Consciousness is a term that has been used to refer to a variety of aspects of the relationship between the mind and the world with which it interacts. It has been defined, at one time or another, as: subjective experience; awareness; the ability to experience feelings; wakefulness; having a sense of selfhood; or as the executive control system of the mind. Despite the difficulty of definition, many philosophers believe that there is a basic underlying intuition about consciousness that is shared by nearly all people. As Max Velmans and Susan Schneider wrote in The Blackwell Companion to Consciousness:
In philosophy, consciousness is often said to imply four characteristics: subjectivity, change, continuity, and selectivity. Philosopher Franz Brentano has also suggested intentionality or aboutness (that consciousness is about something); however, there is no consensus on whether intentionality is a requirement for consciousness. Issues of practical concern in the philosophy of consciousness include whether consciousness can ever be explained mechanistically; whether non-human consciousness exists and if so how it can be recognized; at what point in fetal development consciousness begins; and whether it may ever be possible for computers to achieve a conscious state.
At one time consciousness was viewed with skepticism by many scientists and considered within the domain of philosophers and theologians, but in recent years it has been an increasingly significant topic of scientific research. In psychology and neuroscience, the focus of most research is on understanding what it means biologically and psychologically for information to be present in consciousness—that is, on determining the neural and psychological correlates of consciousness. The majority of experimental studies use human subjects and assess consciousness by asking subjects for a verbal report of their experiences (e.g., "tell me if you notice anything when I do this"). Issues of interest include phenomena such as subliminal perception, blindsight, denial of impairment, and altered states of consciousness produced by psychoactive drugs or spiritual or meditative techniques.
In medicine, consciousness is assessed by observing a patient's arousal and responsiveness, and can be seen as a continuum of states ranging from full alertness and comprehension, through disorientation, then delirium, then loss of any meaningful communication, and ending with loss of movement in response to painful stimuli. Issues of practical concern include how the presence of consciousness can be assessed in severely ill, comatose, or anesthetized people, and how to treat conditions in which consciousness is impaired or disrupted.
Spiritual approaches
Vedanta
According to Vedanta, awareness is not a product of physical processes and can be considered under four aspects. The first is waking consciousness (jagaritasthana), the identification with “I” or “me” in relationship with phenomenal experiences with external objects. The second aspect is dream consciousness (svapna-sthana), which embodies the same subject/object duality as the waking state. The third aspect of consciousness is deep sleep (susupti), which is non-dual as a result of holding in abeyance all feelings, thoughts, and sensations. The final aspect is the consciousness that underlies and transcends the first three aspects (turiya) also referred to as a trans-cognitive state (anubhava) or a state of self-realization or freedom from body-mind identification (moksha). The state of turiya has been correlated with physiological and bio chemical changes in the body. Gaudiya Vedanta recognizes a fifth aspect of consciousness in which God becomes subordinate to bhakti.
Vijñāna
In Buddhism, consciousness (viññāṇa) is included in the five classically defined experiential "aggregates". The aggregates are seen as empty of self-nature; that is, they arise dependent on causes and conditions. The cause for consciousness arising (viññāṇa) is the arising of another aggregate (physical or mental); and, consciousness arising in turn gives rise to one or more of the mental (nāma) aggregates. The causation chain identified in the aggregate (khandha) model overlaps the conditioning chain in Dependent Origination (paticcasamuppāda) model. Consciousness is the third link, between mind body mental formations and name & form in the traditional Twelve Causes (nidāna) of Dependent Origination. The six classes of consciousness are: eye-consciousness, ear-consciousness, nose-consciousness, tongue-consciousness, body-consciousness, intellect-consciousness. The following aspects are traditionally highlighted within Dependent Origination:
