Heinrich Cornelius Agrippa von Nettesheim (1486-1535), as befits a great magician, left behind him a number of mysteries for posterity. In two letters to his friends, in which he discussed the progress of his great treatise on magic De occulta philosophia libri tres1 [Three Books of Occult Philosophy, hereafter DOP ], Agrippa wrote of a “secret key” to the occult philosophy, a key which would be revealed only to his closest friends.2 In the latter half of the sixteenth century, it was commonly believed that this “key” referred to a text of black magic spuriously attributed to Agrippa,3 thus lending credence to the legends of Agrippa the black magician, which in turn led to Agrippa’s importance as a source for the Faust legends. But if the Fourth Book of Occult Philosophy was certainly a spurious work, what was Agrippa’s secret key to the occult philosophy?