The study of Kabbalah. Not the Kabbalah of Madonna, Britney Spears, and these other pieces of trash, but real Kabbalah that only a selected few people from a generation are allowed to study, and practice. To study Kabbalah you have to be over 40 years of age, learned on the ENTIRE Tanach, Shas, have a beard, and be a man. Only a selected few rabbis can study this mystic art, and whoever studys it without being allowed, harm will come onto them. A great book on kabbalah even for goyim, is Faith and Folly: by: Rabbi Yaakov Hillel. In hebrew it is called "Tamim Tihiye" by the same rabbi.