Rahner & Battershaw 1971, "Mystery of the Cross", s. 49–50. Se og Grigg 1977, s. 477 (inkludert note 59) Rahner, Hugo; Battershaw, Brian (translator) (1971). Greek Myths and Christian Mystery. New York, New York: Biblo & Tannen Booksellers and Publishers Incorporated. ISBN0-8196-0270-1. Grigg, Robert (1977). «"Symphōnian Aeidō tēs Basileias": An Image of Imperial Harmony on the Base of the Column of Arcadius». The Art Bulletin. 59 (4): 469–482.
Plato. Timaeus, 8.36b and 8.36c: «And thus the whole mixture out of which he cut these portions was all exhausted by him. This entire compound divided lengthways into two parts, which he joined to one another at the centre like the letter X, and bent them into a circular form, connecting them with themselves and each other at the point opposite to their original meeting-point; and, comprehending them in a uniform revolution upon the same axis, he made the one the outer and the other the inner circle.»