Helena Petrovna Blavatsky was a Russian occultist, philosopher, and author who co-founded the Theosophical Society in 1875. She is best known for her influential writings on esotericism, mysticism, and the spiritual nature of the universe. Blavatsky's most famous work is "The Secret Doctrine," in which she expounded her ideas on theosophy, a belief system that seeks to understand the mysteries of existence and the divine through direct experience and intuition. Blavatsky's teachings have had a lasting impact on the New Age movement and continue to be studied and debated by scholars and spiritual seekers around the world.