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1116: A Great Thinker Converts and Then Turns on the Jews

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13th century Belgian artwork depicting a scene from Petrus Alfonsi's 'Dialogues Against the Jews.'Credit: Sodabottle / Wikimedia Commons

On June 29, 1116 the astronomer, physician and philosopher Moses Sephardi entered the cathedral in Huesca, in what is today the Spanish province of Aragon, as a Jew and emerged as a Christian. In his new identity as Petrus Alfonsi, he became one of the most significant anti-Jewish polemicists of the Middle Ages. Petrus’ most influential work, “The Dialogues against the Jews,” introduced a whole new line of argument into the Christian attempt to prove Judaism wrong, and helped pave the way for increasingly aggressive attacks on Jews in Latin Europe.

Most of the little that is known of the life of Petrus Alfonsi comes from his own writings. He is believed to have been born in Huesca, sometime in the latter half of the 11th century. His uniqueness as a scholar derived from his knowledge of Hebrew, Latin and Arabic, and his learnedness in the traditions of Judaism, Christianity and Islam. (In addition to the role he played as debunker of Judaism – and Islam as well -- Petrus was also significant in making the literary and scientific texts of the Arab world accessible to Christian Europe.)

June 29, the date Moses Sephardi chose for his conversion, is the Feast Day of Peter and Paul in the Roman Catholic church. He later wrote that he chose the names “Petrus” and “Alfonsi,” respectively, in honor of St. Peter and of King Alfonso I of Aragon, his godfather. For this reason, it is thought that he may have had an official role in the court of Alfonso, who ruled 1104-1134. 

In the years before his conversion, between approximately 1110 and 1116, Moses Sephardi lived in England, where, according to a reference in one of his works, he served as physician to King Henry I (reigned 1100-1135). Sephardi is known to have taught astronomy in England and France, and is thought to have collaborated on a eclipse chart with an English colleague named Walcher of Malvern.

His book “Disciplina Clericalis” (A Training School for the Clergy) comprises 39 moral tales from the Orient, translated by the scholar from the Arabic, Persian and Sanskrit. Some of the stories are also known to us from the Persian work “Tales from the Arabian Nights.”

'Petrus' converts 'Moses'

"Dialogues against the Jews” was apparently compiled by the convert Petrus Alfonsi as a response to the harsh criticism he was subjected to by Jews angered by his apostasy. To make the case for the legitimacy of his conversion, and disprove the charge that he was motivated by opportunism, Petrus created a 12-part debate between a Jew he called “Moses” and a Christian named “Petrus.” Although the tone of the conversation is friendly, that only masked the devastating nature of the attack on both Judaism and Islam.

Augustine’s critique of Judaism, which had previously set the tone for the Christian attitude vis-à-vis its sister religion, had argued that Jews, in their blind obedience of the old law – that is, the law of the Hebrew Bible – had not understood that it had been superseded by the New Testament. Petrus introduced his readers to the Talmud, which had hitherto been unknown outside Jewish circles. He claimed that the Jews, having been led astray by their rabbis, had abandoned the Hebrew Bible – which was still valid -- for the laws of the newfangled Talmud, which was both “heretical” and a “fabric of lies.”

This argument both gets around the danger inherent in Augustinian theology that arose from placing the blame for the Jews’ mistakes on the Hebrew Bible, which after all is the basis for Christianity, and transferring it to the Jews’ perfidious leaders, who led them into crucifying Jesus. It also leaves the door open to the Jews’ eventual conversion, since there is nothing inherently satanic about them as a people.

The “Dialogues” then proceeds to systematically debunk Islam, depicting Muhammad as a false prophet and the Koran as heretical.

Finally, Petrus proves to his interlocutor, Moses, the correctness of Christianity, demonstrating how it is compatible with the Old Testament. Naturally, Moses converts at the end of the book.

“The Dialogues against the Jews” became the most widely read anti-Jewish text of medieval times. It also served as the basis for the Christian (mis)understanding and condemnation of Islam.

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