In his Or Zarua, the only primary source of information on his life, Isaac ben Moses mentions as his teachers two Bohemian scholars, Jacob ha-Laban and Isaac ben Jacob ha-Laban. Led by a thirst for Talmudic knowledge, he undertook in his youth extensive journeys to the prominent yeshivot of Germany and France. According to Gross he went to Ratisbon first; but S.N. Bernstein conjectures that previously he stopped for a long time at Vienna, and became closely identified with the city, as he is usually quoted as "Isaac of Vienna." From among the many scholars at Ratisbon he selected for his guide the mysticYehuda ben Samuel HaChasid. About 1217 he went to Paris, where the great Talmudist Judah ben Isaac Messer Leon became his chief teacher. He also visited for a short time the yeshiva of Jacob ben Meir in Provins. Then he returned to Germany, and studied under the mystic Eleazar ben Judah at Worms, and, at Speyer, under Simchah ben Samuel of Speyer, his intimate friend, and Eliezer ben Joel ha-Levi, author of Abi ha-'Ezri and Abi'asaf. At Würzburg, where Rabbi Meir of Rothenburg was his pupil, he became rosh yeshiva. Later on, Isaac returned to Ratisbon, and then settled for some time in Vienna, where he held the position of Av Beth Din and rosh yeshiva. Finally, he went to Saxony and Bohemia. Isaac lived a long but unsteady and troubled life. He saw the law compelling Jews to wear the yellow badge put into force in France, and he deplored the massacres of the Jews in Frankfurt-am-Main and the extortions practised upon them by the nobles of Austria. His son-in-law was Samuel ben Shabbethai of Leipzig; his son Chaim Eliezer, called Or Zarua, like him a scholar, carried on a comprehensive halachic correspondence, a part of which was printed under the title Sefer She'elot u-Teshubot.
Work
Toward the end of his life, about 1260, Isaac composed his ritual work Or Zarua. He is usually quoted as "Isaac Or Zarua." It was printed from the Amsterdam manuscript by Lipa and Höschel in Jitomir, 1862. Other manuscripts are at Oxford and in the Jewish Theological Seminary in New York City. In the edition of Lipa and Höschel Seder Nezikin is wanting; most of the rest of the work was afterward printed at Jerusalem by J.M. Hirschensohn. The Or Zarua comprises the whole ritual, and is arranged according to the Talmudic tractates, while at the same time the halachot are kept together. The author, unlike Maimonides in his Mishneh Torah, does not confine himself to giving the halachic decisions, but gives also the passage of the Talmud, explains the subject matter, and develops the din from it. Thus the Or Zarua is at the same time a ritual code and a Talmudic commentary. As it contains, in addition, explanations of some passages in the Bible, the author is also quoted as a Bible commentator. Moreover, the book contains a part of the halachic correspondence which the author carried on with Talmudic scholars of Italy, France, and Austria. Older collections of halachic decisions which the author had gathered together during his lifetime seem also to be embodied in the work. Isaac explains unknown words in Bohemian, his mother tongue, and cites the Jerusalem Talmud, to which he ascribes great authority in halachic decisions. The work is introduced by a treatise couched in words to whose meanings mystical significance is attached. It is an imitation of the Alphabet of Akiba ben Joseph, and was composed at the order of Isaac's teacher Eleazar ben Judah of Worms. Isaac's son Chaim Eliezer arranged a compendium of this work which exists in several manuscripts. The Or Zarua succeeded in displacing all the older ritual works. It is very important also for the Kulturgeschichte of the German Jews in the Middle Ages. According to Gross, Isaac's chief importance rests upon the fact that he introduced among the Jewish communities in Slavic lands the study of the Talmud from France and the west of Germany. Isaac was of a mild and peace-loving character and it was for this reason, perhaps, that he did not participate in the struggle against the study of secular sciences, though an incorrect ritual decision would rouse him to indignant energy. He carried on a controversy with several rabbis concerning the legal status of a betrothed girl who had been forced by circumstances to adopt Christianity and had afterward returned to Judaism. His anxiety for correct observance led him to counsel the more difficult rather than the easier ritual practise. His mystical studies account for his belief in miracles. He was held in high regard by his pupils, and, like other teachers of the time, was given the title Ha-Kadosh. His contemporary Isaiah di Trani described him as "the wonder of the age".
