Rudolf Steiner, Johanna Collis (2000). “Rosicrucian Wisdom: An Introduction ; Fourteen Lectures Given in Munich Between 22 May and 6 June 1907”, p.11, Rudolf Steiner Press
Rudolf Steiner, Christopher Bamford (2004). “Start Now!: A Book of Soul and Spiritual Exercises : Meditation Instructions, Meditations, Exercises, Verses for Living a Spiritual Year, Prayers for the Dead & Other Practices for Beginning and Experienced Practitioners”, p.124, SteinerBooks
Rudolf Steiner, Alice Wulsin (1983). “The inner nature of music and the experience of tone: selected lectures from the work of Rudolf Steiner”, Rudolf Steiner Pr