Nature in 2025.[24] 256 people looked at clearly visible pictures, shown for different lengths of time, while their brains were recorded with three techniques: functional MRI, magnetoencephalography and electrodes placed inside the skull of patients with epilepsy. The authors summarised the outcome as results that “align with some predictions” of both theories “while substantially challenging key tenets of both”. For integrated information theory, the expected sustained synchronisation within the back of the brain, where it locates consciousness, was missing. For global workspace theory, the predicted second burst of ignition when a stimulus disappeared mostly did not happen, and the front of the brain held only some aspects of what was seen.

The first results had been announced at a public event in the summer of 2023, before the paper was available, and the press reported them as partly supporting integrated information theory. That provoked one of the bitterest episodes in the field’s recent history. In September 2023, 124 researchers posted an open letter arguing that the theory should be called pseudoscience.[25][26] The experiment, they wrote, had tested only “idiosyncratic predictions” not logically tied to the core of the theory, while the theory itself implied that “an inactive grid of connected logic gates” could be conscious, possibly more so than people, and that organoids in a dish and early human foetuses probably were. Until the theory as a whole could be tested, they argued, the label applied, and it mattered because of what the theory might be used to decide: the care of coma patients, AI, research on embryos and animals, abortion.

Many in the field thought the letter went too far. The philosopher Philip Goff replied that the theory is “not perfect, either in its scientific or its philosophical aspects”, and that because consciousness is known privately, from the inside, experiments alone cannot settle which theory is right; opponents of the letter pointed out that no current theory has been tested as a whole either.[27] An expanded version of the letter was published in Nature Neuroscience in 2025.[28] The quarrel showed something that the polite version of the field hides: the theories disagree not only about the brain, but about what would count as testing a theory of consciousness at all.

The hard part

The hard problem, and what philosophers think

Behind the neuroscience sits the question it may not be able to answer. Suppose a theory predicted perfectly which brain states go with which experiences. Would it explain why those states feel like anything? Chalmers thinks not, and has argued that one can conceive of a zombie,[29] a being physically identical to a person, behaving the same, with no experience at all. If that is even coherent, he argues, experience is something over and above the physical facts.

The replies cover every option. Functionalists say that a mental state is defined by what it does, so that anything that does what experience does has experience; the identity theory says that experiences simply are brain states, as lightning is an electrical discharge. Illusionists, among them Keith Frankish and the late Daniel Dennett, say that the special, ineffable quality that seems to make the problem hard is itself a trick the brain plays on itself, so that what needs explaining is the conviction that there is a hard problem.[30] At the other end, dualists keep experience as a separate kind of thing, and panpsychists, like Goff, propose that some simple form of experience is a basic feature of matter, so that brains combine it rather than create it.

In the largest survey of professional philosophers, run in 2020, about 62% accepted or leaned towards the view that there is a hard problem.[31] No single view of consciousness had a majority: functionalism led with a third, dualism had more than a fifth, and panpsychism had 8%. On zombies, the largest group, 37%, thought them conceivable but not actually possible; 24% thought them possible.

Fig. 4 — Philosophers’ answers on the hard problem and on what consciousness is, accept or lean towards, target faculty. Source: PhilPapers Survey 2020.

This is not idle. Each answer changes what the science is looking for. A functionalist expects that a complete map of what the brain does will be a complete theory of consciousness. A dualist or panpsychist expects that it will leave something out. An illusionist expects the map to explain, among other things, why we are so sure it leaves something out.

Beyond us

Why the theories matter beyond the brain

The theories were built on human brains, mostly on vision, mostly in laboratory experiments with people who can say what they saw. Their disagreements about brain areas look technical. Their disagreements about everything else are not, because each one, carried outside the skull, gives a different answer about who else is conscious.

Fig. 5 — Four families of theory and what they imply beyond human brains, as summarised in this essay. Sources: Dehaene, Lau & Kouider 2017; Lau & Rosenthal 2011; Tononi & Koch 2015; Lamme 2006; Seth 2025.

Global workspace theorists have argued that consciousness is a matter of a particular kind of computation, selecting information and broadcasting it, plus monitoring one’s own processing, and that current machines “are still mostly implementing computations that reflect unconscious processing” in the human brain; nothing in principle stops an engineer building the rest.[32] Integrated information theory says the opposite: consciousness is graded and common among living things, can occur in very simple systems, but feed-forward networks are not conscious, and a digital computer, even running a faithful simulation of a human brain, “would experience next to nothing”.[33] On higher-order views the question becomes whether a creature can represent its own mental states, which is itself contested for most animals. And Seth, from the predictive side, argues that consciousness depends on being a living organism, so that real artificial consciousness is “unlikely along current trajectories”, though more plausible as machines become more brain-like or life-like.[22]

So the same evidence about bees, or the same conversation with a chatbot, can count as a sign of consciousness on one theory and as irrelevant on another. That is the situation the next two essays walk into. The animal case rests mostly on behaviour and on similarities of brain and evolution; the machine case rests on behaviour and on architecture, and behaviour is exactly what a language model is trained to imitate.

What we know

What is known, and what is not

Some things are now well established. Wakefulness and awareness are different, and can be separated by injury, sleep and drugs. Much of what the brain does, it does without awareness. Some people who cannot move or speak are aware, perhaps one in four of those who seem unresponsive after brain injury, and that can be detected from outside, by imaging, or by measures like PCI that do not depend on behaviour at all. Being able to measure the level of consciousness, at least in human brains, is a real achievement with real consequences for patients.

Other things are not known. There is no agreed theory of which brain activity makes the difference, and the first serious attempt to test two theories against each other challenged both. There is no agreement on what a theory of consciousness would have to do to count as tested. And there is no agreement, among scientists or philosophers, on whether explaining everything the brain does would explain why it feels like something to be one.

That last gap is the one the bee headline leaps over. Nobody can look inside another being and see experience; we infer it in other people from their resemblance to us, their behaviour and their reports. With patients who cannot report, the inference already strains. With an insect, it has only behaviour and a tiny, very different brain to work with. With a machine, it has behaviour designed to resemble ours and a structure unlike any brain. The next essay looks at what the animal evidence actually is.

On method and tools

This essay was researched and written with Claude Opus 5.5 (Anthropic): human specification, editorial direction and critical review; machine research, drafting and figure generation. Anthropic has a stake in the question of machine consciousness, which Part III takes up and where this is discussed. The subject came from Luis, who found the New Scientist headline on bees far-fetched; the article itself could not be read, so nothing from it is used beyond its headline. Figures and quotations from research come from the abstracts of the papers cited, each checked against its PubMed record, except for the survey of researchers by Francken and colleagues, read in full; the letter on integrated information theory, read in the PsyArXiv version; Chalmers’s 1995 paper, read on his site; and the PhilPapers results, read on an archived copy of the results page (December 2023) because the live site blocks automated access. Nagel’s phrase is quoted as Chalmers quotes it. The 1974 blindsight paper, the split-brain review, and the books and papers by Chalmers (1996) and Frankish are described from their standard accounts, not read for this essay; the two Nature news stories were read only as far as their open first paragraphs. There is no model in this essay. scripts/consciousness.py draws the five figures from the published numbers, which it writes to docs/consciousness-results.json; Figure 1 is a schematic, and the table in Figure 5 is my summary of the sources cited, not theirs. The cover is drawn by the same script: one square for each of the 241 unresponsive patients in the 2024 study, with the 60 who performed the tasks lit, in random positions.

Authored by: Luis Matos Ferreira — Physicist, Developer, Writer

The series on consciousness

1. Something It Is Like — a short guide to the series, and why the evidence thins out from people to machines (forthcoming).
2. Somebody Home — human consciousness: patients, theories and the hard problem (this essay).
3. Who Else Is In There — bees, fish, octopuses and crabs: what the experiments show (forthcoming).
4. The Machine That Says It Feels — language models, the indicator tests, what is inside the models, and the companies (forthcoming).

Related essays on this blog

1. Up to Us — free will, and whether consciousness is the real problem behind it.
2. Your brain hallucinates your conscious reality — Anil Seth’s predictive view of experience, in brief.
3. Many Ways to Be Clever — brains and models compared: octopuses, bees, crows and language models.
4. Two-Way Traffic — what AI and neuroscience take from each other.
5. The Complete Parts List — why knowing every part of a living thing does not explain it.

Sources

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