Ahimsa as an ethical concept evolved in the Vedic texts. The oldest scriptures indirectly mention "Ahimsa", but do not emphasize it. Over time, the Hindu scripts revise ritual practices and the concept of "Ahimsa" is increasingly refined and emphasized, until "Ahimsa" becomes the highest virtue by the late Vedic era (about 500 BCE). For example, hymn 10.22.25 in the Rig Veda uses the words Satya (truthfulness) and "Ahimsa" in a prayer to deity Indra;English: Unto Tähtinen (1964), Non-violence as an Ethical Principle, Turun Yliopisto, Finland, PhD Thesis, pages 23–25; ;For other occurrence of "Ahimsa" in Rigveda, see Rigveda 5.64.3, Rigveda 1.141.5;</ref> later, the Yajur Veda dated to be between 1000 BCE and 600 BCE, states, "may all beings look at me with a friendly eye, may I do likewise, and may we look at each other with the eyes of a friend".For other occurrences of "Ahimsa" in Vedic literature, see A Vedic Concordance Maurice Bloomfield, Harvard University Press, page 151</ref>