What Is Consciousness?

Consciousness is the simple fact that there is something it feels like to be you. It is the redness of a sunset, the sting of a paper cut, the voice of your own thoughts. Science has learned a great deal about which brain activity goes along with conscious experience. But nobody yet knows how, or why, brain activity produces a felt inner life at all.

That makes consciousness an open question, and one of the deepest. It sits where neuroscience, philosophy and, for many people, faith all meet.

What has brain science learned so far?

Quite a lot about the “where” and “when.” Researchers hunt for the neural correlates of consciousness: the smallest set of brain events that together are enough for a particular experience, as the Stanford Encyclopedia of Philosophy describes them. Some striking clues have come from medicine.

• Anesthesia. Doctors switch consciousness off and on every day, yet according to the U.S. National Institute of General Medical Sciences, scientists don’t know exactly how all types of anesthetics work. How the brain “wakes up” afterward is still being studied.
• Split brains. Roger Sperry studied patients whose nerve fibers connecting the brain’s two halves had been cut to treat severe epilepsy. His work showed that each half has somewhat different specialties, and it earned a share of the 1981 Nobel Prize in Physiology or Medicine. It also raised a startling question: can one skull hold two streams of experience?
• Blindsight. Some people with damage to the brain’s primary visual area say they see nothing in part of their field of view. Yet when asked to guess, they can often detect or tell apart things shown there better than chance. Researchers still debate whether this is truly unconscious vision or a faint, degraded kind of seeing.

Together, these findings show that experience is tightly tied to what the brain is doing. Change the brain, and experience changes.

Why is it called the “hard problem”?

In 1995, philosopher David Chalmers drew a famous line in an influential paper. The “easy problems” are about what the brain does: telling objects apart, focusing attention, reporting what we see, controlling behavior. These are hard in practice, but we know what an answer would look like: find the mechanism. The hard problem is different. Why does any of this processing feel like something from the inside, instead of happening “in the dark”?

Think of a thermostat. It senses temperature and responds. We can explain every part of what it does. Yet most of us assume it feels nothing. A brain also senses and responds, only in vastly more complex ways, and somehow it does feel. Explaining that difference is the hard problem. Chalmers himself has even wondered whether a thermostat might have the faintest trace of experience. That is how open the question is.

In 1998, neuroscientist Christof Koch bet Chalmers that science would discover how neurons produce consciousness within 25 years. In June 2023, both agreed the quest was still ongoing, and Chalmers was declared the winner.

Which scientific theories are competing?

Many theories compete. Two of the best known are:

• Global workspace theory. Information becomes conscious when it is broadcast widely across the brain through long-range connections, so memory, planning, language and action can all use it. Picture a stage in a theater: whatever is in the spotlight is shared with the whole audience. Its brain-based version is called global neuronal workspace theory.
• Integrated information theory. Consciousness depends on how much a physical system’s parts are woven together into a unified whole that can’t be broken into independent pieces. The theory even proposes a measure for it.

In an unusual experiment, supporters of both theories agreed in advance on predictions that would tell them apart. The Cogitate Consortium then tested 256 people using several kinds of brain scanning, including recordings from electrodes inside the brain. An early version of the results appeared in 2023, and the full study was published in Nature in 2025. The verdict was humbling. The data matched some predictions of each theory, while substantially challenging key claims of both.

The field also grew heated. In September 2023, a letter signed by 124 scholars argued that integrated information theory should be labeled pseudoscience. It caused an uproar, with other researchers objecting to the label and worrying about polarization. Disagreement this sharp is a sign of a young science, not a settled one.

Could an AI be conscious?

This is open. In 2023, a team of 19 researchers checked AI systems against “indicator properties” drawn from leading scientific theories of consciousness. Their analysis suggested that none of the AI systems of that time was conscious. It also found no obvious technical barriers to building systems that meet those indicators in the future.

There is a catch. Those indicators are only as good as the theories behind them, and the theories are still being tested. A chatbot saying “I feel” is not good evidence on its own, because it learned from human writing full of such words.

What do we still not know?

We don’t know which brain processes are necessary for experience and which are just helpers. We don’t know how to measure consciousness in people who cannot respond, in animals, or in machines. And we don’t know whether the hard problem will ever be solved by more data, or needs a new way of thinking.

Underneath lies an older question: is consciousness entirely produced by the brain? Here science can inform, but belief and philosophy also play a part. Two thoughtful perspectives, offered as perspectives rather than evidence:

• A thoughtful believer might say the hard problem hints that mind is more than matter. Many traditions hold that each person has a soul, and ideas of a self that survives death appear across cultures, as the Stanford Encyclopedia’s entry on dualism notes. The close link between brain and experience, on this view, may show how the soul works through the body, not that no soul exists.
• A thoughtful skeptic might say that every brain change bringing an experience change points to the brain as the source. Science has turned other deep mysteries into ordinary biology before, the skeptic adds, and patience may do the same here.

Both views agree on the evidence we have. They differ on what it means. For more, see Can Science Decide Whether God Exists? and What Happens When We Die?

What would change the answer?

• A theory that makes risky, specific predictions about brain activity, and keeps passing tests that its rivals fail.
• A reliable “consciousness meter” that agrees with what awake people report and works in patients who cannot speak.
• A clear argument, or discovery, showing why certain physical processes must feel like something. That would be a real step on the hard problem itself.

You are using consciousness to read this sentence, and no one can yet fully explain how. That is not a reason for despair. It is an invitation. What would it take to convince you that another being, or a machine, has an inner life? Keep asking.