||Part II, Yoreh De'ah, 403 chapters on Issur ve-Hetter (ritual law), commencing with shehitah and terefot and ending with usury, idolatry, and mourning of the Tur with the commentary of R. Joseph Caro, Bet Joseph. The work is divided into four sections (Turim, "rows"; first complete edition, Piove di Sacco, 1475): Part I, Orah Hayyim, contains 697 chapters and deals with blessings, prayers, the Sabbath, festivals, and fasts; Part II, Yoreh De'ah, 403 chapters on Issur ve-Hetter (ritual law), commencing with shehitah and terefot and ending with usury, idolatry, and mourning; Part III, Even ha-Ezer, 178 chapters on laws affecting women, particularly marriage, divorce, halizah, and ketubbah; Part IV, Hoshen Mishpat, 427 chapters on civil law and personal relations.
The arrangement of the book, its simple style, and its wealth of content, made it a basic work in Hebrew law, and opened a new era in the realm of halakhic codification. R. Jacob invariably quotes the text of the Talmud and its commentaries as well as the opinions of authorities who preceded him, and then lays down the halakhah, mainly following Maimonides and his own father. On questions of faith and belief, however, he does not hesitate expressly to oppose Maimonides. He was aware of the views of the Hasidei Ashkenaz, whose influence is discernible particularly in the Orah Hayyim. The excellence of the work soon led to its dissemination throughout the Diaspora. Its authority has been recognized and accepted by all Jewish scholars throughout the generations, many of whom (including R. Joseph Caro, R. Moses Isserles, R. Isaac Aboab, R. Jacob ibn Habib, R. Joel Sirkes, and R. Hayyim Benveniste) wrote commentaries on it, and made precis of it. When R. Caro wrote his major work, the Beit Yosef, he decided to "base it upon the Turim... because it contains most of the views of the posekim."
R. Jacob b. Asher (1270?–1340), was the son of R. Asher b. Jehiel (the Rosh), under whom he studied. In 1303 he accompanied his father from Germany to Toledo, where he lived in great poverty, shunning rabbinical office and devoting all his time to study. In his learning, he avoided prolixity and casuistry. Typical of his style is his first halakhic work, Sefer ha-Remazim, in which he gave the halakhic rulings deduced from his father's work, Ha-Asheri (under the title Kizzur Piskei ha-Rosh, Constantinople, 1515). R. Jacob's enduring fame rests upon his major work, the Arba'ah Turim, as a result of which, he is commonly referred to as "the Ba'al ha-Turim." Perceiving that "reasoning had become faulty, controversy had increased, opinions had multiplied, so that there is no halakhic ruling which is free from differences of opinion," he decided to compile a work to embrace all halakhot and customs incumbent upon the individual and the community.
Jacob also wrote a comprehensive commentary on the Pentateuch (Zolkiew, 1806), containing the best expositions of the peshat ("literal meaning") by earlier Bible commentators, such as Saadiah Gaon, Rashi, Abraham ibn Ezra, David Kimhi, and others, in particular abstracting "the straightforward explanations" from the commentary of Nahmanides and disregarding the kabbalistic ones, since "my soul has not entered its secret" (cf. Gen. 49:6). To the beginning of each section, he added "as a little appetizer, gematriot and explanations of the masorah, in order to attract the mind." Ironically, it was just these "appetizers" that were published (under the title Perush ha-Torah le-R. Ya'akov Ba'al ha-Turim (Constantinople, 1500 and 1514)) some three centuries before the main part of the work, and it was this portion only which was widely known for many generations.
R. Jacob neither served in any rabbinical post nor received any remuneration from the community but was involved in communal activities. He appended his signature to a sentence of death upon an informer (Judah b. Asher, Responsa Zikhron Yehudah (1846), no. 75). His ethical will to his children (first published Pressburg, 1885) reflects his high spiritual and cultural level. A late tradition, mentioned by H. J. D. Azulai, relates that Jacob set out for Erez Israel but died on the journey.