“ 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