48: "The seven keys open the mysteries, past and future, of the seven great rootraces and of the seven kalpas."  Every occult book, symbol and allegory can be subjected to seven interpretations.  There are three locks to be opened.  Seven keys.  Every book can be read exoterically, subjectively and spiritually.  All the keys are not yet available.  (See Secret Doctrine, I, 330, 343.)  There is the physiological key, the psychological, the astrological and the metaphysical.  The fifth key is the geometrical.