The series was originally based on "Digital Devil Story", a science fiction novel series by Aya Nishitani. The series takes its name from the first book's subtitle. Most "Megami Tensei" titles are stand-alone entries with their own stories and characters. Recurring elements include plot themes, a story shaped by the player's choices, and the ability to fight using and often recruit creatures (demons, Personas) to aid the player in battle. Elements of philosophy, religion, occultism, and science fiction have all been incorporated into the series at different times.