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Consciousness is a quality of the mind generally regarded to comprise such key features as subjectivity, self-awareness, sentience, sapience, and the ability to perceive the relationship between oneself and one's environment. It is a subject of much research in philosophy, psychology, neurology, and cognitive science.

Some philosophers divide consciousness into phenomenal consciousness, which is experience itself, and access consciousness, which is the processing of the things in experience (Block 2004). Others consider this distinction to be mistaken (Dennett 1991). Some philosophers believe that consciousness is part of the physical world whilst others believe it may be separate from the physical world in some sense. Many cultures and religious traditions place the seat of consciousness in a soul that is the mind separate from the body. Many scientists and philosophers consider consciousness to be intimately linked to the neural functioning of the brain, dictating the way by which the world is experienced.

Humans (and often other animals, as well) are variously said to possess consciousness, self-awareness, and a mind that contains our sensations, perceptions, dreams, lucid dreams, inner speech and imagination etc. Each of us has a subjective view. There are many debates about the extent to which the mind constructs or experiences the outer world, the passage of time, and free will.

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 human 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. Panpsychism is the belief that all matter, including rocks for example, is sentient or conscious.

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 among religious groups, that consciousness may exist after death or before birth.


"Consciousness" derives from Latin "conscientia," which primarily means moral conscience. In the literal sense, "conscientia" means knowledge-with, that is, shared knowledge. The word first appears in Latin juridic texts by writers such as Cicero. Here, conscientia is the knowledge that a witness has of the deed of someone else. In Christian theology, conscience stands for the moral conscience in which our actions and intentions are registered and which is only fully known to god. Medieval writers such as Thomas Aquinas describe the conscientia as the act by which we apply practical and moral knowledge to our own actions (Aquinas, De Veritate 17,1 c.a.). René Descartes was the first to use "conscientia" in a way that does not seem to fit this traditional meaning, and, as a consequence, the translators of his writings in other languages like French and English coined new words in order to denote merely psychological consciousness. These are, for instance, conscience, and Bewusstsein. See Catherine G. Davies, Conscience as Consciousness, Oxford 1990, and Hennig, Cartesian Conscientia.

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. At present, the best-supported hypotheses about such cases of loss of consciousness (or loss of time resolution) focus on the 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 alcohol, nicotine, THC, 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. This procedure was first performed by Roger Sperry in the 1960's. Tests of these patients have shown that, after the link is completely severed, the hemispheres are no longer able to communicate, leading to certain problems that usually arise only in test conditions. For example, while the left side of the brain can verbally describe what is going on in the right visual field, the right hemisphere is essentially mute, instead relying on its spatial abilities to interact with the world on the left visual field. Some say that 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.

Neurophysiological studies in awake, behaving monkeys performed by neuroscientists point to advanced cortical areas in prefrontal cortex and temporal lobes as carriers of neuronal correlates of consciousness. Christof Koch and Francis Crick argued that neuronal mechanisms of consciousness are intricatly related to prefrontal cortex — the most advanced cortical area. Experimental work of Steven Wise, Mikhail Lebedev and their colleagues supports this view. They demonstrated that activity of prefrontal cortex neurons reflects illusory perceptions of movements of visual stimuli. Nikos Logothetis and colleagues made similar observations on visually responsive neurons in the temporal lobe. These neurons reflect the visual perception in the situation when conflicting visual images are presented to different eyes (i.e., bistable percepts during binocular rivalry). The studies of blindsight — vision without awareness after lesions to parts of the visual system such as the primary visual cortex — performed by Lawrence Weiskrantz and David P. Carey provided important insights on how conscious perception arises in the brain. In recent years the theory of two visual streams, vision for perception versus vision for action was developed by Melvyn Goodale, David Milner and others. According to this theory, visual perception arises as the result of processing of visual information by the ventral stream areas (located mostly in the temporal lobe), whereas the dorsal stream areas (located mostly in the parietal lobe) process visual information unconsciously. For example, quick catching of the ball would engage mostly the dorsal stream areas, and viewing a painting would be handled by the ventral stream. Overal, these studies show that conscious versus unconscious behaviors can be linked to specific brain areas and patterns of neuronal activation.

Philosophical approaches

Some philosophers suggest that consciousness resists or even defies definition. Others believe it can be usefully distinguished between phenomenal consciousness and access or psychological consciousness, while still others disagree. There are many philosophical stances on consciousness, including: behaviorism, dualism, idealism, functionalism, phenomenalism, physicalism, emergentism, and mysticism. John Locke's chapter XXVII "On Identity and Diversity" in An Essay Concerning Human Understanding (1689) has been said to be one of the first modern conceptualization of consciousness as the repeated self-identification of one-self, through which moral responsibility could be attributed to the subject.

Phenomenal and access consciousness

- Philosophers call our current experience phenomenal consciousness. Phenomenal consciousness is simply experience, it is moving, coloured forms, sounds, sensations, emotions and feelings with our bodies and responses at the center. These experiences, considered independently of any impact on behavior, are called qualia. The hard problem of consciousness was formulated by Chalmers in 1996, dealing with the issue of "how to explain a state of phenomenal consciousness in terms of its neurological basis" (Block 2004). Daniel Dennett(1988) suggests that what people think of as qualia are judgements and consequent behaviour. He extends this analysis (Dennett (1996)) by arguing that phenomenal consciousness can be explained in terms of access consciousness, denying the existence of qualia, hence denying the existence of a "hard problem."

Access consciousness is the phenomenon whereby information in our minds is accessible for verbal report, reasoning, and the control of behavior. So, when we perceive, information about what we perceive is often access conscious; when we introspect, information about our thoughts is access conscious; when we remember, information about the past (e.g., something that we learned) is often access conscious; and so on. Chalmers thinks that access consciousness is less mysterious than phenomenal consciousness, so that it is held to pose one of the easy problems of consciousness. Dennett disagrees, asserting that the totality of consciousness can be understood in terms of impact on behavior, as studied through heterophenomenology.

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

The description and location of phenomenal consciousness

Although it is the conventional wisdom that consciousness cannot be defined, philosophers have been describing phenomenal consciousness for centuries. Rene Descartes wrote Meditations on First Philosophy in the seventeenth century, containing extensive descriptions of what it is to be conscious. Descartes described conscious experience as imaginings and perceptions laid out in space and time that are viewed from a point. Each thing appears as a result of some quality (qualia) such as colour, smell, etc. 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.

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 form and contents of this phenomenal consciousness, there are fierce disagreements. As an example, Descartes proposed that the contents are brain activity seen by a non-physical place without extension (the Res Cogitans), which he identified as the soul. 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. This concept is a type of Direct realism. The precise physical substrate of conscious experience in the world, such as photons, quantum fields, etc. is usually not specified. Other philosophers, such as George Berkeley, have proposed that the contents of consciousness are an aspect of minds and do not involve matter at all. This is a type of Idealism. Yet others, such as Leibniz, have considered that each point in the universe is endowed with conscious content. This is a form of Panpsychism. Panpsychism is the belief that all matter, including rocks for example, is sentient or conscious. The concept of the things in conscious experience being impressions in the brain is a type of representationalism, and representationalism can be a form of indirect realism.

Some philosophers, such as David Armstrong and Daniel Dennett, believe that qualia exist in terms of judgements or beliefs about things in the world, and are therefore meaningless when separated from behavior, while other philosophers insist that qualia cannot be understood in terms of belief. Dennett believes that "ineffable, intrinsic, private" qualia do not exist (Dennett 1988). However, Dennett does not believe that we have no conscious, phenomenal experience he proposes that: "When we marvel, in those moments of heightened self-consciousness, at the glorious richness of our conscious experience, the richness we marvel at is actually the richness of the world outside, in all its ravishing detail. It does not "enter" our conscious minds, but is simply available" (Dennett 1991). This relationship between "conscious experience" and "mind" is similar to that proposed in direct realism but Dennett does not use this term to describe it. Dennett also believes that phenomenal experiences cannot be divorced from their associated behavior and behavioral dispositions, from the judgements we implicitly make about them.

It is sometimes held that consciousness emerges from the complexity of brain processing (see for instance the Multiple Drafts Model of consciousness). The general label 'emergence' applies to new phenomena that emerge from a physical basis without the connection between the two explicitly specified.

Some theorists hold that phenomenal consciousness poses an explanatory gap. Colin McGinn believes that the problem cannot be solved, and Chalmers criticizes purely physical accounts of mental experiences based on the idea that philosophical zombies are logically possible and supports property dualism. But others have proposed scientific theories to explain the explanatory gap, such as Quantum mind, space-time theories of consciousness, and Electromagnetic theories of consciousness to explain the correspondence between brain activity and experience. As yet there is little evidence from brain studies to support these theories.

Evidence from parapsychology of psychokinesis or telepathy, if substantiatied, might support the theory that the location of consciousness is not confined to the brain. However, no evidence has been substantiatied.

Access consciousness

There have been numerous approaches to the processes that act on conscious experience from instant to instant. Philosophers who have explored this problem include Gerald Edelman, G. Spencer-Brown, Edmund Husserl and Daniel Dennett.

Some philosophers have concentrated on reflexive processes to link one instant to the next, some on discriminations and differences between things in conscious experience and others on the overall behaviour of the organism.

G. Spencer-Brown provides an example of the analysis of consciousness as a process, the process in this case being differentiating one thing from another.G. Spencer-Brown proposes in Laws of Form that the root of cognition is the ability to perceive dualism, i.e., in its most simple construct, the capability of differentiating a "this" from a "that." A mathematician, he captured this concept of elementary content-in-context in an abstraction: an algebraic and tautological symbol he referred to as the "Mark," also referred to as a "distinction." Francisco Varela, a co-founder of the Integral Institute, and Humberto Maturana also identify "distinction" as the elementary act of cognition. By definition, this concept extends the notion of "consciousness" well beyond that solely evidenced by humans and lends itself to the idea of a "scale" of consciousness.

Physical approaches

Even at the dawn of Newtonian science, Leibniz and many others were suggesting physical theories of consciousness. Modern physical theories of consciousness can be divided into three types: theories to explain behaviour and access consciousness, theories to explain phenomenal consciousness and theories to explain the quantum mechanical (QM) Quantum mind. Theories that seek to explain behaviour are an everyday part of neuroscience, some of these theories of access consciousness, such as Edelman's theory, contentiously identify phenomenal consciousness with reflex events in the brain. Theories that seek to explain phenomenal consciousness directly, such as Space-time theories of consciousness and Electromagnetic theories of consciousness, have been available for almost a century, but have not as yet been confirmed by experiment. Theories that attempt to explain the QM measurement problem include Pribram and Bohm's Holonomic brain theory, Hameroff and Penrose's Orch-OR theory, Spin-Mediated Consciousness Theory and the Many-minds interpretation. Some of these QM theories offer descriptions of phenomenal consciousness, as well as QM interpretations of access consciousness. None of the quantum mechanical theories has been confirmed by experiment, and there are philosophers who argue that QM has no bearing on consciousness.

There is also a concerted effort in the field of Artificial Intelligence to create digital computer programs that can simulate consciousness.

Spiritual approaches

Spiritual approaches to consciousness involve the idea of altered states of consciousness or religious experience. Changes in the state of consciousness or a religious experience can occur spontaneously or as a result of religious observance. It is also maintained by some religions and religious factions that the universe itself is consciousness.

In shamanic practices, changes in states of consciousness are induced by activities that create trance states, such as drumming, dancing, fasting, sensory deprivation, exposure to extremes of temperature, or the use of psychoactive drugs. The experience that occurs is interpreted as entering a real, but parallel, world. In many polytheistic religions a change in emotional state is often attributed to the action of a god, for instance love was ruled by Aphrodite and Eros in Ancient Greek polytheism. In Hinduism the change in state is induced by the practice of yoga. Yoga means "joining" and is intended to produce a state of oneness between the practitioner and the divine. In Islam and Christianity, the change of state can occur as a result of prayer or as a religious experience.

The change in state of consciousness in Hinduism, Buddhism, Christianity and Islam is reported to be quite similar. The pursuit of yoga and the Buddhist Jhanas involve feelings of oneness with the world that give rise to a state of rapture. This is also reported by those undergoing some forms of Christian (or Islamic) religious experience; for instance, James (1902) provides the following report:

I cannot express it in any other way than to say that I did "lie down in the stream of life and let it flow over me." I gave up all fear of any impending disease; I was perfectly willing and obedient. There was no intellectual effort, or train of thought. My dominant idea was: "Behold the handmaid of the Lord: be it unto me even as thou wilt," and a perfect confidence that all would be well, that all was well. The creative life was flowing into me every instant, and I felt myself allied with the Infinite, in harmony, and full of the peace that passeth understanding. There was no place in my mind for a jarring body. I had no consciousness of time or space or persons, but only of love and happiness and faith.

Meditation is used in some forms of yoga such as Raja Yoga, Hatha Yoga, Transcendental meditation, the Buddhist Jhanas, the Buddhist Immaterial Jhanas (there are several versions of the jhanas in different types of Buddhism), in the practices of Christian monks and Islamic scholars such as Sufis. Meditation can have a calming influence on practitioners, as well as changing the state of consciousness. Therevada Buddhism views the Jhanas and some yogic practices view the early stages of meditation as a preliminary "serenity meditation" in which it is demonstrated that states such as rapture are delusions, products of mind rather than the soul. In most types of Buddhism, serenity meditation is followed by a philosophical "insight meditation" that focuses on the idea that the universe is consciousness only, one that is perhaps indistinguishable from Monism.

Functions of consciousness

We generally agree that our fellow human beings are conscious, and that much simpler 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 the extent to which 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 consciousness. 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 that 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, it 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 that, according to some philosophers, anything capable of passing the Turing test as well as a person is necessarily conscious. Other philosophers say that a philosophical zombie could pass the test yet fail to be conscious. This matter is heavily disputed. Still others take it for granted that computers can think, since this is what they were designed to do; Edsger Dijkstra commented that "The question of whether a computer can think is no more interesting than the question of whether a submarine can swim."

A thought experiment, which is intended to show problems with the Turing Test, is as follows. Imagine a computer in which are stored a very large number of questions and a very large number of actual human responses to these questions. If the number of questions and answers was large enough, then the computer would be able to mimic consciousness by a purely mechanical procedure. Of course, this is a purely hypothetical example, because any attempt to create a lookup table for all possible responses would entail a device of truly gigantic proportions. For this reasons, some consider this thought experiment to be misleading. See Chinese room.

Mirror test

With the mirror test, devised by Gordon Gallup in the 1970s, one is interested in whether 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

Wikibooks Consciousness studies has more about this subject:


  • Baars, B. (1997). In the Theater of Consciousness: The Workspace of the Mind. New York, NY: Oxford University Press. 2001 reprint: ISBN 0195147030
  • Blackmore, S. (2003). Consciousness: an Introduction. Oxford: Oxford University Press. ISBN 019515343X
  • Block, N. (2004). The Encyclopedia of Cognitive Science.
  • Chalmers, D. (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: Binding, Integration, and Dissociation. Oxford: Oxford Univerisity Press. ISBN 0198508573
  • Damasio, A. (1999). The Feeling of What Happens: Body and Emotion in the Making of Consciousness. New York: Harcourt Press. ISBN 0156010755
  • Dennett, D. (1991). Consciousness Explained, Boston: Little & Company. ISBN 0316180661
  • Harnad, S. (2005) What is Consciousness? New York Review of Books 52(11).
  • James, W. (1902) The Varieties of Religious Experience
  • Koch, C. (2004). The Quest for Consciousness. Englewood, CO: Roberts & Company. ISBN 0974707708
  • Libet, B., Freeman, A. & Sutherland, K. 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. ISBN 0262133709
  • Searle, J. (2004). Mind: A Brief Introduction. New York: Oxford University Press.
  • Sperry, R. (1968) Hemispheric deconnection and unity in conscious awareness. American Psychologist, 23. 723-33.
  • Sperry. R. (1969) A modified concept of consciousness. Psychological Review, 76, 532-6.
  • Sperry. R. (1995) The riddle of consciousness and the changing scientific worldview, Journal of Humanistic Psychology, 35. 7-3 3.

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).
  • McFarlane, Thomas J. (1988) Consciousness and Quantum Mechanics
  • 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.
  • Walker, E.H. (1997), ‘Quantum Mechanical Tunneling in Synaptic and Ephaptic Transmission’, Int. J. Quantum Chemistry, 11, 103-127.

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