Well, according to Freudian theory, there is the id, the ego, and the superego. Read this article.
Freud's Division of the Mind
David B. Stevenson '96, Brown University
Freud understood the mind as constantly in conflict with itself, and understood this conflict as the primary cause of human anxiety and unhappiness. His classic example is the patient Anna O, who displayed a rash of psychological and physiological symptoms: assorted paralyses, hysterical squints, coughs, speech disorders, and others. Under hypnosis, Freud and Josef Breuer, a fellow physician, traced many of these symptoms to memories of a period when she nursed her dying father. One symptom, a nervous cough, they related to a particular event at her father's bedside. Upon hearing dance music drifting from a neighbor's house, she felt an urge to be there, gone from her father's bedside. Immediately, she was struck with guilt and self-reproach. She covered this internal conflict with a nervous cough, and from that day on, coughed reflexively at the sound of rhythmic music. Freud's investigations into internal conflicts such as this led him to an eventual division of the mind into three parts, three conflicting internal tendencies, the well-known id, ego, and super-ego.
This division, it is important to note, is not the separation of the mind into three structures and functions which exist in physical partitions in the brain; they are not even truly structures, but rather separate aspects and elements of the single structure of the mind. Although it is convenient to say, for example, that the id "demands" immediate gratification, the mind has no three distinct little men who engage in a constant fisticuffs of conflict. The personification of these elements merely serves as a convenient guide through a complex psychoanalytic theory.
The id, the ego and the superego function in different levels of consciousness: indeed, Freud's theory of the mind hinges upon the ability of impulses or memories to "float" from one level to another. The interaction between the three functions of the mind represents a constant movement of items from one level to another.
As the baby emerges from the womb into the reality of life, he wants only to eat, drink, urinate, defecate, be warm, and gain sexual pleasure. These urges are the demands of the id, the most primitive motivational force. In pursuit of these ends, the id demands immediate gratification: it is ruled by the pleasure principle, demanding satisfaction now, regardless of circumstances and possible undesirable effects. If a young child was ruled entirely by his id, he would steal and eat a piece of chocolate from a store regardless of the menacing owner watching above him or even his parents scolding beside him.
The id will not stand for a delay in gratification. For some urges, such as urination, this is easily satisfied. However, if the urge is not immediately discharaged, the id will form a memory of the end of the motivation: the thirsty infant will form an image of the mother's breast. This act of wish-fulfillment satisfies the id's desire for the moment, though obviously it does not reduce the tension of the unfulfilled urge.
The eventual understanding that immediate gratification is usually impossible (and often unwise) comes with the formation of the ego, which is ruled by the reality principle. The ego acts as a go-between in the id's relations with reality, often supressing the id's urges until an appropriate situation arises. This repression of inappropriate desires and urges represents the greatest strain on, and the most important function of, the mind. The ego often utilizes defense mechanisms to achieve and aid this repression. Where the id may have an urge and form a picture which satisfies this urge, the ego engages in a strategy to actually fulfill the urge. The thirsty five-year-old now not only identifies water as the satisfaction of his urge, but forms a plan to obtain water, perhaps by finding a drinking fountain. While the ego is still in the service of the id, it borrows some of its psychic energy in an effort to control the urge until it is feasibly satisfied. The ego's efforts at pragmatic satisfaction of urges eventually builds a great number of skills and memories and becomes aware of itself as an entity. With the formation of the ego, the individual becomes a self, instead of an amalgamation of urges and needs.
While the ego may temporarily repress certain urges of the id in fear of punishment, eventually these external sources of punishment are internalized, and the child will not steal the chocolate, even unwatched, because he has taken punishment, right, and wrong into himself. The superego uses guilt and self-reproach as its primary means of enforcement for these rules. But if a person does something which is acceptable to the superego, he experiences pride and self-satisfaction.
The superego is sub-dividable into two parts: conscience and ego ideal. Conscience tells what is right and wrong, and forces the ego to inhibit the id in pursuit of morally acceptable, not pleasurable or even realistic, goals. The ego ideal aims the individual's path of life toward the ideal, perfect goals instilled by society. In the pursuit, the mind attempts to make up for the loss of the perfect life experienced as a baby.