The psyche is not an indivisible unity but a divisible and more or less divided whole. Although the separate parts are connected with one another, they are relatively independent, so much so that certain parts of the psyche never become associated with the ego at all, or only very rarely.
CW, Volume 8, “The Psychological Foundation of the Belief in Spirits”, par 582
Carl Jung, part 1: Taking inner life seriously
Mark Vernon on achieving the right balance between what Jung called the ego and the Self and how central it is to his theory of personality development: