The naive pre-scientific beliefs of sages whose reflections on metaphysical topics continue to stimulate displays a contrast which demonstrates the perennial aspects of philosophical thought. Their errors are not just a matter of knowledge but of faulty connections. Every schoolboy now knows what sorts of explanations are likely and what are not and will not accept the incredible simply on the basis that it is widely believed. Augustine in his City of God uses the concept of the ‘incredible’ in ways which appear to run counter to each other. In Vol.II Part XXI Chap. 4 he writes about wonders which, unless you had experienced them, you would discount as incredible. Some of his examples, the salamander who lives in fire and the incorruptible flesh of the peacock for instance are simply superstitions, others such as the nature of charcoal and lime used in mortar have properties that astound and baffle by reason of their virtue. Diamonds which resist all forces of destruction are easily wrought by the application of goat’s blood. A diamond will also negate the power of a magnet.
The diamond is a stone possessed by many among ourselves, especially by jewellers and lapidaries, and the stone is so hard that it can be wrought neither by iron nor fire, nor, they say, by anything at all except goat's blood.
Yet far more astonishing is what I heard about this stone from my brother in the episcopate, Severus bishop of Milevis. He told me that Bathanarius, once count of Africa, when the bishop was dining with him, produced a magnet, and held it under a silver plate on which he placed a bit of iron; then as he moved his hand with the magnet underneath the plate, the iron upon the plate moved about accordingly. The intervening silver was not affected at all, but precisely as the magnet was moved backwards and forwards below it, no matter how quickly, so was the iron attracted above. I have related what I myself have witnessed; I have related what I was told by one whom I trust as I trust my own eyes. Let me further say what I have read about this magnet. When a diamond is laid near it, it does not lift iron; or if it has already lifted it, as soon as the diamond approaches, it drops it.
It would be invidious to chide Augustine for his credulity in these instances but his use of the notion of the incredible as giving some sort of warrant to theological beliefs is paradoxical.
A full quote is necessary to give the flavour of this:
But granting that this was once incredible, behold, now, the world has come to the belief that the earthly body of Christ was received up into heaven. Already both the learned and unlearned have believed in the resurrection of the flesh and its ascension to the heavenly places, while only a very few either of the educated or uneducated are still staggered by it. If this is a credible thing which is believed, then let those who do not believe see how stolid they are; and if it is incredible, then this also is an incredible thing, that what is incredible should have received such credit. Here then we have two incredibles,—to wit, the resurrection of our body to eternity, and that the world should believe so incredible a thing; and both these incredibles the same God predicted should come to pass before either had as yet occurred. We see that already one of the two has come to pass, for the world has believed what was incredible; why should we despair that the remaining one shall also come to pass, and that this which the world believed, though it was incredible, shall itself occur? For already that which was equally incredible has come to pass, in the world's believing an incredible thing. Both were incredible: the one we see accomplished, the other we believe shall be; for both were predicted in those same Scriptures by means of which the world believed. And the very manner in which the world's faith was won is found to be even more incredible, if we consider it.(Vol.II. 22:5)
Believing in the diamond cutting efficacy of goat’s blood is a good preparation for that which of its nature cannot be tested. In its way this is a proleptic trumping of Hume’s ‘the truth of a miracle is a greater miracle than the miracle itself’. The incredible general acceptance of the incredible is for Augustine made rational by the fact of miracles. Can anyone doubt their centrality?