The spirit self is a revelation of the world of spirit within the I, in the same way that, from the other direction, sense experience is a revelation of the physical world within the I.

 

Rudolf Steiner:  "The I lives in the soul. While the highest expression of the I belongs to the consciousness soul, we must say nevertheless that this I, emanating from there, fills the whole soul, and exercises its effect upon the body through the soul. And in the I the spirit is living. The spirit shines into the I and lives within it as in its sheath, as the I lives in body and soul as its sheaths. The spirit forms the I from within outward, the mineral world from without inward. We will call the I-creating spirit, the spirit that lives as I, 'spirit self," because it appears as the I or self of the human being.... The spirit self is a revelation of the world of spirit within the I, in the same way that, from the other direction, sense experience is a revelation of the physical world within the I. In what is red, green, yellow, hard, soft, warm, cold, we discern the revelations of the corporeal world; in what is true and good, we discern the revelations of the world of spirit."

Source: from Anthroposophy. quoted on p. 210 of The Mission of the New Spirit Revelation.