What’s the difference between conscience and conscious? They stem from the same Latin root, but their usage is distinct. Writers occasionally confuse the two words, but if you remain conscious, you’ll likely be able to say with a clear conscience that you know the difference.
Conscience and conscious both come from the Latin word conscius; the word elements mean “with” and “to know.” (Yes, the -science in conscience means the same thing as science itself.)
Conscience is a noun meaning “sense of the quality of one’s character and conduct,” “adherence to moral principles,” and “consideration of fairness and justice.” Confusion between conscience and conscious occurs because the latter word is sometimes used as a noun synonymous with consciousness, meaning “mental awareness,” though the longer form is usually employed.
More often, however, conscious appears as an adjective meaning “aware” or “awake,” or “involving perception or thought.” It also appears in combination with a noun in phrasal adjectives such as “budget conscious” to refer to someone who is concerned, sensitive, or vigilant about something.
Conscience and conscious can be distinguished because the former word is qualitative — people have various degrees of moral strength — while conscious, as its antonym, unconscious, indicates, is quantitative: You’re either one or the other, whether the word is used as a noun or an adjective.
However, consciousness, as the word is usually applied, like conscious refers to a continuum: We speak of raising one’s consciousness and of higher consciousness, because this quality can be improved or increased. Like the noun conscious, though, consciousness has a quantitative sense as well, referring to a state of mental activity, as opposed to unconsciousness caused by illness or injury.
Other words descended from the Latin word are self-conscious, which literally means “self-aware” but has acquired a connotation of “preoccupied with how one is perceived by others,” an attitude that leads to shyness and stress, and conscionable and its more common antonym unconscionable; the latter means “inexcusable, reprehensible.”
Conscientious means “scrupulous” or “careful”; a conscientious objector is someone who objects to a requirement on religious grounds. Originally, around the turn of the twentieth century, the context was mandatory vaccination, but ever since World War I, the primary sense has been of a person who refuses military conscription.