Henricus Cornelius Agrippa (1486-1535) was also an astrologer and writer. He authored one of the most influential books on the occult, 'De occulta philosophia libri tres'; a study of elemental, celestial, and intellectual magic. The book discusses the four classical elements, (earth, air, fire, and water), astrology, numbers, angels, gods, mystical names and virtues, and how they relate to medicine and alchemy.