Consciousness is the perception of what passes in a man’s own mind.
 John Locke, An Essay Concerning Human Understanding (1689). copy citation

Context

“They who talk thus may, with as much reason, if it be necessary to their hypothesis, say that a man is always hungry, but that he does not always feel it; whereas hunger consists in that very sensation, as thinking consists in being conscious that one thinks. If they say that a man is always conscious to himself of thinking, I ask, How they know it? Consciousness is the perception of what passes in a man’s own mind. Can another man perceive that I am conscious of anything, when I perceive it not myself? No man’s knowledge here can go beyond his experience. Wake a man out of a sound sleep, and ask him what he was that moment thinking of.” source