What is consciousness?
Why is there something it is like to be you?
The question
Right now you are having experiences: the shapes of these letters, a sound in the room. The brain is a physical organ, and everything it does is electrical and chemical activity. Why does some of that activity come with an inner point of view — why is there “something it is like” to be you, in the philosopher Thomas Nagel’s phrase, rather than information processed in the dark?
It helps to split this in two. Which brain processes go together with experience? That question is being studied scientifically. Why do those processes feel like anything at all? David Chalmers called this the “hard problem”.
What we know
- Consciousness depends closely on the brain. Anaesthesia switches it off and on, it fades in deep dreamless sleep, and brain injuries can remove specific parts of experience, such as the perception of colour or motion.
- Most of what the brain does is not conscious. The cerebellum holds most of the brain’s neurons, yet damage to it barely changes experience. Images shown too briefly or masked can influence behaviour without being noticed.
- Researchers compare trials in which the input is the same but awareness differs — a faint picture seen on some trials and missed on others. This is the search for the “neural correlates of consciousness”. The back of the cortex appears central; the role of the front is disputed.
- Consciousness can now be estimated in patients who cannot respond: a magnetic pulse stimulates the cortex, and the complexity of the brain’s “echo” separates wakefulness from deep sleep and anaesthesia.
Main hypotheses
Global workspace
Consciousness is the brain-wide broadcast of information. When a representation “ignites” a network of frontal and parietal areas, it becomes available to memory, speech and planning at once; that availability is what being conscious of it means.
Integrated information theory
Consciousness is a particular kind of causal structure: a system whose parts constrain one another as a whole (“integrated information”). It implies that some very simple systems have a little consciousness, while ordinary computers, however clever, may have almost none. It is also among the most disputed theories: its central quantity is extremely hard to measure.
Higher-order theories
A mental state becomes conscious when the brain represents itself as being in that state: a kind of internal monitoring.
Other views
Predictive processing treats experience as the brain’s best guess about the causes of its signals. Illusionists argue that the sense of an ineffable inner “show” is itself constructed by the brain, and that explaining this dissolves the hard problem. Panpsychists propose that experience is a basic feature of matter.
Where understanding ends
We can list correlations between brain activity and what people report, but not why any of them should be accompanied by experience at all. Nor can we rule out that an animal very unlike us, or a machine, has experiences it cannot tell us about.
This creates a measurement problem at the root of the field. Every test relies on reports, behaviour or brain signals — exactly what a system without experience could, in principle, also produce. Theories that disagree deeply about who is conscious often agree on the cases a laboratory can check.
In 2025 the first large “adversarial collaboration” reported its results. Supporters of the two leading theories had agreed in advance on predictions that would favour one over the other. The data matched some predictions of each and substantially challenged key claims of both.
Even the basic boundaries are unclear. Are newborns conscious? Fish? Insects? Is consciousness all-or-nothing, or does it come in degrees? Is it even a single phenomenon?
What would settle it
- A theory that makes risky, precise predictions — which lesions, drugs or stimulations will change experience, and how — and survives them better than its rivals.
- A measure of consciousness that works without reports — in patients, infants, animals — validated where the answer is known.
- For the hard problem itself, it is unclear what evidence would count. Some expect it to fade as the science matures, the way “what is life?” faded once biochemistry explained metabolism and heredity. Others think a gap will remain whatever we learn. Even what an answer would look like is disputed.
Sources
- Nagel, T. (1974). What is it like to be a bat? The Philosophical Review, 83(4), 435–450.
- Chalmers, D. J. (1995). Facing up to the problem of consciousness. Journal of Consciousness Studies, 2(3), 200–219.
- Casali, A. G., et al. (2013). A theoretically based index of consciousness independent of sensory processing and behavior. Science Translational Medicine, 5(198), 198ra105.
- Koch, C., Massimini, M., Boly, M., & Tononi, G. (2016). Neural correlates of consciousness: progress and problems. Nature Reviews Neuroscience, 17(5), 307–321.
- Tononi, G., Boly, M., Massimini, M., & Koch, C. (2016). Integrated information theory: from consciousness to its physical substrate. Nature Reviews Neuroscience, 17(7), 450–461.
- Mashour, G. A., Roelfsema, P., Changeux, J.-P., & Dehaene, S. (2020). Conscious processing and the global neuronal workspace hypothesis. Neuron, 105(5), 776–798.
- Seth, A. K., & Bayne, T. (2022). Theories of consciousness. Nature Reviews Neuroscience, 23(7), 439–452.
- Cogitate Consortium, et al. (2025). Adversarial testing of global neuronal workspace and integrated information theories of consciousness. Nature, 642, 133–142.