Sigmund Freud didn't exactly invent the idea of the conscious versus unconscious mind, but he certainly was responsible for making it popular and this was one of his main contributions to psychology.
Freud compared himself to an archaeologist digging away layers of the human mind, and identified three separate areas of the mind.
Freud (1915) described conscious mind, which consists of all the mental processes of which we are aware. For example, you may be feeling thirsty at this moment and decide to get a drink.
Unconscious Mind The unconscious mind contains our biologically based instincts (Eros and Thanatos) for the primitive urges for sex and aggression.
While we are fully aware of what is going on in the conscious mind, we have no idea of what information is stored in the unconscious mind.
The unconscious contains all sorts of significant and disturbing material which we need to keep out of awareness because they are too threatening to acknowledge fully.
So, the unconscious is not like a dust bin containing unimportant or irrelevant thoughts. Rather, it is precisely because they are so powerful that they are kept buried. Nevertheless, they exert a significant influence.
Subconscious / Pre-conscious Mind The pre-conscious contains thoughts and feelings that a person is not currently aware of, but which can easily be brought to consciousness. It exists just below the level of consciousness before the unconscious mind.
This is what we mean in our everyday usage of the word available memory. For example, you are presently not thinking about your mobile telephone number, but now it is mentioned you can recall it with ease. Mild emotional experiences may be in the subconscious but sometimes traumatic and powerful negative emotions are repressed and hence not available in the subconscious.
Our feelings, motives and decisions are actually powerfully influenced by our past experiences, stored in the pre-conscious and instincts from the unconscious.
Freud applied these three systems to his structure of the personality, or psyche – the id, ego and superego. Here the id is regarded as entirely unconscious whilst the ego and superego have conscious, preconscious, and unconscious aspect. Freud also regarded the mind to be like an iceberg, where the greatest part is hidden beneath the water or unconscious.
Freud believed that the influences of the pre-conscious and unconscious reveal themselves in a variety of ways, including dreams, and in slips of the tongue, now popularly known as 'Freudian slips'. Freud (1920) gave an example of such a slip when a British Member of Parliament referred to a colleague with whom he was irritated as 'the honorable member from Hell' instead of from Hull.
However, to other psychologists determined to be scientific in their approach (e.g. behaviorists) the concept of the unconscious mind has proved a source of considerable frustration because it defies objective description and is extremely difficult to objectively test or measure.