In these much-valued lectures, Rudolf Steiner begins by positing the question, Why investigate the spiritual worlds at all? He goes on to explore the contemporary need for spiritual knowledge and the authentic paths that can lead to it. Speaking in Torquay, England, at the International Summer School organized by his friend and colleague D.N. Dunlop, Steiner surveys the differences in various types of consciousness from ancient to modern times, in waking and dreaming, from space into time and the changes that have taken place in relation to knowledge and science in the course of history. He goes on to discuss the Mystery nature of crystallized minerals and metals such as copper and silver, and their relationship to the planets. He also describes how the ages of life can become organs of perception. In a dramatic conclusion Steiner explains the role of moon beings, ahrimanic elemental beings, and the true nature of mediumship, ectoplasm and spiritual possession. Presented here in a fresh translation that corrects many errors in previous editions, the text is complemented with notes, an introduction by Paul King and an index. Lectures include: Nature is the Great Illusion. Know Thyself; The Three Worlds and their Reflected Images; Form and Substantiality of the Mineral Kingdom in relation to Human Levels of Consciousness; The Secret of Research into other Realms through the Metamorphosis of Consciousness; The Inner Enlivening of the Soul through the Qualities of Metals; Initiation Science; Star Knowledge; Possible Aberrations in Spiritual Research. (Eleven lectures, 11-22 Aug. 1924, GA 243)