Marci Shimoff, Carol Kline, Marianne Williamson (2012). “Love For No Reason: 7 Steps to Creating a Life of Unconditional Love”, p.49, Simon and Schuster
Attributed to Aristotle in Diogenes Laertius "The Lives and Opinions of Eminent Philosophers" translated by C. D. Yonge and Henry G. Bohn (p. 188), 1853.