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
Thomas Hobbes (1839). “Of Liberty and Necessity; a treatise, wherein all controversy concerning predestination, election, free will, grace, merits, reprobation, etc. is fully decided and cleared. New edition”, p.11