Abraham bar Ḥiyya ha-Nasi was a Jewish mathematician, astronomer, and philosopher, also in Arabic he was known as Sahib al-Shurta, “Elder of the Royal Suite”. He occupies an important place in the history of Jewish science and became one of the most important figures in the scientific movement which made the Jews of Provence, Spain, and Italy the intermediaries between Mohammedan science and the Christian world.
Background
Abraham bar Ḥiyya ha-Nasi was born circa 1070 in Catalonia, and was the great-grandson of the Hezekiah Gaon. The birthplace might be Soria; this would be indicated by the fact that his astronomical tables are prepared for this location. Others, however, suggest that he was born in Barcelona. He was called by the title Savasorda, which is a Spanish derivation of the Arabic title of Sahib as-Shurta, which literally means something like 'chief of police' but probably indicates a position of a courtier. It has been suggested that he received the title as he worked for Alfonso, the king of Aragon, or for the counts of Barcelona. Opinions have differed as to his living in France towards the end of his life. The year of his death is not known, either some assume 1136, some as late as 1148.
Education
The level and extent of Bar Hiyyas learning requires special attention. We must realise that we are dealing with a person which has been held to be one of the brightest minds of his day, and whose name is still found in works of history of both mathematics and astronomy. While in Muslim Spain such learning was not unheard of, in Christian Europe something comparable simply did not yet exist. Bar Hiyya himself played a part in transferring the Arabic science over to the rest of Europe, both with his Hebrew works and through his cooperation with Plato of Tivoli to produce Latin versions of Arabic scientific works. While it is in principle possible that Bar Hiyya had learned from Arabic-speaking persons and books within Christian territory in Catalonia or Aragon, it is much more likely, that he would have obtained his scientific knowledge in an active center of Arabic learning such as Saragossa in some Muslim center of learning. According to Adolph Drechsler, bar Ḥiyya was a pupil of Rabbi Moshe haDarshan and teacher of Abraham Ibn Ezra.
Likewise Bar Hiyya could have obtained the title of Sahib as-shurta in the court of Saragossa, the main Muslim capital and the main center of Arabic learning in the area close to Barcelona and Soria as well if we would consider that as his birthplace. Saragossa was also one of the foremost seats of Jewish learning Ibn Gabirol wrote the most part of his work in this city in the mid-11th century and Bahya ibn Pakuda wrote his Duties of the Heart there in the 1080s.
Career
Savasorda’s most influential work by far is his Hebrew treatise on practical geometry, the Hibbur ha-meshlhah we-ha-lishboret. Translated into Latin as Liber embadorum by Plato of Tivoli, the work holds an unusual position in the history of mathematics. It is the earliest exposition of Arab algebra written in Europe, and it contains the first complete solution in Europe of the quadratic equation, x2 — ax + b — 0.
The year the Hibbur was translated (1145) also saw the Robert of Chester translation of al-Khwarizmi's algebra and so may well be regarded as the birth year of European algebra. Thus the Hibbur was among the earliest works to introduce Arab trigonometry into Europe, and it was also the earliest to treat of Euclid’s Book of Divisions. Leonardo Fibonacci was influenced by Savasorda and devoted an entire section of his Practica geometriae to division of figures. Savasorda made a novel contribution when he included the division of geometric figures in a practical treatise, thus effecting a synthesis of Greek theory with the pragmatic aspects of mathematics.
Savasorda himself recommended Euclid, Theodosius of Bithynia, Menelaus, Autolycus, Apollonius of Perga, Eudemus of Rhodes, and Hero of Alexandria for study in geometry. He knew well al-Khwarizml and al-Karajl. Following Hero and not Euclid he did not accept the Pythagorean figurate numbers in his explanation of plane and square numbers. In general, Savasorda preferred those definitions and explanations that may be aligned more easily and closely with reality.
To understand this approach, it is necessary to go back to the earliest known Hebrew geometry, the Mishnat ha-Middol (ca. A.D. 150). This work may be considered as a link in the chain of transmission of mathematics between Palestine and the early medieval Arab civilization. The Arab mathematicians al-Khwarizml and al-Karajl, and later Savasorda, followed the methodological lines of this old Mishna. Savasorda himself provided a new cross-cultural bridge a thousand years after the Mishna. In his Encyclopedia there is the same teaching of both theory and practice, including not only the art of practical reckoning and business arithmetic but also the theory of numbers and geometric definition. This book is probably the earliest algorismic work written in Western Europe, but knowledge of the work is not apparent in the arithmetical works of either Abraham ibn Ezra or Levi ben Gerson, although they may have had a common origin.
In the history of decimal theory and practice, the two mainstreams of development in the Middle Ages came from the Jewish and Christian cultures. Savasorda, however, did not belong definitely to any one mathematical group.
He spent most of his life in Barcelona, an area of both Arab and Christian learning, and was active in translating the masterpieces of Arab science. In an apologetic epistle on astrology to Jehuda ben Barsillae al-Barceloni, he deplored the lack of knowledge of Arab science and language among the people of Provence.
He wrote his own works in Hebrew, but he helped translate the following works into Latin: al-Imranfs De horarum electionibus (1133-1134), al-Khayyat’s De nativitatibus (1136), and Almansori’s Judicia seu propositiones . . . (1136). Savasorda may have worked on translations of the Quadripartitum of Ptolemy, the Spherics of Theodosius, the De motu siellarum of al-Battani, and others, with Plato of Tivoli. It is also possible that he worked with Rudolf of Bruges on the De astrolabia.
Religion
Abraham bar Hiya, known to non-Jewish writers as Abraham Judaeus and to Muslims as savasorda (captain of the guards) found a delicate balance between his neo-Platonic philosophical leanings and his loyalty to the main precepts of Judaism.
Views
Avraham bar Hiyya believed in the preeminence of matter, i.e. pre-creationism, but held that all was the work of an almighty force that governed all matters. He was extremely knowledgeable in the science of al-Andalus, he fully belonged to the traditionally minded religious world which was dominant for instance in Provence. That Bar Hiyya was proficient in the Arabic-language scientific culture is testified by his scientific works and the references to philosophical ideas in his other works. But we also know that he was writing in Hebrew and that he most probably wrote to an audience he knew. Therefore, with full justification, we can say that Bar Hiyya was standing with one foot in both worlds and his eyes turned to the north.
Quotations:
Avraham bar Hiyya said about Barcelona: " Where there is a holy congregation, including sages, wise and illustrious men, such as R. Sheshet, R. Shealtiel, R. Solomon and R. Abraham, son of Chisdai".
"The major part of the mathematical "classics" in Hebrew were translated from Arabic between the second third of the thirteenth century and the first third of the fourteenth century, within the northern littoral of the western Mediterranean. This movement occurred after the original works by Abraham bar Hiyya and Abraham ibn Ezra became available to a wide readership."
Personality
Quotes from others about the person
As Levey writes of Abraham:
"... did not definitely belong definitely to one mathematical group. He spent most of his life in Barcelona, an area of both Arab and Christian learning, and was active in translating the masterpieces of Arab science. ... he deplored the lack of knowledge of Arab science and language among the people of Provence. He wrote his own works in Hebrew, but he helped translate ... works into Latin...."
