The Theosophical manual by Annie Besant helps us understand Reincarnation as the law of necessity for human progression and perfection, and has been constantly gaining ground among the more thoughtful students of life and evolution. Reincarnation for a human is the modus operandi of the vast evolutionary journey. Reincarnation and Karma are the twin doctrine of hope and responsibility. The fundamental cause of Reincarnation, as of all manifestation, is the desire for active life,  the thirst for sentient existence. A human,  being self-conscious, may build up to do evil or may build up to do good by his or her own thoughts. The resistance of wrong thought and action, patient service to others, a scrupulous devotion to unselfish ends - these build up for the reincarnating human a brain which is the fit instrument for their increased manifestation, a brain in which good tendencies will find grooves ready for their easy working.