(Latin: conscire, to know)
In an objective sense, signifies the totality of psychic phenomena which distinguish the state of wakefulness from that of a dreamless sleep.
It includes the whole range of cognitive, emotional, and appetitive states that can be apprehended.
In a subjective sense it denotes the cognitive act by which psychic states actually present are known and recognized as one's own.
This consciousness may be exercised in three ways.
It may accompany any act of knowing; in the very act of perceiving an object, concomitantly with the knowledge of the object the mind knows the perception as its own though attention is not centered on this subjective phase of the act.
Or it may be the act of a faculty expressly attending to the operations of other faculties, as when the mind is aware of the operations of the senses, movements of the. body, etc.
Or it may be the reflexive act, called self-consciousness, by which the mind applies itself to observe its own activities.
This reflex consciousness marks the essential difference between man and lower animals; since the latter have a faculty whose act renders them conscious of the operations of other faculties, but have no faculty capable of knowing its own act.
New Catholic Dictionary