Stanislav Grof, Hal Zina Bennett (2009). “The Holotropic Mind: The Three Levels of Human Consciousness and How They Shape Our Lives”, p.18, Harper Collins
Richard Pascale, Jerry Sternin, Monique Sternin (2013). “The Power of Positive Deviance: How Unlikely Innovators Solve the World's Toughest Problems”, p.120, Harvard Business Press
Richard Chenevix Trench (1867). “On the Study of Words Lectures Addressed (originally) to the Pupils at the Diocesan Training-school, Winchester by Richard Chenevix Trench”, p.24