Consciousness is a quality of the mind generally regarded to comprise qualities such as subjectivity, self-awareness, sentience, sapience, and the ability to perceive the relationship between oneself and one's environment. Philosophers divide consciousness into phenomenal consciousness which is experience itself and access consciousness which is the processing of the things in experience (Block 2004).

In common parlance, consciousness denotes being awake and responsive to one's environment; this contrasts with being asleep or being in a coma. The term 'level of consciousness' denotes how consciousness seems to vary during anesthesia and during various states of mind such as day dreaming, lucid dreaming, imagining etc. Nonconsciousness exists when consciousness is not present. There is speculation, especially amongst religious groups, that consciousness may exist after death or before birth.

Consciousness is notoriously difficult to define or locate. Many cultures and religious traditions place the seat of consciousness in a soul separate from the body. Conversely, many scientists and philosophers consider consciousness to be intimately linked to the neural functioning of the brain.

An understanding of necessary preconditions for consciousness in the human brain may allow us to address important ethical questions. For instance, to what extent are non-human animals conscious? At what point in fetal development does consciousness begin? Can machines ever achieve conscious states? These issues are of great interest to those concerned with the ethical treatment of other beings, be they animals, fetuses, or in the future, machines.


Consciousness and language

Because humans express their conscious states using language, it is tempting to equate language abilities and consciousness. There are, however, speechless humans (infants, feral children, aphasics), to whom consciousness is attributed despite language lost or not yet acquired. Moreover, the study of brain states of non-linguistic primates, in particular the macaques, has been used extensively by scientists and philosophers in their quest for the neural correlates of the contents of consciousness.

Cognitive neuroscience approaches

Modern investigations into and discoveries about consciousness are based on psychological statistical studies and case studies of consciousness states and the deficits caused by lesions, stroke, injury, or surgery that disrupt the normal functioning of human senses and cognition. These discoveries suggest that the mind is a complex structure derived from various localized functions that are bound together with a unitary awareness.

Several studies point to common mechanisms in different clinical conditions that lead to loss of consciousness. Persistent vegetative state (PVS) is a condition in which an individual loses the higher cerebral powers of the brain, but maintains sleep-wake cycles with full or partial autonomic functions. Studies comparing PVS with healthy, awake subjects consistently demonstrate an impaired connectivity between the deeper (brainstem and thalamic) and the upper (cortical) areas of the brain. In addition, it is agreed that the general brain activity in the cortex is lower in the PVS state. Some electroneurobiological interpretations of consciousness characterize this loss of consciousness as a loss of the ability to resolve time (similar to playing an old phonographic record at very slow or very rapid speed), along a continuum that starts with inattention, continues on sleep and arrives to coma and death.

Loss of consciousness also occurs in other conditions, such as general (tonic-clonic) epileptic seizures, in general anaesthesia, maybe even in deep (slow wave) sleep. The currently best suhe need for 1) a widespread cortical network, including particularly the frontal, parietal and temporal cortices, and 2) cooperation between the deep layers of the brain, especially the thalamus, and the upper layers; the cortex. Such hypotheses go under the common term "globalist theories" of consciousness, due to the claim for a widespread, global network necessary for consciousness to interact with non-mental reality in the first place.

Brain chemistry affects human consciousness. Sleeping drugs (such as Midazolam = Dormicum) can bring the brain from the awake condition (conscious) to the sleep (unconscious). Wake-up drugs such as Anexate reverse this process. Many other drugs (such as heroin, cocaine, LSD, MDMA) have a consciousness-changing effect.

There is a neural link between the left and right hemispheres of the brain, known as the corpus callosum. This link is sometimes surgically severed to control severe seizures in epilepsy patients. Tests of these patients have shown that after the link is completely severed, each hemisphere possesses its own sense of self and each has a separate awareness from the other. It is as if two separate minds now share the same skull, but both still represent themselves as a single "I" to the outside world.

The bilateral removal of the Centromedian nucleus (part of the Intra-laminar nucleus of the Thalamus) appears to abolish consciousness, causing coma, PVS, severe mutism and other features that mimic brain death. The centromedian nucleus is also one of the principal sites of action of general anaesthetics and anti-psychotic drugs.

Philosophical approaches

Philosophers distinguish between phenomenal consciousness and access or psychological consciousness. Some suggest that consciousness resists or even defies definition. There are many philosophical stances on consciousness, sometimes known as 'isms', including: behaviorism, cognitivism, dualism, idealism, functionalism, phenomenalism, physicalism, pseudonomenalism, emergentism, and mysticism.

Phenomenal and access consciousness

Philosophers call mental events themselves phenomenal consciousness. Phenomenal consciousness is simply experience, it is moving, coloured forms, sounds, sensations, emotions and feelings with our bodies and responses at the centre. The Hard problem of consciousness is how to explain a state of phenomenal consciousness in terms of its neurological basis (Chalmers 1996). Some philosophers, such as Descartes in his famous phrase "cogito, ergo sum", believe that phenomenal consciousness is incorrigible, meaning that it cannot be doubted.

Access consciousness means something like awareness, or that a mind is directed at something. (That sounds more like a definition of that philosophical term "intentionality" often referred to with the layman's term "aboutness".) So when we perceive, we are conscious of what we perceive; when we introspect, we are conscious of our thoughts; when we remember, we are conscious of something that happened in the past, or of some piece of information that we learned; and so on. Naive and Direct Realists believe that access consciousness is all that needs to be known about consciousness because they regard phenomenal consciousness as the world itself.

Events that occur in the mind or brain that are not within phenomenal consciousness are known as unconscious events.

The description and location of consciousness

Although it is the conventional wisdom that consciousness cannot be defined, philosophers have been describing it for centuries. Rene Descartes wrote Meditations on First Philosophy in the seventeenth century, and this contains extensive descriptions of what it is to be conscious. Descartes described consciousness as things laid out in space and time that are viewed from a point. Each thing appears as a result of some quality such as colour, smell etc. (philosophers call these qualities 'qualia'). Other philosophers such as Nicholas Malebranche, John Locke, David Hume and Immanuel Kant also agreed with much of this description although some avoid mentioning the viewing point. The extension of things in time was considered in more detail by Kant and James. Kant wrote that "only on the presupposition of time can we represent to ourselves a number of things as existing at one and the same time (simultaneously) or at different times (successively)". William James stressed the extension of experience in time and said that time is "the short duration of which we are immediately and incessantly sensible". These philosophers also go on to describe dreams, thoughts, emotions etc.

Philosophers have provided a description of consciousness that is like our own experience. When we look around a room or have a dream, things are laid out in space and time and viewed as if from a point. However, when philosophers and scientists consider the location of the contents of consciousness there are fierce disagreements. Some philosophers and scientists do not hold that every mental event has a direct physical event (weak or no 'Supervenience'). As an example, Descartes proposed that the contents of consciousness are images in the brain and the viewing point is some special, non-physical place without extension (the Res Cogitans). This idea is known as 'Cartesian Dualism'. Another example is found in the work of Thomas Reid who thought the contents of consciousness are the world itself which becomes conscious experience in some way through a chain of cause and effect. The precise physical substrate of conscious experience in the world, such as photons, photochemicals, quantum fields etc. is not specified. This idea of a chain of cause and effect or chain of relations causing conscious experience to supervene on the world is found in post-modernism and some forms of behaviourism. There are few examples of scientists and philosophers who adhere to the idea that mental events are directly physical events in the brain. Those who do propose this usually argue that we only think that the descriptions of consciousness occur (eg: Daniel Dennett) although some proponents of Quantum mind, space-time theories of consciousness and Electromagnetic theories of consciousness suggest a direct correspondence between brain activity and experience.

The concept of supervenience is closely related to the idea of emergentism. It is sometimes held that consciousness will emerge from the complexity of brain processing (see for instance the Multiple Drafts Model of consciousness). The general label 'emergence' allows a new physical phenomenon to be implied by physicalist theorists without specifying the exact nature of the phenomenon. This leaves an explanatory gap.

Quantum mechanical approaches

The physicist Roger Penrose, in his book Shadows of the Mind, argued for a quantum mind approach, suggesting that non-local quantum mechanical effects within sub-neural structures give rise to conscious states. He has argued for the need for a fundamentally new physics in order to explain consciousness, which he conceives as a fungible material: one of which any portion can substitute another. ('Shadows' is effectively a second edition of The Emperor's New Mind. Penrose is keen to stress that it replaces that older work).

This central feature of the quantum approach is of much importance: quantum mechanical theories of consciousness investigate such an entity, namely an experience having the property of being fungible, and their work has not hitherto faced the problems set by experience as it is found, namely as distinctively exclusive and "unbarterable". This for other researchers makes a main controversial point.

Penrose was not the first to suggest a link between consciousness and QM; Michael Lockwood and Henry Stapp got there first, and so did Brian Flanagan. Before them there was Bohr, the father of quantum mechanics (QM), who, as David Bohm tells us, "suggests that thought involves such small amounts of energy that quantum-theoretical limitations play an essential role in determining its character." Also of interest are the ideas of Weyl, Wigner, and Schrodinger. All of them shared in the view of consciousness as a fungible reality; adversaries of this stance call it "antipersonalism" and argue that such a construct has never been factually found.

The Uncollapsing theorem is an unproven conjecture that if an observer is in a deep altered state of consciousness they will not collapse the wavefunction of a system when they observe it but rather uncollapse it, meaning they can move the wavefunction into a wider possibility states. This would explain certain experiences of meditators who feel they move into a larger space of possibilities when they go into deep meditation.

In sum, no real evidence has been found to support any specific relationship between quantum mechanics and the occurrence of consciousness.

Spiritual approaches


In Buddhism, consciousness-only (Sanskrit vijapti-mātratā, vijapti-mātra, citta-mātra; Chinese 唯心 pinyin wei xin) is a theory according to which all existence is nothing but consciousness, and therefore there is nothing that lies outside of the mind. This means that conscious-experience is nothing but false discriminations, imaginations; a provisional antidote; thus, the notion of consciousness-only is an indictment of the problems the activities of consciousness engender.

Integral approach to consciousness

Especially in his book Integral Psychology, Ken Wilber has attempted to develop an integral approach to consciousness.

Functions of consciousness

We generally agree that our fellow human beings are conscious and that lower life forms such as bacteria are not. Many of us attribute consciousness to higher-order animals such as dolphins and primates; academic research is investigating which rather than whether animals are conscious. This suggests the hypothesis that consciousness has co-evolved with life, which would require it to have some sort of added value. People have therefore looked for specific functions of consiousness. Bernard Baars (1997) for instance states that “consciousness is a supremely functional adaptation” and suggests a variety of functions in which consciousness plays a role: prioritization of alternatives, problem solving, decision making, brain processes recruiting, action control, error detection, planning, learning, adaptation, context creation, and access to information. Antonio Damasio (1999) regards consciousness as part of an organism’s survival kit, allowing planned rather than instinctual responses. He also points out that awareness of self allows a concern for one’s own survival, which increases the drive to survive. Although how far consciousness is involved in behaviour is an actively debated issue. Many psychologists, such as radical behaviourists, and many philosophers, such as those who support Ryle's approach, would maintain that behaviour can be explained by non-conscious processes akin to artificial intelligence and might consider consciousness to be epiphenomenal or only weakly related to function.

Tests of consciousness

As there is still not a clear definition of consciousness, no empirical tests currently exist to test consciousness as a whole. Some have even argued that empirical tests of consciousness are intrinsically impossible. However, some researchers have devised tests to detect what they feel are certain aspects of consciousness. A test similar to this was used in the novel "Do Androids Dream of Electric Sheep" by Philip K. Dick to see if a person was a robot or an actual human. In the Ridley Scott movie Blade Runner which was inspired by that book, the test is known as the "Voigt-Kampf" test and tests the subject for empathy.

Turing Test

Alan Turing proposed what is now known as the Turing test to determine if a computer could simulate human conversation undetectably. This test is commonly cited in discussion of artificial intelligence. The application to consciousness is highly suggestive, but not clear. One is reminded of Edsger Dijkstra's comment "The question of whether a computer can think is no more interesting than the question of whether a submarine can swim".

Mirror test

With the mirror test, devised by Gordon Gallup in the 1970s, one is interested in whether or not animals are able to recognize themselves in a mirror. Such self-recognition is said to be an indicator of consciousness. Humans (older than 18 months), great apes (except for gorillas), and bottlenose dolphins have all been observed to pass this test.

See also

Cognitive Neuroscience


Physical Theories of Consciousness



Further reading


  • Baars, B. (1997). In the Theater of Consciousness. New York, NY: Oxford University Press.
  • Blackmore, S. (2004). Consciousness: an Introduction. Oxford: Oxford University Press.
  • Block, N. (2004). The Encylopedia of Cognitive Science. (
  • Chalmers, David. (1996). The Conscious Mind: In Search of a Fundamental Theory. New York: Oxford University Press. ISBN 0-19-511789-1
  • Cleermans, A. (Ed.) (2003). The Unity of Consciousness. Oxford: Oxford Univerisity Press
  • Damasio, A. (1999). The Feeling of What Happens. New York: Harcourt Press.
  • Dennett, D. (1991). Consciousness Explained, Boston: Little & Company.
  • Koch, C. (2004). The Quest for Consciousness. Englewood, CO: Roberts & Company.
  • Libet, Benjamin, Anthony Freeman & Keith Sutherland, ed. (1999). The Volitional Brain: Towards a neuroscience of free will. Exeter, UK: Short Run Press, Ltd.
  • Metzinger, T. (2003). Being No One: the Self-model Theory of Subjectivity. Cambridge, MA: MIT Press.
  • Metzinger, T. (Ed.) (2000). The Neural Correlates of Consciousness. Cambridge, MA: MIT Press.

(Consciousness and quantum mechanics)

  • Bourget, D. (2004), ‘Quantum Leaps in Philosophy of Mind’, Journal of Consciousness Studies 11 (12).
  • Eccles, J.C. (1994), How the Self Controls its Brain, (Springer-Verlag).
  • Hagan, S., Hameroff, S.R., and Tuszyński, E. (2000), ‘Quantum Computation in Brain Microtubules: Decoherence and biological feasibility’, Physical Review E, 65.
  • Hodgson, D. (2002), ‘Quantum Physics, Consciousness, and Free Will’, in R. Kane (ed.) The Oxford Handbook of Freewill (Oxford University Press).
  • Lockwood, M. (1989), Mind, Brain and Quantum (Oxford: Oxford University Press).
  • Mulhauser, G. R. (1995), ‘On the End of the Quantum Mechanical Romance’, Psyche, 2 (5).
  • Penrose, R., Hameroff, S. R. (1996), ‘Conscious Events as Orchestrated Space-Time Selections’, Journal of Consciousness Studies, 3 (1), pp. 36-53.
  • Stapp, H.P. (1993), Mind, Matter, and Quantum Mechanics, First Edition (Springer-Verlag).
  • Tegmark, M. (1999), ‘The importance of Decoherence in Brain Processes’, Physical Review E, 61, pp. 4194-4206.

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