The soul, which in reality is one and undifferentiated, is apparently individualized through the limitations of the mental body, which is the seat of the ego-mind. The ego-mind is formed by the accumulated impressions of past experiences and actions. And it is this ego-mind that constitutes the kernel of the existence of the reincarnating individual. The ego-mind, as a reservoir of latent impressions, is the state of the mental body. The ego-mind, becoming spirit and experiencing activated and manifested impressions, is the state of the subtle body. The ego-mind, as descended in the gross sphere for creative action, is the state of a physical incarnation. Thus the ego-mind, which is seated in the mental body, is the entity that contains all the phases of continued existence as a separate individual.
-Discourses 7th Ed., p328