Reincarnation is the return of the spiritual Principle in a new fleshly envelope. For a human being, this envelope is always a human body. This return can take place on the same planet where one lived his last existence, or on another planet. The return to a material body cannot be fixed in advance, just as one cannot fix a time for terrestrial life. Some human beings spend 3 years on earth and others 90. If one were to say: man lives 30 years above the earth, one would speak as an amateur of statistics, but not as an observer of real laws. The length of life on earth is a personal fact, so the length of time one spends before returning to earth is also a special fact that depends on many circumstances. It must be said at once that before reincarnating on a planet, the spiritual being prepares himself for the loss of the memory of his previous existences. We will be able to give all the testimonies of the classical authors concerning this fact and symbolized by the immersion in the water of the river Lete, which gives oblivion before the return to the earth. The reincarnation, really, has been marked as an esoteric mystery in all the initiations of the Antiquity.