Conscience - conscientious - conscious - consciousness
These are words that give rise to many poor spellings, and - perhaps because their form is confused - some confusion of use.
In spellings, it may help you if you know that they come from the Latin word that also gave us science (a word that few people mis-spell). The original Latin word was scio, which means 'I know': it was possibly pronounced with a '-k-' sound after the '-s-'. Modern church Latin (which is like Italian, in sound) pronounces '-sc-' as English pronounces '-sh-' (IPA: /ʃ/) -- which is the pronunciation of the '-sci-' group of letters in all these words.
Scio gave the noun scientia, the root of 'science' and 'scientist': it originally meant 'knowledge', and it did not have to have anything to do with what we should call nowadays 'the scientific method', or what are sometimes called 'the natural sciences' (Physics Chemistry, Biology and so on).
- The noun conscience means 'the awareness of being right or wrong in one's behaviour'. To have a bad conscience about something is to be aware that one has not done something - or that one has done something wrong. "My conscience is clear" means that 'I am not aware that I have done anything wrong'.
- 'Conscience' gives us the adjective conscientious. This has a slightly different meaning. It is used only for the idea of 'good conscience', and it means 'behaving in such a way as to deserve a good conscience'. A conscientious student is one who always does the work assigned, does it well and hands it in on time.
- The adjective conscious on the other hand means aware, or awake. "When the alarm clock rang, she was conscious in a moment".
- If you are conscious of something, you know that it exists. "I am conscious that you are an expert, so can I ask you...?"
- Consciousness is the state of being conscious. The opposite of conscious is unconscious, which is used to describe the state of sleep, or of a person who has had a bang on the head. It can also be used figuratively: "Throughout the lecture, the student was unconscious."
- Some modern novels use a technique called the stream of consciousness, where the writer tries to communicate exactly what is going through a character's head.
- The negative forms of conscious can give difficulty, particularly in Psychology. I understand that followers of the psychologist Sigmund Freud call the part of the mind that lies below our conscious control the subconscious, to make it clear that it is a higher level of mental operation than the unconscious, which means 'lacking consciousness' (i.e. asleep, or knocked out, like a boxer who has lost a fight). Followers of Jung, on the other hand, use unconscious to mean the part of the mind that is active, but below our conscious control. Ask an expert for better or more detailed explanation.
- Conscientious and conscious are two of the 117 mis-spellings listed as 'Common difficulties' in the section on 'Spelling' within 'Writing' in UEfAP.