It is an analytical study of the human psyche outlining his theories of the psychodynamics of the id, ego and super-ego, which is of fundamental importance in the development of psychoanalysis.

[1] The Ego and the Id develops a line of reasoning as a groundwork for explaining various (or perhaps all) psychological conditions, pathological and non-pathological alike.

Freud argues that (according to his work with psychoanalysis) the supposedly conscious ego can be shown to possess unconscious thoughts (16) when it unknowingly resists parts of itself.

It is this idea of perception that leads Freud to call the ego a "body-ego" (31)—a mental projection of the surface of one's physical body.

Sexual instincts that stem from the id and bring about the Oedipus complex, are what dictate the shape and structure of the super-ego.

He props up his argument for these forces by appealing to cosmology and by implicitly invoking ideas of entropy and Newton's third law of motion (that of equal and opposite forces): “the task of [the death instinct] is to lead organic matter back to the inorganic state; on the other hand... Eros aims at more far-reaching coalescence of the particles into which living matter has been dispersed” (56).

Besides this purely aesthetic reasoning, Freud gives no further argument for the existence of these two opposing instincts—save to (parenthetically) mention "anabolism and katabolism" (56), the cellular processes of building up and breaking down molecules.

The answer may lie in sexuality—in a “narcissistic reservoir of libido... [that is] desexualized Eros.” This process of desexualization occurs, according to Freud, when libidinal energy passes from the id (its origin) into the ego—which (through a process called “sublimation”) abandons the original sexual aims and utilizes the energy to fuel thought and self-interested motility (62).

If the ideas he posits here are accurate, then the ego, indeed, finds itself a victim to the stronger super-ego and id (which tend to work together).

Freud cites his experiences in psychoanalysis, in which people exhibit a sense of guilt that makes them resistant to conquering their pathology.

His explanation is that the super-ego condemns the ego—"[displaying] particular severity and [raging] against the ego with the utmost cruelty" (73) and giving it a deep-seated, mysterious feeling of guilt.

Sometimes (in the case of melancholia) the ego has identified with a forbidden love-object so strongly, that it can't bear the super-ego's criticism and gives up—with suicide.

And finally (in cases of hysteria) both the object, the feelings for it, and resulting guilt (caused by the super-ego's criticism) are repressed—causing hysteric reactions.

This task falls to the ego because it is the only part of the mind capable of exercising direct control over the actions of the body.

Thus the ego finds itself the seat of anxiety, beset by potential dangers from three directions (84)—by the super-ego, the id, and the external world.