Nicene and Post-Nicene Fathers, Vol. II:
On Christian Doctrine: Chapter 24

Chapter 24.—The Intercourse and Agreement with Demons Which Superstitious Observances Maintain.

37.  And all these omens are of force just so far as has been arranged with the devils by that previous understanding in the mind which is, as it were, the common language, but they are all full of hurtful curiosity, torturing anxiety, and deadly slavery.  For it was not because they had meaning that they were attended to, but it was by attending to and marking them that they came to have meaning.  And so they are made different for different people, according to their several notions and prejudices.  For those spirits which are bent upon deceiving, take care to provide for each person the same sort of omens as they see his own conjectures and preconceptions have already entangled him in.  For, to take an illustration, the same figure of the letter X, which is made in the shape of a cross, means one thing among the Greeks and another among the Latins, not by nature, but by agreement and pre-arrangement as to its signification; and so, any one who knows both languages uses this letter in a different sense when writing to a Greek from that in which he uses it when writing to a Latin.  And the same sound, beta, which is the name of a letter among the Greeks, is the name of a vegetable among the Latins; and when I say, lege, these two syllables mean one thing to a Greek and another to a Latin.  Now, just as all these signs affect the mind according to the arrangements of the community in which each man lives, and affect different men’s minds differently, because these arrangements are different; and as, further, men did not agree upon them as p. 548 signs because they were already significant, but on the contrary they are now significant because men have agreed upon them; in the same way also, those signs by which the ruinous intercourse with devils is maintained have meaning just in proportion to each man’s observations.  And this appears quite plainly in the rites of the augurs; for they, both before they observe the omens and after they have completed their observations, take pains not to see the flight or hear the cries of birds, because these omens are of no significance apart from the previous arrangement in the mind of the observer.