Consciousness is variously defined as subjective experience, awareness, the ability to experience "feeling", wakefulness, or the executive control system of the mind. It is an umbrella term that may refer to a variety of mental phenomena. Although humans realize what everyday experiences are, consciousness refuses to be defined, philosophers note (e.g. John Searle in The Oxford Companion to Philosophy):
"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."—Schneider and Velmans, 2007
Consciousness in medicine (e.g., anesthesiology) is assessed by observing a patient's alertness and responsiveness, and can be seen as a continuum of states ranging from alert, oriented to time and place, and communicative, through disorientation, then delirium, then loss of any meaningful communication, and ending with loss of movement in response to painful stimulation.
Consciousness in psychology and philosophy typically means something beyond what it means for anesthesiology, and may be said in many contexts to imply four characteristics: subjectivity, change, continuity, and selectivity. Philosopher Franz Brentano has suggested intentionality or aboutness (that consciousness is about something). However, within the philosophy of mind there is no consensus on whether intentionality is a requirement for consciousness.
Consciousness is the subject of much research in philosophy of mind, psychology, neuroscience, cognitive science,cognitive neuroscience and artificial intelligence. Issues of practical concern include how the presence of consciousness can be assessed in severely ill or comatose people; whether non-human consciousness exists and if so how it can be measured; at what point in fetal development consciousness begins; and whether computers can achieve a conscious state.
The word "conscious" is derived from Latin conscius meaning "1. having joint or common knowledge with another, privy to, cognizant of; 2. conscious to oneself; esp., conscious of guilt". A related word was conscientia, which primarily means moral conscience. In the literal sense, "conscientia" means knowledge-with, that is, shared knowledge. The word first appears in Latin juridical texts by writers such as Cicero. Here, conscientia is the knowledge that a witness has of the deed of someone else.
René Descartes (1596–1650) is generally taken to be the first philosopher to use "conscientia" in a way that does not seem to fit this traditional meaning. Descartes used "conscientia" the way modern speakers would use "conscience." In Search after Truth he says "conscience or internal testimony" (conscientia vel interno testimonio).
Shortly thereafter, in Britain, the neo-Platonist theologian Ralph Cudworth used the modern meaning of consciousness in his "True Intellectual System of the Universe" (1678) and associated the concept with personal identity, which is assured by the repeated consciousness of oneself. Cudworth's use of the term also remained intertwined with moral agency. While there were no elaborate theories of consciousness in the seventeenth century, there was an awareness of the idea of consciousness. Cudworth was the first English philosopher to make extensive use of the noun "consciousness" with a specific philosophical meaning.
Twelve years later, Locke in Essay Concerning Human Understanding (1690) connected consciousness with personal identity. Locke argued that being the same person from one time to another was not dependent upon having the same soul or same body, but instead the same consciousness. Locke defined consciousness as “the perception of what passes in a man’s own mind.” Locke had much influence on the 18th Century view of consciousness: in Samuel Johnson's celebrated Dictionary (1755), Johnson uses this definition of "consciousness."