It varies from person to person and from time to time. It has no autonomy. Ego's roots are to be found in the electro-chemical functions of the physical brain. Personality is reduced to the status of being a by-product of the body. The psychology of Freud retains the awareness of a three dimensional experience of our human-being even as it collapses the three into local variations of the single dimension, Time.
Modern psychology can thus appear to have continuity with the psychology of the past.
Freudian psychology appears to be a scientific refinement of the ancient view. Simultaneously, the modern rejection of the past is now justified. The shamanistic, the prophetic, and the mystical can be rejected as illusory, since they depend upon the existence of independent, transcendental psychic and spiritual dimensions. Wisdom is replaced by Science. For Freud, Psyche is an illusory and temporary by-product of a purely temporal struggle between the spirit of society and the instinctual urges.