Representation of consciousness from the seventeenth century by Robert Fludd, an English Paracelsian physician Consciousness, at its simplest, is sentience and awareness of internal and external existence. However, its nature has led to millennia of analyses, explanations and debates by philosophers, theologians, linguists, and scientists. Opinions differ about what exactly needs to be studied or even considered consciousness. In some explanations, it is synonymous with the mind, and at other times, an aspect of mind. In the past, it was one's "inner life", the world of introspection, of private thought, imagination and volition. Today, it often includes any kind of cognition, experience, feeling or perception. It may be awareness, awareness of awareness, or self-awareness either continuously changing or not. The disparate range of research, notions and speculations raises a curiosity about whether the right questions are being asked.
The origin of the modern concept of consciousness is often attributed to Locke's Essay Concerning Human Understanding, published in 1690.Locke defined consciousness as "the perception of what passes in a man's own mind".His essay influenced the 18th-century view of consciousness, and his definition appeared in Samuel Johnson's celebrated Dictionary (1755). "Consciousness" (French: conscience) is also defined 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". Consciousness has also become a significant topic of interdisciplinary research in cognitive science, involving fields such as psychology, linguistics, anthropology,[6] 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.
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.The degree of consciousness is measured by standardized behavior observation scales such as the Glasgow Coma Scale.
In the late 20th century, philosophers like Hamlyn, Rorty, and Wilkes have disagreed with Kahn, Hardie and Modrak as to whether Aristotle even had a concept of consciousness. Aristotle does not use any single word or terminology to name the phenomenon; it is used only much later, especially by John Locke. Caston contends that for Aristotle, perceptual awareness was somewhat the same as what modern philosophers call consciousness.