Petrus Alphonsi

He was born Moses Sephardi, a Jew of Huesca in Aragon, in northern Spain, probably around 1076. He was a physician and an astronomer, trained in the Arabic learning that made Spain the most scientifically advanced corner of Europe.

On 29 June 1106 he was baptized at Huesca. The king of Aragon, Alfonso I, stood as his godfather, and he took the name Petrus Alfonsi, Peter, for the feast day of Saints Peter and Paul, and Alfonsi for his sponsor. He was about thirty.

He then had to explain himself, and he did it in an unusual way. The Dialogi contra Iudaeos is a debate in twelve parts between two characters. One is called Moses, the other Peter. They are the same man: his old self and his new one, arguing.

Moses gets real arguments. He accuses Peter of abandoning his people for worldly advantage, presses on the incarnation, the Trinity, and the resurrection, and does not simply fold at the first reply. The book also turns on Islam, Peter had lived among Muslims and read Arabic, and his account of Muhammad's life is one of the earliest in Latin Europe drawn from actual Arabic sources rather than rumour. It is a polemic, written to persuade, and it was used for centuries in ways that hardened Christian hostility to Jews. That is part of its record and should be said.

He went north. He was in England, apparently as a physician at the court of Henry I, and taught astronomy there; his work on the astrolabe and his translation of al-Khwarizmi's astronomical tables were among the first channels by which Arabic science reached England. He wrote a Letter to the Peripatetics of France urging them to abandon the old learning and take up the astronomy of the Arabs. Adelard of Bath and the Latin translators who followed built on what he brought.

His most-read book was none of these. The Disciplina clericalis is a collection of thirty-odd short tales and proverbs drawn from Arabic, Persian, and Indian sources, framed as a father instructing his son. It is the first collection of Eastern stories to circulate in Latin Europe, and its tales turn up afterwards in Boccaccio, in Chaucer, in the Gesta Romanorum, and in Cervantes.

He was still alive in 1130 and probably some years after; the date of his death is not recorded.

Sources: Petrus Alfonsi, Dialogi contra Iudaeos and Disciplina clericalis; John Tolan, Petrus Alfonsi and His Medieval Readers; Charles Homer Haskins, Studies in the History of Mediaeval Science; The Jewish Encyclopedia (1906).