29 June 1106 Moses Sephardi becomes Petrus Alphonsi – philosopher, scientist and polemicist #otdimjh


A 13th-century Belgian manuscript showing the dialogue between the Jew “Moyses” and the Christian “Petrus”

Petrus Alphonsi (also known as Peter Alfonsi; born Moses Sephardi) was a Jewish Spanish physician, writer, astronomer, and polemicist, who converted to Christianity. [Wikipedia summarizing Tolan]


Petrus Alfonsi’s Tetragrammaton-Trinity diagram, which influenced Joachim of Fiore and the development of the Shield of the Trinity diagram.

Born at an unknown date in the 11th century in Huesca, when the city still was part of al-Andalus (Islamic Spain), he embraced Christianity and was baptized at Huesca on St. Peter’s Day, 29 June 1106. In honor of the saint Peter, and of his royal patron the Aragonese King Alfonso I and godfather he took the name of Petrus Alfonsi (Alfonso’s Peter).

Petrus was born a Jew while living in al-Andalus, and after he rose to prominence, he converted to Christianity. In his life and writing he carried the tension of dual Christian and Jewish identity with great awareness.


Alphonsi “was probably the first to connect the ‘ineffable’ trinity with the ‘ineffable’ Tetragram”.

His environment gave him an advantageous knowledge of Christianity, Judaism and Islam that would later prove useful in his polemics. John Tolan mentioned in his book Petrus Alfonsi and His Medieval Readers that “Alfonsi’s texts were received enthusiastically—he became an auctor, an authority to be quoted. His success was due in large part to his ability to bridge several cultures: a Jew from the [Muslim] world of al-Andalus.” His knowledge of these different religions is what makes Alfonsi unique and why he is essential to be studied when looking at Jewish-Christian and Christian-Moslem debate.


According to Tolan, Petrus Alfonsi was reared in a society in turmoil: a place of chaos and political instability, where Judaism was in conflict with science, and Islam and Christianity were becoming a larger influence. His background was conveniently placed in the center of contention between religions and circumstances that surrounded his upbringing, and provided the framework for apologetics and polemics that would shape Medieval Judaic perception.


Like many converts of his time, Alfonsi was accused of bad faith by the Jewish community and to counter this, as well as to show his zeal for his new faith he wrote a work attacking Judaism and defending the truths of the Christian faith. It became one of the most widely read and used anti-Jewish polemical texts of the Middle Ages, as Tolan shows. Alfonsi wrote the Dialogues in 1110; he presents them as a disputation between his former Jewish self (Moses) and his current Christian self (Peter). He divides it into twelve “Dialogues” or chapters: and the first four attack Judaism, the fifth attacks Islam, and the last seven defend Christianity. (see below for further details)

Prayer and reflection: Petrus Alfonsi faced the challenge of combining his Jewish identity and Christian faith in a way that harnessed and developed the tools and resources of Jewish-Christian polemics. This strategy would continue with disastrous effects for many centuries. His own sharp mind, and ability to master several fields of emerging knowledge, science and philosophy was not able to reconcile the disjuncture between the church and Israel. May Messianic Jews today have wisdom and insight to see the right relationship between Israel and the church, and their own particular role, without falling victim to prejudice, stereotyping and the ‘teaching of contempt’. In Yeshua’s name we pray. Amen.

Petrus Alfonsi and His Medieval Readers – John Victor Tolan – University Press of Florida (30 Nov. 1993)




Author’s Postprint version. Please cite published version: Wacks, David. “Conflicted Identity and Colonial Adaptation in Petrus Alfonsi’s Dialogus Contra Judaeos and Disciplina Clericalis.” Marginal Voices: Studies in Converso Literature of Medieval and Golden Age Spain. Ed. Gregory B. Kaplan & Amy Aronson-Friedman. Leiden: Brill, 2012. 69–90. http://www.brill.nl/marginal-voices

From Peter Chessworth

Petrus Alfonsi is one of the key actors in the transmission and assimilation of Arabic scientific, literary and religious texts and ideas to Latin Europe in the early 12th century. His impact is attested in the survival of roughly 160 manuscripts of his works, in the frequent use made of them by key authors from the 12th century to the 16th, and in their wide diffusion through early printed editions.

Petrus Alfonsi was born Moses, a Jew from al-Andalus. He was educated in Hebrew and Arabic; his writings show familiarity with the Talmud, with texts of Arabic astronomy, medicine and philos- ophy, and with the Arabic wisdom traditions. Moses converted to Christianity, and the first date associated with his life is that of his baptism, on June 29, 1106, in the cathedral (and former mosque) of Huesca. He explains that he took the name Petrus in honor of St Peter and Alfonsi in honor of his godfather, King Alfonso I of Aragon. This probably indicates that he played a role in Alfonso’s court, perhaps as royal physician and astrologer.

At some point between 1110 and 1116 Alfonsi went to England, where he taught astronomy, and in 1116 produced his Tabulae astro- nomicae (‘Astronomical tables’), a somewhat flawed Latin version of al-Khwārazmī’s Zīj al-Sindhind, a set of astronomical tables with accompanying ‘canons’ or explanatory texts. Two of his students in England are known by name: Walcher of Malvern and Adelard of Bath. Walcher composed a text on how to predict eclipses, based on the teachings of Alfonsi, and Adelard revised and improved Alfonsi’s Latin version of al-Khwārazmī’s text. According to one manuscript of Alfonsi’s Disciplina clericalis (‘Clerical instruction’), he served for a time as royal physician to King Henry I of England. Sometime in the 1120s, it seems, he was in France, as he wrote an Epistola ad peripateticos in Francia (‘Letter to the peripatetics in France’), in which he complains of his lack of students, professes his expertise in the art of astronomy, and lambastes Latin intellectuals for preferring the study of grammar and logic to the ‘hard science’ of astronomy.

The works of Petrus Alfonsi provide a fascinating glimpse at how the Latin West adapted and transformed the intellectual and cultural legacy of the Arab world. The historical Alfonsi himself imported new texts and new ideas into England and France: the aphorisms and fables of the eastern Wisdom traditions, astronomical texts and knowledge, and his own interpretations of the Qur’an and Talmud suffused with Hispano-Arab religious polemics. He shaped this knowledge to fit the needs and desires of his pan-European Latin readers. His rationalistic religious disquisitions reflect the concerns of the theologians of the 12th-century renaissance, of faith seeking under- standing. He passionately defended astronomy and affirmed that the study of nature could reveal God’s designs for creation. And the moral aphorisms of the Disciplina are directed to the edification of a proud new educated clerical elite.

His readers, copyists and continuers were to perpetuate the pro- cess of ‘naturalization’ of the Jewish and Arabic elements of Alfonsi’s thought, using the Dialogues to inform a new, harsher anti-Judaism, mining the Disciplina as grist for their sermon tales and instructive fables.

Like many converts of his time, Alfonsi was accused of bad faith by the Jewish community and to counter this, as well as to show his zeal for his new faith he wrote a work attacking Judaism and defending the truths of the Christian faith. It became one of the most widely read and used anti-Jewish polemical texts of the Middle Ages, as Tolan shows. Alfonsi wrote the Dialogues in 1110; he presents them as a disputation between his former Jewish self (Moses) and his current Christian self (Peter). He divides it into twelve “Dialogues” or chapters: and the first four attack Judaism, the fifth attacks Islam, and the last seven defend Christianity.

Up until the Dialogi contra Iudaeos, the Augustinian tradition was followed in Christendom which allowed relative tolerance to the Jewish people, and for the most part up until this point the attacks on the Jewish people were localized and more importantly, not organized. There was no literature before Petrus Alfonsi’sDialogi condemning Judaism as a whole. There was no document for people to latch on to and group up against the Jewish people.

Alfonsi attempted to prove Christianity by disproving Judaism. The difficulty in proving Christianity through the invalidity of Judaism is that the basic tenets of Christianity originate in the Old Testament; if a polemicist proves the Old Testament is invalid, then ipso facto he also proves the invalidity of Christianity. Petrus attempted to avoid this problem, and refutes Judaism with their own weapons by challenging the Talmud and rabbis.

This work presented a point of view contrary to previous Christian philosophy because Christians claimed that the Jews were blindly practicing the Old Law. Petrus Alfonsi initiated a differing idea that “the Jews no longer followed the Old Law; they follow a new and heretical law, that of the Talmud.” Petrus’ belief was that the Jewish leaders were knowingly and willfully leading their flock astray. He believed that they purposely lied in order to conceal their sin of killing Jesus, in spite of the fact that they knew that he was the Son of God. Petrus Alfonsi also claimed that the Talmud was written to keep the Jewish people from seeing that Jesus was the Son of God; he called the Talmud “a fabric of lies” and a “heretical book.”

What makes this doctrine so radically different from previous Christian polemics is that they tried to prove the validity of Christianity by pointing out scriptures in the Old Law that confirmed that Jesus was the Son of God. With this belief, it portrayed the Jews as a people who would eventually see the truth and would ultimately convert to Christianity. Petrus’ new concept claimed that the Jewish leaders were blatantly lying and had attempted to cover up the truth. This new concept obviously would create a new type of tension between Christians and Jews.

The Augustinian tradition afforded Jews in Europe a tolerance throughout the Latin West that was not shared among other religions. This tradition did not place any emphasis on Judaism being heretical, but rather pointed to the fact that the Jews had a pivotal part to play in the spreading of Christianity. This doctrine was originally written to explain why Jews were not converting to Christianity. Since the Jews were the ones who had kept the law, it would seem logical that they would know whether the savior had come, and this presented a problem within Christian society. Daniel Lasker said of Petrus’ ideology that “These innovations signaled the beginning of the end of the relative Christian tolerance of Jews and Judaism inspired by the writings of Augustine.” Other authors before Petrus had used harsher rhetoric; there was seldom any deviation from the Augustinian tradition.

Alphonsi “was probably the first to connect the ‘ineffable’ trinity with the ‘ineffable’ Tetragram“.[10]

The Augustinian tradition assumed that, once the Jews’ purpose was served, they would convert to Christianity, but the Jews were not converting and people were looking for a new explanation. Alfonsi attempted to explain this discrepancy by stating that Judaism is heretical, and that the Jewish leaders have knowingly covered up the truth. He made his claim specific to the religion and Jewish leader, but not to the people as a whole. He did this through pointing out scientific inconsistencies in the belief of Judaism.

Alfonsi’s polemical work did not signify that the twelfth century was filled with violence between religions, or that the Christians were actively crusading against the Jews for conversion. At this time the Augustinian tradition remained and Christians assumed that the Jews would just progress towards becoming Christians. During Alfonsi’s life, his work set the stage and afforded the language that would enable later persecutions, rather than his polemics developing out of Jewish persecution. Although Alfonsi may not have been the man who was forcibly converting Jews, his writings did enable later polemicist to fabricate even bolder claims of the Talmud including that it was satanic. These new writings and ideals influenced the thought of many others in the Latin West for years to come.

Petrus Alfonsi’s Dialogi contra Iudaeos was not an entirely new polemical concept; he used the same arguments and cited the same Old Testament prophecies that polemicists before him had been using. Before Alfonsi’s Dialogi contra Iudaeos, Medieval Latin knew very little about the religious beliefs and practices of the Jews living within their own city. Most Christians did not know the contents of the Talmud, and some did not even know of its existence. This lack of knowledge provided a problem for Christians who were trying to prove the superiority of Christianity over Judaism, and they were doing this without even knowing the basics of Judaism.

What made Petrus Alfonsi’s work unique and gave him a level of influence that was unmatched by any of the preceding polemicists was his knowledge of Judaism combined with his new concept on how to perceive it. As stated earlier, Petrus’ unique upbringing gave him a particular advantage to be an authority on polemics. Because Petrus came from Iberia, a place where polemics were initiated from actual dialogue and actual knowledge of rival religions, he was able to bring his Andalusian polemic with his firsthand knowledge of Judaism out of Iberia to Latin Europe, and transform the Latin polemical tradition.

The Dialogi contra Iudaeos represented a turning point in not only polemical strategy, but also the perception of Judaism. In the Dialogi, Alfonsi argued with himself as his old Jewish self (Moses) and his new converted Christian self (Peter). What made this particular strategy of polemics so influential was the ability to control the argument legitimately without the need of a second party. Since the argument was between Judaism and Christianity, and Alfonsi was once a Jew and then a Christian, he was able to argue both sides with accuracy. By arguing against himself in the Dialogi, he was able to set the parameters of the argument without any unforeseen issues from a second party. Petrus was able to make each side say what he wanted; because of this, it was authoritative, and became a damaging piece to the perception of the Jews.

The polemics between Moses and Peter seemed to have a friendly tone in their voices, but the arguments that the Dialogi presented were a radically new way to attack Judaism. It was far more negative than any of the Latin works influenced by the Augustinian tradition. Alfonsi viewed Judaism as a conspiratorial, anti-Christian sect. Although he claimed that Judaism did follow the Old Law, he said that it is “only in part, and that part is not pleasing to God.” He also challenged the general idea that the Jews unknowingly killed the son of God, and said that they killed Jesus out of envy. He said that, “God revealed to their priests the Temple would be destroyed and the Jews scattered as punishment for the Crucifixion; the priests, out of malice and envy, hid this revelation from their people.”

This was a clear deviation from the Augustinian tradition, and was only successful because of the unique position as a Jewish convert that Petrus Alfonsi occupied. Because of his knowledge of the Talmud and Judaism, that until then was unprecedented by Christian polemicists, it validated his anti-Judaic position. This knowledge made him an authority, and allowed some people to begin to question the longstanding Augustinian tradition of tolerance, which was problematic for the Jews on many different levels. This not only became a threat to Jewish communities in creating new contentions between Jews and Christians that had not previously existed, but also Jews had to worry about the possibility of losing their position of tolerance with Christendom. With these new polemical works came the issue of what was the purpose of the Jew. If the Jews’ position no longer fell in line with acceptance within Christendom, then they would be forced into a new role, that of intolerance.

Various Arguments of the Dialogi contra Iudaeos[edit]

Alfonsi’s claim that was the most deleterious to Jewish-Christian relations was that the Jews knew that Christ was the Son of God and still killed him. John Tolan says that “Alfonsi was the first Latin writer of anti-Jewish polemic to assert that the Jews were guilty of deicide .” In the tenth titulus of the Dialogi contra Iudaeos, Alfonsi declares “that Christ was crucified and killed by the Jews of their own spontaneous will.” He claimed that the Jewish leaders were a deceitful people that should not be trusted, and since Alfonsi used to be a Jew, he was qualified to reveal their thinking process.

When the Jews were accused of killing the Son of God, there were three responses given in an attempt to justify why this action was done. The first response was that the Crucifixion was necessary, according to Moses, for it “fulfilled his will.” The second point that Moses makes is that many of the Jews’ ancestors were not a part of the Crucifixion and were already living somewhere else in the world; Judah killed Christ, not Israel. The last point Moses makes is that the Jews had a right to kill him because they had a just judgment of Jesus being a magician. Peter retaliates with valid counterpoints that are clearly better constructed than Moses’ points. This is not to say that Moses’ arguments were not well thought out; it is merely that Peter puts together a better articulated argument. Once Moses conceded that Peter was making valid points, he questioned then why the Jews would kill Jesus, because there were many Jews that were known for their wisdom. Peter then says that “since they denied him and slew him from envy, this is why they are guilty of such a great crime He said they decided to kill Christ “not in order to fulfill his will, but from the poison of hatred and envy.” Previous polemicists have claimed that the exile of the Jews was due to the Crucifixion, but what was new was the idea that at least a small number of rabbis knew that Jesus was the Son of God before they killed him and that the rabbis also knew this was the reason they were in exile.

Petrus’ attack, although directed at Judaism, does not attempt to challenge the Jewish people; he reserves his polemics for the rabbis and rabbinical Jewish writings. This is particularly interesting because his polemics demonstrate that the Jewish people were not impenitently heretical but rather misguided by envious rabbis who wanted to retain power over the Jews. If this was the case, then there was hope for the Christians that the Jews could convert.

This perception of the Jews being capable of conversion if they were just enlightened of the truth about the deceitful rabbis was not injurious to the Jews in the immediate future, but rather to Judaism as a whole over a long period of time. These concepts that flipped the Augustinian tradition upside-down laid the groundwork and afforded the language that would enable Christians to persecute the Jews for the purpose of conversion. According to Christians, once the Jews had discovered the truth that Alfonsi had, they would convert because the truth was self-evident. However, this was not the case and it gave Christians and later polemicist the impetus for developing a culture that would require a new position for the Jews.

When Alfonsi used the Talmud in his arguments, his goal was to expose it as “devoid of divine inspiration” and he did this through proving the Talmud was “contrary to logical and scientific fact.” The way that Alfonsi used the Talmud was completely different from how Christians in the past had used it. Previously Christians would merely peruse the Talmud for inflammatory references to Jesus in order to invoke Christian disdain towards the Jews. When Petrus Alfonsi quoted from the Talmud, he ignored any such slanderous language, and focused on references that would contradict philosophical logic or scientific fact. He proved philosophical fact in his polemics by discussing how the corporeality of God could not exist because it contradicted the dominant Aristotelian theory, and that the Talmudic rabbis saw such scriptures as “God created man in his own image,” as literal. In the Dialogi contra Iudaeos Petrus attacked the mystical tradition called Shi’ur Qomah. He showed how science of his day clearly contradicted the Talmudic claim in hopes of discrediting the validity of it being divinely inspired.

ALFONSI, PETRUS (called before baptism Moses Sephardi, “the Spaniard”):

A controversialist and physician in ordinary to King Alfonso VI. of Castile; born at Huesca, Aragon, in 1062, and died in 1110 at the age of forty-eight. He embraced Christianity and was baptized at Huesca on St. Peter’s day, June 29, 1106, in his forty-fifth year. In honor of the saint and of his royal patron and godfather he took the name of Petrus Alfonsi (Alfonso’s Peter). Like all the apostates of his time, he sought to show his zeal for the new faith by attacking Judaism and defending the truths of the Christian faith. He composed a series of twelve dialogues against the Jews, the supposed disputants being Mose and Pedro (= Moses Sephardi and Petrus Alfonsi, or, in other words, himself before and after conversion). Though the work is overpraised by Raymund Martin, in his “Pugio Fidei,” and others equally biased, it is but little known to-day; and, as Steinschneider observes (“Hebr. Uebers.” p. 933), fully merits the oblivion into which it has fallen. The “Dialogi in quibus impiæ Judæorum . . . opiniones . . . confutantur,” the full title of which is given in Wolf, “Biblioteca Hebræa” (i. 971) and Fürst, “Bibl. Jud.” (i. 36), appeared at Cologne in 1536 and later in “Biblioteca Patrum” (xii. 358, xxi.; ed. Lugdunensis, p. 172; ed. Migne, t. 157, p. 535). Other books are ascribed to him, and he is sometimes confounded with Petrus Hispanus of the thirteenth century. See Steinschneider (l.c. p. 470, § 282; p. 934, § 557, note 208), who regards him as the probable translator of the “Canones Tabularum” (“Cod. Corp. Chr.” 283, 13; f. 141b) from the Arabic. It is ascribed to one Petrus Anfulsus, who is very likely identical with Alfonsi (see Steinschneider, “Hebr. Bibl.” 1882, xxi. 38; “Hebr. Uebers.” pp. 985, 986, § 589).

Another controversial tract, described as a dialogue “Inter Petrum Christianum et Moysem Hæreticum” (Codex Merton, 175b, f. 281; in Coxe’s “Cat.” p. 69), is said to have been written by Petrus Alphonsi (compare “Hebr. Bibl.” xxi. 38). In Cambridge University, England, there is a manuscript of the fifteenth century bearing the title: “De Conversione Petri Alfonsi Quondam Judæi et Libro Ejus in Judæos et Saracenos,” which is mentioned in Steinschneider’s “Polemische und Apologetische Literatur,” 1877, p. 224 (compare p. 235, No. 5, s.v. Epistola).

Alfonsi’s fame rests chiefly on a collection of thirty-three tales, composed in Latin. This collection has enjoyed a most remarkable popularity, and is, on that account, an interesting subject of study in comparative literature. It is entitled “Disciplina Clericalis,” or “A Training-school for the Clergy,” and was often used by clergymen in their discourses, notwithstanding the questionable moral tone of some of the stories. The work is important as throwing light on the migration of fables, and is almost indispensable to the student of medieval folk-lore. Translations of it into French, Spanish, and German are extant; and Joseph Jacobs has recently discovered some of the stories at the end of Caxton’s translation of the fables of Æsop, where thirteen apologues of “Alfonce” are taken from the “Disciplina Clericalis.”

An outline of the tales, by Douce, is prefixed to Ellis’ “Early English Metrical Romances.” Nearly all the stories are adopted in the “Gesta Romanorum.” Chapters ii. and iii. were done into Hebrew and issued under the title , “Book of Enoch,” Constantinople, 1516; Venice, 1544 and 1605. An early French translation of this Hebrew extract was made prior to 1698 by Piques, and August Pichard published another version in Paris, 1838.


  • The whole literature is put together and discussed in Steinschneider’s Hebr. Uebers. (pp. 934-935). Mention should be made of the scholarly edition of F. W. V. Schmidt, Berlin, 1827, to whose notes Steinschneider offers very valuable emendations and parallels from Oriental and Western folk-lore.
  • Steinschneider, Manna, 1847, pp. 102, 114;
  • idem, Cat. Bodl. cols. 549, 550, 733, 734;
  • idem, Jewish Literature, p. 174;
  • the authorities mentioned in B. P[ick]’s article, Pedro Alfonso, in McClintock and Strong’s Cyclopedia, vii. 864, 865;
  • A. Clouston, Flowers from a Persian Garden, p. 100, London, 1890;
  • Jacobs, Jewish Ideals, 1896, pp. 141-143, lays stress on Alfonsi’s importance as one of the intermediaries between Eastern and Western folk-lore, and quotes one of Caxton’s stories from “Alfonce.”

About richardsh

Messianic Jewish teacher in UK
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