Louis Renou, Religions of Ancient India (London, 1953), p. 20; Michael Witzel, ‘Vedas and Upanishads’, in Gavin Flood, ed., Blackwell Companion to Hinduism (Oxford, 2003), pp. 70–71; J. C. Heesterman, ‘Ritual, Revelation and the Axial Age’, in S. N. Eisenstadt, ed., The Origins and Diversity of Axial Age Civilizations (Albany, NY, 1986), p. 398