Consciousness is being aware of something internal to one's self, or of states or objects in one's external environment. It has been the topic of extensive explanations, analyses, and debate among philosophers, scientists, and theologians for millennia. There is no consensus on what exactly needs to be studied, or whether consciousness can be considered a scientific concept. In some explanations it is synonymous with mind, while in others it is considered an aspect of it. In the past, consciousness meant one's "inner life": the world of introspection, private thought, imagination, and volition. Today, it often includes various forms of cognition, experience, feeling, or perception. It may be awareness, awareness of awareness, metacognition, or self-awareness, either continuously changing or not. There is also a medical definition that helps, for example, to discern "coma" from other states. The disparate range of research, notions, and speculations raises some curiosity about whether the right questions are being asked. Examples of the range of descriptions, definitions and explanations are: ordered distinction between self and environment, simple wakefulness, one's sense of selfhood or soul explored by "looking within", being a metaphorical "stream" of contents, or being a mental state, mental event, or mental process of the brain.
Etymology The words "conscious" and "consciousness" in the English language date to the 17th century, and the first recorded use of "conscious" as a simple adjective was applied figuratively to inanimate objects ("the conscious Groves", 1643). It derived from the Latin conscius (con- "together" and scio "to know") which meant "knowing with" or "having joint or common knowledge with another", especially as in sharing a secret. Thomas Hobbes in Leviathan (1651) wrote: "Where two, or more men, know of one and the same fact, they are said to be Conscious of it one to another". There were also many occurrences in Latin writings of the phrase conscius sibi, which translates literally as "knowing with oneself", or in other words "sharing knowledge with oneself about something". This phrase has the figurative sense of "knowing that one knows", which is something like the modern English word "conscious", but it was rendered into English as "conscious to oneself" or "conscious unto oneself". For example, Archbishop Ussher wrote in 1613 of "being so conscious unto myself of my great weakness". The Latin conscientia, literally "knowledge-with", first appears in Roman juridical texts by writers such as Cicero. It means a kind of shared knowledge with moral value, specifically what a witness knows of someone else's deeds. Although René Descartes (1596–1650), writing in Latin, is generally taken to be the first philosopher to use conscientia in a way less like the traditional meaning and more like the way modern English speakers would use "conscience", his meaning is nowhere defined. In Search after Truth (Regulæ ad directionem ingenii ut et inquisitio veritatis per lumen naturale, Amsterdam 1701) he wrote the word with a gloss: conscientiâ, vel interno testimonio (translatable as "conscience, or internal testimony"). It might mean the knowledge of the value of one's own thoughts. One way that this shift during the seventeenth century from "conscience" to "consciousness" took place was through the poetry of John Milton, as the scholar Timothy M. Harrison has shown.
The origin of the modern concept of consciousness is often attributed to John Locke who defined the word in his Essay Concerning Human Understanding, published in 1690, as "the perception of what passes in a man's own mind". The essay strongly influenced 18th-century British philosophy, and Locke's definition appeared in Samuel Johnson's celebrated Dictionary (1755). The French term conscience is defined roughly like English "consciousness" in the 1753 volume of Diderot and d'Alembert's Encyclopédie as "the opinion or internal feeling that we ourselves have from what we do".
Problem of definition Scholars are divided as to whether Aristotle had a concept of consciousness. He does not use any single word or terminology that is clearly similar to the phenomenon or concept defined by John Locke. Victor Caston contends that Aristotle did have a concept more clearly similar to perception. Modern dictionary definitions of the word consciousness evolved over several centuries and reflect a range of seemingly related meanings, with some differences that have been controversial, such as the distinction between inward awareness and perception of the physical world, or the distinction between conscious and unconscious, or the notion of a mental entity or mental activity that is not physical. The common-usage definitions of consciousness in Webster's Third New International Dictionary (1966) are as follows:
awareness or perception of an inward psychological or spiritual fact; intuitively perceived knowledge of something in one's inner self inward awareness of an external object, state, or fact concerned awareness; INTEREST, CONCERN—often used with an attributive noun [e.g. class consciousness] the state or activity that is characterized by sensation, emotion, volition, or thought; mind in the broadest possible sense; something in nature that is distinguished from the physical the totality in psychology of sensations, perceptions, ideas, attitudes, and feelings of which an individual or a group is aware at any given time or within a particular time span— waking life (as that to which one returns after sleep, trance, fever) wherein all one's mental powers have returned . . . the part of mental life or psychic content in psychoanalysis that is immediately available to the ego— The Cambridge English Dictionary defines consciousness as "the state of being awake, thinking, and knowing what is happening around you", as well as "the state of understanding and realizing something". The Oxford Living Dictionary defines consciousness as "[t]he state of being aware of and responsive to one's surroundings", "[a] person's awareness or perception of something", and "[t]he fact of awareness by the mind of itself and the world". Philosophers have attempted to clarify technical distinctions by using a jargon of their own. The corresponding entry in the Routledge Encyclopedia of Philosophy (1998) reads:
Consciousness Philosophers have used the term consciousness for four main topics: knowledge in general, intentionality, introspection (and the knowledge it specifically generates) and phenomenal experience... Something within one's mind is 'introspectively conscious' just in case one introspects it (or is poised to do so). Introspection is often thought to deliver one's primary knowledge of one's mental life. An experience or other mental entity is 'phenomenally conscious' just in case there is 'something it is like' for one to have it. The clearest examples are: perceptual experience, such as tastings and seeings; bodily-sensational experiences, such as those of pains, tickles and itches; imaginative experiences, such as those of one's own actions or perceptions; and streams of thought, as in the experience of thinking 'in words' or 'in images'. Introspection and phenomenality seem independent, or dissociable, although this is controversial.
Traditional metaphors for mind During the early 19th century, the emerging field of geology inspired a popular metaphor that the mind likewise had hidden layers "which recorded the past of the individual". By 1875, most psychologists believed that "consciousness was but a small part of mental life", and this idea underlies the goal of Freudian therapy, to expose the unconscious layer of the mind. Other metaphors from various sciences inspired other analyses of the mind, for example: Johann Friedrich Herbart described ideas as being attracted and repulsed like magnets; John Stuart Mill developed the idea of "mental chemistry" and "mental compounds", and Edward B. Titchener sought the "structure" of the mind by analyzing its "elements". The abstract idea of states of consciousness mirrored the concept of states of matter. In 1892, William James noted that the "ambiguous word 'content' has been recently invented instead of 'object'" and that the metaphor of mind as a container seemed to minimize the dualistic problem of how "states of consciousness can know" things, or objects; by 1899 psychologists were busily studying the "contents of conscious experience by introspection and experiment". Another popular metaphor was James's doctrine of the stream of consciousness, with continuity, fringes, and transitions. James discussed the difficulties of describing and studying psychological phenomena, recognizing that commonly used terminology was a necessary and acceptable starting point towards more precise, scientifically justified language. Prime examples were phrases like inner experience and personal consciousness:
The first and foremost concrete fact which every one will affirm to belong to his inner experience is the fact that consciousness of some sort goes on. 'States of mind' succeed each other in him. [...] But everyone knows what the terms mean [only] in a rough way; [...] When I say every 'state' or 'thought' is part of a personal consciousness, 'personal consciousness' is one of the terms in question. Its meaning we know so long as no one asks us to define it, but to give an accurate account of it is the most difficult of philosophic tasks. [...] The only states of consciousness that we naturally deal with are found in personal consciousnesses, minds, selves, concrete particular I's and you's.
From introspection to awareness and experience Prior to the 20th century, philosophers treated the phenomenon of consciousness as the "inner world [of] one's own mind", and introspection was the mind "attending to" itself, an activity seemingly distinct from that of perceiving the "outer world" and its physical phenomena. In 1892 William James noted the distinction along with doubts about the inward character of the mind:'Things' have been doubted, but thoughts and feelings have never been doubted. The outer world, but never the inner world, has been denied. Everyone assumes that we have direct introspective acquaintance with our thinking activity as such, with our consciousness as something inward and contrasted with the outer objects which it knows. Yet I must confess that for my part I cannot feel sure of this conclusion. [...] It seems as if consciousness as an inner activity were rather a postulate than a sensibly given fact... By the 1960s, for many philosophers and psychologists who talked about consciousness, the word no longer meant the 'inner world' but an indefinite, large category called awareness, as in the following example:
It is difficult for modern Western man to grasp that the Greeks really had no concept of consciousness in that they did not class together phenomena as varied as problem solving, remembering, imagining, perceiving, feeling pain, dreaming, and acting on the grounds that all these are manifestations of being aware or being conscious. Many philosophers and scientists have been unhappy about the difficulty of producing a definition that does not involve circularity or fuzziness. In The Macmillan Dictionary of Psychology (1989 edition), Stuart Sutherland emphasized external awareness, and expressed a skeptical attitude more than a definition:
Consciousness—The having of perceptions, thoughts, and feelings; awareness. The term is impossible to define except in terms that are unintelligible without a grasp of what consciousness means. Many fall into the trap of equating consciousness with self-consciousness—to be conscious it is only necessary to be aware of the external world. Consciousness is a fascinating but elusive phenomenon: it is impossible to specify what it is, what it does, or why it has evolved. Nothing worth reading has been written on it. Using 'awareness', however, as a definition or synonym of consciousness is not a simple matter:
If awareness of the environment . . . is the criterion of consciousness, then even the protozoans are conscious. If awareness of awareness is required, then it is doubtful whether the great apes and human infants are conscious. In 1974, philosopher Thomas Nagel used 'consciousness', 'conscious experience', 'subjective experience' and the 'subjective character of experience' as synonyms for something that "occurs at many levels of animal life ... [although] it is difficult to say in general what provides evidence of it." Nagel's terminology also included what has been described as "the standard 'what it's like' locution" in reference to the impenetrable subjectivity of any organism's experience which Nagel referred to as "inner life" without implying any kind of introspection. On Nagel's approach, philosopher Peter Hacker commented: "Consciousness, thus conceived, is extended to the whole domain of 'experience'—of 'Life' subjectively understood." He regarded this as a "novel analysis of consciousness" and has been particularly critical of Nagel's terminology and its philosophical consequences. In 2002 he attacked Nagel's 'what it's like' phrase as "malconstructed" and meaningless English—it sounds as if it asks for an analogy, but does not—and he called Nagel's approach logically "misconceived" as a definition of consciousness. In 2012 Hacker went further and asserted that Nagel had "laid the groundwork for ... forty years of fresh confusion about consciousness" and that "the contemporary philosophical conception of consciousness that is embraced by the 'consciousness studies community' is incoherent".
Influence on research Many philosophers have argued that consciousness is a unitary concept that is understood by the majority of people despite the difficulty philosophers have had defining it. The term 'subjective experience', following Nagel, is ambiguous, as philosophers seem to differ from non-philosophers in their intuitions about its meaning. Max Velmans proposed that the "everyday understanding of consciousness" uncontroversially "refers to experience itself rather than any particular thing that we observe or experience" and he added that consciousness "is [therefore] exemplified by all the things that we observe or experience", whether thoughts, feelings, or perceptions. Velmans noted however, as of 2009, that there was a deep level of "confusion and internal division" among experts about the phenomenon of consciousness, because researchers lacked "a sufficiently well-specified use of the term...to agree that they are investigating the same thing". He argued additionally that "pre-existing theoretical commitments" to competing explanations of consciousness might be a source of bias. Within the "modern consciousness studies" community the technical phrase 'phenomenal consciousness' is a common synonym for all forms of awareness, or simply 'experience', without differentiating between inner and outer, or between higher and lower types. With advances in brain research, "the presence or absence of experienced phenomena" of any kind underlies the work of those neuroscientists who seek "to analyze the precise relation of conscious phenomenology to its associated information processing" in the brain. This neuroscientific goal is to find the "neural correlates of consciousness" (NCC). One criticism of this goal is that it begins with a theoretical commitment to the neurological origin of all "experienced phenomena" whether inner or outer. Also, the fact that the easiest 'content of consciousness' to be so analyzed is "the experienced three-dimensional world (the phenomenal world) beyond the body surface" invites another criticism, that most consciousness research since the 1990s, perhaps because of bias, has focused on processes of external perception. From a history of psychology perspective, Julian Jaynes rejected popular but "superficial views of consciousness" especially those which equate it with "that vaguest of terms, experience". In 1976 he insisted that if not for introspection, which for decades had been ignored or taken for granted rather than explained, there could be no "conception of what consciousness is" and in 1990, he reaffirmed the traditional idea of the phenomenon called 'consciousness', writing that "its denotative definition is, as it was for René Descartes, John Locke, and David Hume, what is introspectable". Jaynes saw consciousness as an important but small part of human mentality, and he asserted: "there can be no progress in the science of consciousness until ... what is introspectable [is] sharply distinguished" from the unconscious processes of cognition such as perception, reactive awareness and attention, and automatic forms of learning, problem-solving, and decision-making. The cognitive science point of view—with an inter-disciplinary perspective involving fields such as psychology, linguistics and anthropology—requires no agreed definition of "consciousness" but studies the interaction of many processes besides perception. For some researchers, consciousness is linked to some kind of "selfhood", for example to certain pragmatic issues such as the feeling of agency and the effects of regret and action on experience of one's own body or social identity. Similarly Daniel Kahneman, who focused on systematic errors in perception, memory and decision-making, has differentiated between two kinds of mental processes, or cognitive "systems": the "fast" activities that are primary, automatic and "cannot be turned off", and the "slow", deliberate, effortful activities of a secondary system "often associated with the subjective experience of agency, choice, and concentration". Kahneman's two systems have been described as "roughly corresponding to unconscious and conscious processes". The two systems can interact, for example in sharing the control of attention. While System 1 can be impulsive, "System 2 is in charge of self-control", and "When we think of ourselves, we identify with System 2, the conscious, reasoning self that has beliefs, makes choices, and decides what to think about and what to do". Some have argued that we should eliminate the concept from our understanding of the mind, a position known as consciousness semanticism.
Medical definition In medicine, a "level of consciousness" terminology is used to describe a patient's arousal and responsiveness, which can be seen as a continuum of states ranging from full alertness and comprehension, through disorientation, delirium, loss of meaningful communication, and finally loss of movement in response to painful stimuli. Issues of practical concern include how the level of consciousness can be assessed in severely ill, comatose, or anesthetized people, and how to treat conditions in which consciousness is impaired or disrupted. The degree or level of consciousness is measured by standardized behavior observation scales such as the Glasgow Coma Scale.
Philosophy of mind While historically philosophers have defended various views on consciousness, surveys indicate that physicalism is now the dominant position among contemporary philosophers of mind. For an overview of the field, approaches often include both historical perspectives (e.g., Descartes, Locke, Kant) and organization by key issues in contemporary debates. An alternative is to focus primarily on current philosophical stances and empirical findings.
Coherence of the concept Philosophers differ from non-philosophers in their intuitions about what consciousness is. While most people have a strong intuition for the existence of what they refer to as consciousness, skeptics argue that this intuition is too narrow, either because the concept of consciousness is embedded in our intuitions, or because we all are illusions. Gilbert Ryle, for example, argued that traditional understanding of consciousness depends on a Cartesian dualist outlook that improperly distinguishes between mind and body, or between mind and world. He proposed that we speak not of minds, bodies, and the world, but of entities, or identities, acting in the world. Thus, by speaking of "consciousness" we end up leading ourselves by thinking that there is any sort of thing as consciousness separated from behavioral and linguistic understandings.
Types Ned Block argues that discussions on consciousness often fail to properly distinguish phenomenal consciousness from access consciousness. These terms had been used before Block, but he adopted the short forms P-consciousness and A-consciousness. According to Block:
P-consciousness is raw experience: it is moving, colored 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. A-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 access conscious; when we introspect, information about our thoughts is access conscious; when we remember, information about the past is access conscious, and so on. Block adds that P-consciousness does not allow of easy definition: he admits that he "cannot define P-consciousness in any remotely noncircular way. Although some philosophers, such as Daniel Dennett, have disputed the validity of this distinction, others have broadly accepted it. David Chalmers has argued that A-consciousness can in principle be understood in mechanistic terms, but that understanding P-consciousness is much more challenging: he calls this the hard problem of consciousness. Some philosophers believe that Block's two types of consciousness are not the end of the story. William Lycan, for example, argued in his book Consciousness and Experience that at least eight clearly distinct types of consciousness can be identified (organism consciousness; control consciousness; consciousness of; state/event consciousness; reportability; introspective consciousness; subjective consciousness; self-consciousness)—and that even this list omits several more obscure forms. There is also debate over whether or not A-consciousness and P-consciousness always coexist or if they can exist separately. Although P-consciousness without A-consciousness is more widely accepted, there have been some hypothetical examples of A-consciousness without P-consciousness. Block, for instance, suggests the case of a "zombie" that is computationally identical to a person but without any subjectivity. However, he remains somewhat skeptical concluding "I don't know whether there are any actual cases of A-consciousness without P-consciousness, but I hope I have illustrated their conceptual possibility".
Distinguishing consciousness from its contents Sam Harris observes: "At the level of your experience, you are not a body of cells, organelles, and atoms; you are consciousness and its ever-changing contents". Seen in this way, consciousness is a subjectively experienced, ever-present field in which things (the contents of consciousness) come and go. Christopher Tricker argues that this field of consciousness is symbolized by the mythical bird that opens the Daoist classic the Zhuangzi. This bird's name is Of a Flock (peng 鵬), yet its back is countless thousands of miles across and its wings are like clouds arcing across the heavens. "Like Of a Flock, whose wings arc across the heavens, the wings of your consciousness span to the horizon. At the same time, the wings of every other being's consciousness span to the horizon. You are of a flock, one bird among kin."
Mind–body problem
Mental processes (such as consciousness) and physical processes (such as brain events) seem to be correlated, however the specific nature of the connection is unknown. The first influential philosopher to discuss this question specifically was Descartes, and the answer he gave is known as mind–body dualism. Descartes proposed that consciousness resides within an immaterial domain he called res cogitans (the realm of thought), in contrast to the domain of material things, which he called res extensa (the realm of extension). He suggested that the interaction between these two domains occurs inside the brain, perhaps in a small midline structure called the pineal gland. Although it is widely accepted that Descartes explained the problem cogently, few later philosophers have been happy with his solution, and his ideas about the pineal gland have especially been ridiculed. However, no alternative solution has gained general acceptance. Proposed solutions can be divided broadly into two categories: dualist solutions that maintain Descartes' rigid distinction between the realm of consciousness and the realm of matter but give different answers for how the two realms relate to each other; and monist solutions that maintain that there is really only one realm of being, of which consciousness and matter are both aspects. Each of these categories itself contains numerous variants. The two main types of dualism are substance dualism (which holds that the mind is formed of a distinct type of substance not governed by the laws of physics), and property dualism (which holds that the laws of physics are universally valid but cannot be used to explain the mind). The three main types of monism are physicalism (which holds that the mind is made out of matter), idealism (which holds that only thought or experience truly exists, and matter is merely an illusion), and neutral monism (which holds that both mind and matter are aspects of a distinct essence that is itself identical to neither of them). There are also, however, a large number of idiosyncratic theories that cannot cleanly be assigned to any of these schools of thought. Since the dawn of Newtonian science with its vision of simple mechanical principles governing the entire universe, some philosophers have been tempted by the idea that consciousness could be explained in purely physical terms. The first influential writer to propose such an idea explicitly was Julien Offray de La Mettrie, in his book Man a Machine (L'homme machine). His arguments, however, were very abstract. The most influential modern physical theories of consciousness are based on psychology and neuroscience. Theories proposed by neuroscientists such as Gerald Edelman and Antonio Damasio, and by philosophers such as Daniel Dennett, seek to explain consciousness in terms of neural events occurring within the brain. Many other neuroscientists, such as Christof Koch, have explored the neural basis of consciousness without attempting to frame all-encompassing global theories. At the same time, computer scientists working in the field of artificial intelligence have pursued the goal of creating digital computer programs that can simulate or embody consciousness. A few theoretical physicists have argued that classical physics is intrinsically incapable of explaining the holistic aspects of consciousness, but that quantum theory may provide the missing ingredients. Several theorists have therefore proposed quantum mind (QM) theories of consciousness. Notable theories falling into this category include the holonomic brain theory of Karl Pribram and David Bohm, and the Orch-OR theory formulated by Stuart Hameroff and Roger Penrose. Some of these QM theories offer descriptions of phenomenal consciousness, as well as QM interpretations of access consciousness. None of the quantum mechanical theories have been confirmed by experiment. Recent publications by G. Guerreshi, J. Cia, S. Popescu, and H. Briegel could falsify proposals such as those of Hameroff, which rely on quantum entanglement in protein. At the present time many scientists and philosophers consider the arguments for an important role of quantum phenomena to be unconvincing. Empirical evidence is against the notion of quantum consciousness, an experiment about wave function collapse led by Catalina Curceanu in 2022 suggests that quantum consciousness, as suggested by Roger Penrose and Stuart Hameroff, is highly implausible. Max Tegmark's calculation of decoherence timescales in the brain also suggests that the operation of the brain is adequately described by classical physics. Apart from the general question of the "hard problem" of consciousness (which is, roughly speaking, the question of how mental experience can arise from a physical basis), a more specialized question is how to square the subjective notion that we are in control of our decisions (at least in some small measure) with the customary view of causality that subsequent events are caused by prior events. The topic of free will is the philosophical and scientific examination of this conundrum.
Problem of other minds
Many philosophers consider experience to be the essence of consciousness, and believe that experience can only fully be known from the inside, subjectively. The problem of other minds is a philosophical problem traditionally stated as the following epistemological question: Given that I can only observe the behavior of others, how can I know that others have minds? The problem of other minds is particularly acute for people who believe in the possibility of philosophical zombies, that is, people who think it is possible in principle to have an entity that is physically indistinguishable from a human being and behaves like a human being in every way but nevertheless lacks consciousness. Related issues have also been studied extensively by Greg Littmann of the University of Illinois, and by Colin Allen (a professor at the University of Pittsburgh) regarding the literature and research studying artificial intelligence in androids. The most commonly given answer is that we attribute consciousness to other people because we see that they resemble us in appearance and behavior; we reason that if they look like us and act like us, they must be like us in other ways, including having experiences of the sort that we do. There are, however, a variety of problems with that explanation. For one thing, it seems to violate the principle of parsimony, by postulating an invisible entity that is not necessary to explain what we observe. Some philosophers, such as Daniel Dennett in a research paper titled "The Unimagined Preposterousness of Zombies", argue that people who give this explanation do not really understand what they are saying. More broadly, philosophers who do not accept the possibility of zombies generally believe that consciousness is reflected in behavior (including verbal behavior), and that we attribute consciousness on the basis of behavior. A more straightforward way of saying this is that we attribute experiences to people because of what they can do, including the fact that they can tell us about their experiences.
Qualia
The term "qualia" was introduced in philosophical literature by C. I. Lewis. The word is derived from Latin and means "of what sort". It is basically a quantity or property of something as perceived or experienced by an individual, like the scent of rose, the taste of wine, or the pain of a headache. They are difficult to articulate or describe. The philosopher and scientist Daniel Dennett describes them as "the way things seem to us", while philosopher and cognitive scientist David Chalmers expanded on qualia as the "hard problem of consciousness" in the 1990s. When qualia are experienced, activity is simulated in the brain, and these processes are called neural correlates of consciousness (NCCs). Many scientific studies have been done to attempt to link particular brain regions with emotions or experiences. Species which experience qualia are said to have sentience, which is central to the animal rights movement, because it includes the ability to experience pain and suffering.
Identity
An unsolved problem in the philosophy of consciousness is how it relates to the nature of personal identity. This includes questions regarding whether someone is the "same person" from moment to moment. If that is the case, another question is what exactly the "identity carrier" is that makes a conscious being "the same" being from one moment to the next. The problem of determining personal identity also includes questions such as Benj Hellie's vertiginous question, which can be summarized as "Why am I me and not someone else?". The philosophical problems regarding the nature of personal identity have been extensively discussed by Thomas Nagel in his book The View from Nowhere. A common view of personal identity is that an individual has a continuous identity that persists from moment to moment, with an individual having a continuous identity consisting of a line segment stretching across time from birth to death. In the case of an afterlife as described in Abrahamic religions, one's personal identity is believed to stretch infinitely into the future, forming a ray or line. This notion of identity is similar to the form of dualism advocated by René Descartes. However, some philosophers argue that this common notion of personal identity is unfounded. Daniel Kolak has argued extensively against it in his book I am You. Kolak refers to the aforementioned notion of personal identity being linear as "Closed individualism". Another view of personal identity according to Kolak is "empty individualism", in which one's personal identity only exists for a single moment of time. However, Kolak advocates for a view of personal identity called "open individualism", in which all consciousness is in reality a single being and individual personal identity in reality does not exist at all. Another philosopher who has contested the notion of personal identity is Derek Parfit. In his book Reasons and Persons, he describes a thought experiment known as the teletransportation paradox. In Buddhist philosophy, the concept of anattā refers to the idea that the self is an illusion. Other philosophers have argued that Hellie's vertiginous question has a number of philosophical implications relating to the metaphysical nature of consciousness. Christian List argues that the vertiginous question and the existence of first-personal facts is evidence against physicalism, and evidence against other third-personal metaphysical pictures, including standard versions of dualism. List also argues that the vertiginous question implies a "quadrilemma" for theories of consciousness. He claims that at most three of the following metaphysical claims can be true: 'first-person realism', 'non-solipsism', 'non-fragmentation', and 'one world'—and at least one of these four must be false. List has proposed a model he calls the "many-worlds theory of consciousness" in order to reconcile the subjective nature of consciousness without lapsing into solipsism. Vincent Conitzer argues that the nature of identity is connected to A series and B series theories of time, and that A-theory being true implies that the "I" is metaphysically distinguished from other perspectives. Other philosophical theories regarding the metaphysical nature of self are Caspar Hare's theories of perspectival realism, in which things within perceptual awareness have a defining intrinsic property that exists absolutely and not relative to anything, and egocentric presentism, in which the experiences of other individuals are not present in the way that one's current perspective is.
Scientific study For many decades, consciousness as a research topic was avoided by the majority of mainstream scientists, because of a general feeling that a phenomenon defined in subjective terms could not properly be studied using objective experimental methods. In 1975 George Mandler published an influential psychological study which distinguished between slow, serial, and limited conscious processes and fast, parallel and extensive unconscious ones. The Science and Religion Forum 1984 annual conference, 'From Artificial Intelligence to Human Consciousness' identified the nature of consciousness as a matter for investigation; Donald Michie was a keynote speaker. A decade later Michie presented an engineering view of consciousness in the computer science context. Starting in the 1980s, an expanding community of neuroscientists and psychologists have associated themselves with a field called Consciousness Studies, giving rise to a stream of experimental work published in books, journals such as Consciousness and Cognition, Frontiers in Consciousness Research, Psyche, and the Journal of Consciousness Studies, along with regular conferences organized by groups such as the Association for the Scientific Study of Consciousness and the Society for Consciousness Studies. Modern medical and psychological investigations into consciousness are based on psychological experiments (including, for example, the investigation of priming effects using subliminal stimuli), and on case studies of alterations in consciousness produced by trauma, illness, or drugs. Broadly viewed, scientific approaches are based on two core concepts. The first identifies the content of consciousness with the experiences that are reported by human subjects; the second makes use of the concept of consciousness that has been developed by neurologists and other medical professionals who deal with patients whose behavior is impaired. In either case, the ultimate goals are to develop techniques for assessing consciousness objectively in humans as well as other animals, and to understand the neural and psychological mechanisms that underlie it.
Measurement via verbal report
Experimental research on consciousness presents special difficulties, due to the lack of a universally accepted operational definition. In the majority of experiments that are specifically about consciousness, the subjects are human, and the criterion used is verbal report: in other words, subjects are asked to describe their experiences, and their descriptions are treated as observations of the contents of consciousness. For example, subjects who stare continuously at a Necker cube usually report that they experience it "flipping" between two 3D configurations, even though the stimulus itself remains the same. The objective is to understand the relationship between the conscious awareness of stimuli (as indicated by verbal report) and the effects the stimuli have on brain activity and behavior. In several paradigms, such as the technique of response priming, the behavior of subjects is clearly influenced by stimuli for which they report no awareness, and suitable experimental manipulations can lead to increasing priming effects despite decreasing prime identification (double dissociation). Verbal report is widely considered to be the most reliable indicator of consciousness, but it raises a number of issues. For one thing, if verbal reports are treated as observations, akin to observations in other branches of science, then the possibility arises that they may contain errors—but it is difficult to make sense of the idea that subjects could be wrong about their own experiences, and even more difficult to see how such an error could be detected. Daniel Dennett has argued for an approach he calls heterophenomenology, which means treating verbal reports as stories that may or may not be true, but his ideas about how to do this have not been widely adopted. Another issue with verbal report as a criterion is that it restricts the field of study to humans who have language: this approach cannot be used to study consciousness in other species, pre-linguistic children, or people with types of brain damage that impair language. As a third issue, philosophers who dispute the validity of the Turing test may feel that it is possible, at least in principle, for verbal report to be dissociated from consciousness entirely: