p-consciousness without a-consciousness is more widely accepted, there have been some hypothetical examples of a without p. 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 symbolised 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 illustration of dualism by rené descartes. inputs are passed by the sensory organs to the pineal gland and from there to the immaterial spirit. 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 cartesian 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's 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 consists of matter organized in a particular way), 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. 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. but if consciousness is subjective and not visible from the outside, why do the vast majority of people believe that other people are conscious, but rocks and trees are not? this is called the problem of other minds. it 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. 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. 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 the necker cube, an ambiguous image 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: a philosophical zombie may give detailed verbal reports of awareness in the absence of any genuine awareness. although verbal report is in practice the "gold standard" for ascribing consciousness, it is not the only possible criterion. in medicine, consciousness is assessed as a combination of verbal behavior, arousal, brain activity and purposeful movement. the last three of these can be used as indicators of consciousness when verbal behavior is absent. the scientific literature regarding the neural bases of arousal and purposeful movement is very extensive. their reliability as indicators of consciousness is disputed, however, due to numerous studies showing that alert human subjects can be induced to behave purposefully in a