20. ↑ Robbins, Ptolemy Tetrabiblos, 'Introduction' p. xii. ↑ "The History of Astrology". Retrieved 2016-12-28. ↑ FA Robbins, 1940; Thorndike 1923) ↑ Firmicus (4th century) (III.4) 'Proemium'. ↑ Roberts (1906) p.488. ↑ Campion (2008) p.