Consciousness is defined in the dictionary as: 1. the state of being conscious; awareness of one's own existence, sensations, thoughts, surroundings, etc. 2. the thoughts and feelings, collectively, of an individual or of an aggregate of people: the moral consciousness of a nation.3. full activity of the mind and senses, as in waking life: to regain consciousness after fainting. 4. awareness of something for what it is; internal knowledge: consciousness of wrongdoing. 5. concern, interest, or acute awareness: class consciousness. 6. the mental activity of which a person is aware as contrasted with unconscious mental processes. 7. Philosophy. the mind or the mental faculties as characterized by thought, feelings, and volition. Idioms: raise one's consciousness: to increase one's awareness and understanding of one's own needs, behavior, attitudes, etc., especially as a member of a particular social or political group.
Psychologists define consciousness as the quality or state of awareness, or, of being aware of an external object or something within oneself. The English philosopher John Locke defined it as “the perception of what passes in a man's own mind.” It is also defined as: sentience, awareness, subjectivity, the ability to experience or to feel, wakefulness, having a sense of selfhood, and the executive control system of the mind. Despite the difficulty in definition, many philosophers believe that there is a broadly shared underlying intuition about what consciousness is. As Max Velmans and Susan Schneider wrote in The Blackwell Companion to Consciousness: "Anything that we are aware of at a given moment forms part of our consciousness, making conscious experience at once the most familiar and most mysterious aspect of our lives."
In the early 19th century the concept was variously considered. Some philosophers regarded it as a kind of substance, or “mental stuff,” quite different from the material substance of the physical world. Others thought of it as an attribute characterized by sensation and voluntary movement, which separated animals and men from lower forms of life and also described the difference between the normal waking state of animals and men and their condition when asleep, in a coma, or under anesthesia (the latter condition was described as unconsciousness). Other descriptions included an analysis of consciousness as a form of relationship or act of the mind toward objects in nature, and a view that consciousness was a continuous field or stream of essentially mental “sense data,” roughly similar to the “ideas” of earlier empirical philosophers.
Philosophers since the time of Descartes and Locke have struggled to comprehend the nature of consciousness and pin down its essential properties. Issues of concern in the philosophy of consciousness include whether the concept is fundamentally coherent; whether consciousness can ever be explained mechanistically; whether non-human consciousness exists and if so how it can be recognized; how consciousness relates to language; whether consciousness can be understood in a way that does not require a dualistic distinction between mental and physical states or properties; and whether it may ever be possible for computing machines like computers or robots to be conscious, a topic studied in the field of artificial intelligence.
At one time consciousness was viewed with skepticism by many scientists, but in recent years it has become a significant topic of research in psychology, neuropsychology and neuroscience. The primary focus 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 assess consciousness by asking human 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, blind sight, denial of impairment, and altered states of consciousness produced by drugs and alcohol, or spiritual or meditative techniques.
That consciousness depends on the function of the brain has been known from ancient times. Although detailed understanding of the neural mechanisms of consciousness has not been achieved, correlations between states of consciousness and functions of the brain are possible. Levels of consciousness in terms of levels of alertness or responsiveness are correlated with patterns of electrical activity of the brain (brain waves) recorded by an electroencephalograph. During wide-awake consciousness the pattern of brain waves consists of rapid irregular waves of low amplitude or voltage. In contrast, during sleep, when consciousness can be said to be minimal, the brain waves are much slower and of greater amplitude, often coming in periodic bursts of slow waxing and waning amplitude.
Both behavioral levels of consciousness and the correlated patterns of electrical activity are related to the function of a part of the brainstem called the reticular formation. Electrical stimulation of the ascending reticular systems arouses a sleeping cat to alert consciousness and simultaneously activates its brain waves to the waking pattern. It was once supposed that the neurophysiological mechanisms subserving consciousness and the higher mental processes must reside in the cortex. It is more likely, however, that the cortex serves the more specialized functions of integrating patterns of sensory experience and organizing motor patterns and that the ascending reticular system represents the neural structures most critically related to consciousness. The brainstem reticular formation should not, however, be called the seat of consciousness. It represents an integrative focus, functioning through its widespread interconnections with the cortex and other regions of the brain. See also introspection; unconscious.
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, delirium, loss of meaningful communication, and finally 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.
(Adapted from Encyclopedia of Britannica and Wikipedia)