1. Aristotle, Politics A11, 1259 a9 (before 322AD)
2. Plato, Theaetetus 174A (before 347AD)
3. G. Kirk, J. Raven, The Presocratic Philosophers: A Critical History with a Selection of Their Texts. Sect. 74, 75 (Cambridge Univ. Press, Cambridge England, 1957), p. 78
4. J. Esmark, Mag. Naturvidenskaberne 2, 29 (1824)
5. J. Esmark, Edinb. New Philos. J. 2, 107 (1926)