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The id, ego, and super-ego are three distinct, yet interacting agents in the psychic apparatus defined in Sigmund Freud's structural model of the psyche.

The three parts are the theoretical constructs in terms of whose activity and interaction our mental life is described. According to this Freudian model of the psyche, the id is the set of uncoordinated instinctual trends; the super-ego plays the critical and moralizing role; and the ego is the organized, realistic part that mediates between the desires of the id and the super-ego.[1]

As Freud explained:
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 it 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 it 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. (p. 19).[2]
Although the model is structural and makes reference to an apparatus, the id, ego and super-ego are purely psychological concepts and do not correspond to (somatic) structures of the brain such as the kind dealt with by neuroscience. The super-ego is observable in how someone can view themselves as guilty, bad, shameful, weak, and feel compelled to do certain things. Freud in The Ego and the Id discusses "the general character of harshness and cruelty exhibited by the [ego] ideal – its dictatorial 'Thou shalt.'"

Freud (1933) hypothesizes different levels of ego ideal or superego development with increasingly greater ideals:
...nor must it be forgotten that a child has a different estimate of [their] parents at different periods of [their] 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.
— New Introductory Lectures on Psychoanalysis, p. 64
The earlier in development, the greater the estimate of parental power. When one defuses into rivalry with the parental imago, then one feels the 'dictatorial thou shalt' to manifest the power the imago represents. Four general levels are found in Freud's work: the auto-erotic, the narcissistic, the anal, and the phallic.[3] These different levels of development and the relations to parental imagos correspond to specific id forms of aggression and affection. For example, aggressive desires to decapitate, to dismember, to cannibalize, to swallow whole, to suck dry, to make disappear, to blow away, etc. animate myths, are enjoyed in fantasy and horror movies, and are observable in the fantasies and repressions of patients across cultures.

The concepts themselves arose at a late stage in the development of Freud's thought as the "structural model" (which succeeded his "economic model" and "topographical model") and was first discussed in his 1920 essay Beyond the Pleasure Principle and was formalized and elaborated upon three years later in his The Ego and the Id. Freud's proposal was influenced by the ambiguity of the term "unconscious" and its many conflicting uses.