Incarnations governed by law of karma: In the successive incarnations of an individual soul, there is not only a thread of continuity and identity-manifested in personal memory and revived in the case of advanced souls-but there is also an uninterrupted reign of the law of cause and effect through the persistence and operation of karma. The successive incarnations with all their particulars are closely and unfailingly determined by rational law, so that it becomes possible for the individual soul to mold its future through wise and intelligent action. The actions of past lives determine the conditions and circumstances of the present life, and the actions of the present life have their share in determining the conditions and circumstances of future lives. Successive incarnations of the individual soul yield their full significance only in the light of the operation of the law of karma.
Karma persists through mental body: The intermittent incarnations in the gross world are only apparently disconnected. Karma persists as a connecting link and determining factor through the mental body, which remains a permanent and constant factor through all the lives of the soul. The law of karma and its manner of operation cannot be fully intelligible as long as the gross body and the gross world are considered to be the only facts of existence. Karmic determination is made possible by the existence of the subtle and mental bodies and worlds.
-Discourses 7th Ed., p327