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Onychomancy is the art of Divination using finger-nails. The future would reveal itself in a series of images that would appear on the finger-nails of a boy when the suns reflection on them is studied. The location of the images on the finger nail determined how soon the divined events would occur.
The following passage is from Policraticus by John of Salisbury circa 1159 (an early book on political philosophy), where he describes this type of divination. “During my boyhood I was placed under the direction of a priest, to teach me psalms. As he practised the art of crystal gazing, it chanced that he after preliminary magical rites made use of me and a boy somewhat older, as we sat at his feet, for his sacrilegious art, in order that what he was seeking by means of finger nails moistened with some sort of sacred oil of crism, or the smooth polished surface of a basin, might be made manifest to him by information imparted by us, and so after pronouncing names which by the horror they inspired seemed to me, child though I was, to belong to demons, and after administering oaths of which, at God's instance, I know nothing, my companion asserted that he saw certain misty figures, but dimly, while I was so blind to all this that nothing appeared to me except the nails or basin and the other objects I had seen there before."
This indictment against Abbot William Sadyngton of Leicester Abbey describes his use of this divination art. " ... The same, whether as one wavering in faith or straying from the faith and the fired judgement of the catholic church, did practice in his own person, contrary to such faith and fixed judgement, divinations or incantations after this manner, to wit, an the eve or on the day of the feast of St Matthew (either 20th or 21st September 1439) at Ingarsby, he took to himself a boy, Maurice by name, and, observing a damnable superstition, smeared the boy's thumb-nail, bidding him look upon his nail and discover to him what sort of things he saw there, and, reading or saying his charms the while, asked the same boy what he had seen, incurring the sentence of the greater excommunication passed against such persons in general; and knowing himself to be thus excommunicate, has since then celebrated masses, even in solemn wise, and otherwise has taken part in divine service, incurring irregularity."