Consciousness

• 1. Introduction

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  Consciousness is the fact of being aware of something inside yourself, or of states and objects in the world around you. Philosophers, scientists, and theologians have argued over it for millennia, and they still have not agreed on what exactly should be studied, or even whether it counts as a scientific concept. In 1989, the psychologist Stuart Sutherland wrote a dictionary entry on it that ended with a startling line: "Nothing worth reading has been written on it." That verdict sits oddly beside the sheer volume of writing the subject has produced. People reach for metaphors to pin it down. They call it a stream, an inner world, a small midline gland, a theater stage, even a giant mythical bird. So what is the thing all these metaphors are reaching for? Why has a word in plain daily use proven so slippery to define? And what happens when you try to find it inside a living brain?

• 2. A Word Born In Secrets

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  In 1643, an English writer applied the adjective "conscious" to inanimate objects, describing "the conscious Groves." That figurative use is the first recorded appearance of the word in English, where both "conscious" and "consciousness" date only to the 17th century. The root lay in the Latin conscius, joining con, meaning "together," with scio, "to know." It meant knowing something with another person, especially the way two people share a secret.

  Thomas Hobbes captured this older sense in Leviathan in 1651, writing that when two or more men know one and the same fact, "they are said to be Conscious of it one to another." Latin writers also used the phrase conscius sibi, literally "knowing with oneself," which carried the figurative meaning of knowing that one knows. English translators rendered it as "conscious to oneself." Archbishop Ussher used exactly that form in 1613, describing himself as "so conscious unto myself of my great weakness."

  Cicero and other Roman legal writers used a related word, conscientia, meaning "knowledge-with." In their juridical texts it described what a witness knows of another person's deeds, a shared knowledge carrying moral weight. René Descartes, writing in Latin in the first half of the 17th century, is usually credited as the first philosopher to bend conscientia toward the modern idea of conscience, though he never defined his meaning. In one work he glossed the word as conscientiâ, vel interno testimonio, translatable as "conscience, or internal testimony."

  The modern concept is most often traced to John Locke. In his Essay Concerning Human Understanding, published in 1690, he defined the word as "the perception of what passes in a man's own mind." That definition shaped 18th-century British philosophy and reappeared in Samuel Johnson's celebrated Dictionary of 1755. The scholar Timothy M. Harrison has shown that the shift from "conscience" to "consciousness" ran partly through the poetry of John Milton.

• 3. The Definition Problem

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  Scholars cannot even agree on whether Aristotle had a concept of consciousness at all. He used no single term clearly matching what Locke later defined, though Victor Caston argues he held a notion closer to perception. The trouble is that the modern dictionary meanings evolved over centuries and still pull in different directions. Some stress inward awareness of a psychological or spiritual fact; others stress awareness of an external object or state.

  The Cambridge English Dictionary keeps it plain, calling consciousness "the state of being awake, thinking, and knowing what is happening around you." The Oxford Living Dictionary offers "the state of being aware of and responsive to one's surroundings." Philosophers, dissatisfied with such breadth, built their own jargon. The Routledge Encyclopedia of Philosophy lists four main topics the word has covered: knowledge in general, intentionality, introspection, and phenomenal experience.

  Stuart Sutherland's skeptical entry put the problem bluntly, warning that many people fall into the trap of equating consciousness with self-consciousness. To be conscious, he noted, it is only necessary to be aware of the external world. Yet using "awareness" as a synonym creates its own tangle. If mere awareness of the environment is the test, then even protozoans qualify as conscious. If awareness of awareness is required, then it becomes doubtful whether the great apes or human infants make the cut.

  In 1974, the philosopher Thomas Nagel used "consciousness," "conscious experience," and "subjective experience" as synonyms, and introduced what became known as the "what it's like" locution to point at the impenetrable subjectivity of any organism. Peter Hacker turned sharply against this. In 2002 he attacked the phrase as "malconstructed" and meaningless English. By 2012 he claimed Nagel had laid the groundwork for forty years of fresh confusion, and called the contemporary conception embraced by the consciousness studies community "incoherent."

• 4. From Inner World To Stream

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  During the early 19th century, the young science of geology suggested a tempting picture: that the mind, like the earth, had hidden layers recording the individual's past. By 1875, most psychologists believed consciousness was only a small part of mental life, an idea that underlies the goal of Freudian therapy. Other sciences lent their own metaphors. Johann Friedrich Herbart described ideas as attracting and repelling like magnets. John Stuart Mill spoke of "mental chemistry" and "mental compounds." Edward B. Titchener searched for the "structure" of the mind by breaking it into elements.

  In 1892, William James noticed that the "ambiguous word 'content' has been recently invented instead of 'object'." James gave the field one of its most enduring images, the stream of consciousness, complete with continuity, fringes, and transitions. He was honest about the difficulty. The terms "inner experience" and "personal consciousness," he admitted, are known only in a rough way; to give an accurate account of personal consciousness, he wrote, "is the most difficult of philosophic tasks."

  James also voiced a deeper doubt about the whole notion of an inner realm. Thoughts and feelings, he observed, have never been doubted, while the outer world has often been denied. Yet for his part, he could not feel sure that consciousness as an inner activity was "a sensibly given fact" rather than a postulate. By the 1960s, for many philosophers and psychologists, the word had stopped meaning the inner world and become an indefinite, large category called awareness. One writer noted that ancient Greeks really had no concept of consciousness, since they did not lump together problem solving, remembering, imagining, perceiving, feeling pain, dreaming, and acting as varieties of one thing.

• 5. Slicing Up Awareness

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  Ned Block argued that debates on consciousness often fail to separate two very different things, which he labeled P-consciousness and A-consciousness. P-consciousness is raw experience: moving colored forms, sounds, sensations, emotions, and feelings, considered apart from any effect on behavior. Experiences taken this way are called qualia. A-consciousness is different. It is the phenomenon by which information in our minds becomes accessible for verbal report, reasoning, and the control of behavior.

  Block conceded he could not define P-consciousness "in any remotely noncircular way." Daniel Dennett disputed whether the distinction was even valid, while others broadly accepted it. David Chalmers argued that A-consciousness can in principle be understood in mechanistic terms, but that P-consciousness is far harder. He named this the hard problem of consciousness. Block himself wondered whether A-consciousness could exist without P-consciousness, imagining a "zombie" that is computationally identical to a person yet has no subjectivity, though he admitted he did not know whether any actual cases exist.

  William Lycan pushed the carving further. In his book Consciousness and Experience he identified at least eight clearly distinct types, including organism consciousness, control consciousness, reportability, introspective consciousness, and self-consciousness, and said even that list left out more obscure forms. The philosopher Sam Harris framed the whole matter from the inside, observing that at the level of your experience you are not a body of cells and atoms but "consciousness and its ever-changing contents."

  Christopher Tricker found a symbol for this ever-present field in the mythical bird that opens the Daoist classic the Zhuangzi. The bird's name is Of a Flock, peng, yet its back is countless thousands of miles across and its wings are like clouds arcing across the heavens. The wings of your consciousness, Tricker writes, span to the horizon, and so do the wings of every other being's consciousness; you are one bird among kin.

• 6. Mind Or Matter

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  Mental processes and brain events seem to be correlated, yet the exact nature of the link remains unknown. Descartes was the first influential philosopher to tackle the question head-on, and his answer is known as mind-body dualism. He placed consciousness in an immaterial domain he called res cogitans, the realm of thought, set against res extensa, the realm of extension, or material things. He proposed the two interact inside the brain, perhaps in a small midline structure called the pineal gland. Later philosophers generally found his account of the problem cogent, but his pineal-gland solution has been especially ridiculed, and no alternative has won general acceptance.

  Proposed solutions split into two broad camps. Dualist solutions keep Descartes's hard line between consciousness and matter but differ on how the realms relate. Monist solutions hold there is really only one realm, of which mind and matter are both aspects. The two main dualisms are substance dualism and property dualism. The three main monisms are physicalism, idealism, and neutral monism, and surveys indicate that physicalism is now the dominant position among contemporary philosophers of mind.

  Julien Offray de La Mettrie was the first influential writer to argue explicitly that consciousness could be explained in purely physical terms, in his book Man a Machine, though his arguments were very abstract. The most influential modern physical theories rest on psychology and neuroscience. Neuroscientists such as Gerald Edelman and Antonio Damasio, and the philosopher Daniel Dennett, sought to explain consciousness through neural events, while Christof Koch explored its neural basis without building an all-encompassing theory.

  A few theoretical physicists insisted that classical physics cannot capture the holistic aspects of consciousness and looked to quantum theory. The resulting quantum mind theories include the holonomic brain theory of Karl Pribram and David Bohm, and the Orch-OR theory of Stuart Hameroff and Roger Penrose. None has been confirmed by experiment. An experiment on wave function collapse led by Catalina Curceanu in 2022 suggested that quantum consciousness is highly implausible, and Max Tegmark's calculation of decoherence timescales concluded that the brain is adequately described by classical physics.

• 7. Hunting For The Trace

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  For many decades mainstream scientists avoided consciousness as a research topic, on the feeling that a subjectively defined phenomenon could not be studied by objective methods. That changed gradually. In 1975 George Mandler published an influential study distinguishing slow, serial, limited conscious processes from fast, parallel, extensive unconscious ones. The 1984 annual conference of the Science and Religion Forum, titled "From Artificial Intelligence to Human Consciousness," named the subject a matter for investigation, with Donald Michie as a keynote speaker. From the 1980s, a growing community formed the field of Consciousness Studies, with journals and groups such as the Association for the Scientific Study of Consciousness.

  Verbal report became the gold standard for ascribing consciousness. Subjects staring at a Necker cube report it "flipping" between two 3D configurations even though the drawing never changes. But verbal report restricts study to humans with language, leaving out other species, pre-linguistic children, and people whose brain damage impairs speech. In medicine, consciousness is instead assessed as a combination of verbal behavior, arousal, brain activity, and purposeful movement, and the degree of consciousness is measured on scales such as the Glasgow Coma Scale.

  Much of the literature consists of studies seeking the neural correlates of consciousness, the brain activity strongly predictive of awareness. Brain imaging techniques such as EEG and fMRI provide physical measures. Activity in primary sensory areas turns out not to be enough: subjects can report no awareness even when the primary visual cortex, V1, responds clearly to a stimulus. Higher regions look more promising, especially the prefrontal cortex, and a top-down flow of activity from the frontal cortex to sensory areas predicts awareness better than a bottom-up flow. In 2013 the perturbational complexity index was proposed, measuring the cortex's response to transcranial magnetic stimulation, and it ran higher in people who were awake, in REM sleep, or locked-in than in those in deep sleep or a vegetative state.

  Francis Crick, co-discoverer of the double helix, proposed in 2004 that binding an individual's experience requires a conductor, like an orchestra's, to collate information rapidly across the brain. With Christof Koch he nominated a structure called the claustrum, and Crick died while working on the idea. A 2014 study at George Washington University lent support when stimulating the claustrum induced unconsciousness in a 54-year-old woman with intractable epilepsy. A study published in 2022 pushed back, casting the claustrum less as a seat of consciousness than as a "router" that transfers commands and information across the brain.

• 8. Did It Help Us Survive

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  Thomas Henry Huxley argued that consciousness does nothing at all. In an essay on the hypothesis that animals are automata, he defended an epiphenomenalist view in which consciousness is a causally inert effect of neural activity, like "the steam-whistle which accompanies the work of a locomotive engine" without influencing its machinery. William James objected in his essay Are We Automata, reasoning that if natural selection preserved and developed consciousness, it likely had survival value, and could only have had such value if it actually did something. Karl Popper developed a similar evolutionary argument in The Self and Its Brain.

  Opinions divide sharply over when consciousness first arose. It has been argued that it emerged only with the first humans, only with the first mammals, independently in mammals and birds, or with the first reptiles. Others trace it to the first animals with nervous systems, or to early vertebrates in the Cambrian over 500 million years ago. Donald Griffin suggested a gradual evolution in his book Animal Minds, while Peter Godfrey Smith explored the origins in molluscs in his book Metazoa.

  Peter Carruthers proposed one possible advantage: that consciousness lets an individual tell appearance from reality, recognizing that water in the distance might be a mirage, and even manipulating how things appear to others. Critics counter that no one has explained why a functionally equivalent non-conscious organism, a philosophical zombie, could not gain the very same advantages. If evolution is blind to whether a function is performed consciously, it is unclear what edge consciousness adds. This pushed some theorists toward an exaptive account, in which consciousness did not evolve as an adaptation but arose as a by-product of other changes such as increases in brain size. They compared it to the blind spot in the retina, not an adaptation but a consequence of how the retinal axons were wired.

  Gordon Gallup turned the question of self-awareness into an experiment in the 1970s with the mirror test. A spot of coloring is placed near the forehead, and the animal passes if it tries to touch or remove the spot on its own body. Humans older than 18 months, other great apes, bottlenose dolphins, orcas, pigeons, European magpies, and elephants have all passed, while pigs instead use the mirror to find food.