The id, ego, and super-ego are a set of three concepts in psychoanalytic theory describing distinct, interacting agents in the psychic apparatus (defined in Sigmund Freud's structural model of the psyche). The three agents are theoretical constructs that describe the activities and interactions of the mental life of a person. In the ego psychology model of the psyche, the id is the set of uncoordinated instinctual desires; the super-ego plays the critical and moralizing role; and the ego is the organized, realistic agent that mediates between the instinctual desires of the id and the critical super-ego; Freud explained that:

The functional importance of the ego is manifested in the fact that, normally, control over the approaches to motility devolves upon it. Thus, in its relation to the id, [the ego] is like a man on horseback, who has to hold in check the superior strength of the horse; with this difference, that the rider tries to do so with his own strength, while the ego uses borrowed forces. The analogy may be carried a little further. Often, a rider, if he is not to be parted from his horse, is obliged to guide [the horse] where it wants to go; so, in the same way, the ego is in the habit of transforming the id's will into action, as if it were its own.

The existence of the super-ego is observable in how people can view themselves as guilty and bad, shameful and weak, and feel compelled to do certain things. In The Ego and the Id (1923), Freud presents "the general character of harshness and cruelty exhibited by the [ego] ideal — its dictatorial Thou shalt"; thus, in the psychology of the ego, Freud hypothesized different levels of ego ideal or superego development with greater ideals:

. . . nor must it be forgotten that a child has a different estimate of his parents at different periods of his life. At the time at which the Oedipus complex gives place to the super-ego they are something quite magnificent; but later, they lose much of this. Identifications then come about with these later parents as well, and indeed they regularly make important contributions to the formation of character; but in that case they only affect the ego, they no longer influence the super-ego, which has been determined by the earliest parental images.

The earlier in the child's development, the greater the estimate of parental power; thus, when the child is in rivalry with the parental imago, the child then feels the dictatorial Thou shalt, which is the manifest power that the imago represents on four levels: (i) the auto-erotic, (ii) the narcissistic, (iii) the anal, and (iv) the phallic. Those different levels of mental development, and their relations to parental imagos, correspond to specific id forms of aggression and affection; thus aggressive and destructive desires animate the myths in the fantasies and repressions of patients, in all cultures. In response to the unstructured ambiguity and conflicting uses of the term "the unconscious mind", Freud introduced the structured model of ego psychology (id, ego, super-ego) in the essay Beyond the Pleasure Principle (1920) and elaborated, refined, and made that model formal in the essay The Ego and the Id.