Isaac ben Sheshet Perfet (1326–1408) (Hebrew: יצחק בן ששת) was a Spanish Talmudic authority, also known by his acronym, Rivash
Rabbi Yitzchak Bar Sheshes, the Rivash (1408 which is 5168 in the Hebrew Calendar).He was born at Valencia and settled early in life at Barcelona, where he studied under Perez ha-Kohen, under Hasdai ben Judah, and especially under R. Nissim ben Reuben (known as the RaN), for whom he professed throughout his life the greatest veneration. He became known as a Talmudic authority and is studied as a Sephardic Talmudic authority.
While young Isaac acquired a world-wide reputation as a Talmudic authority, and halakic inquiries were addressed to him from all quarters. Generally he led a private life, earning his livelihood in commerce until he was about fifty years old, when he was compelled to accept a position as a rabbi. Together with six other prominent men of Barcelona, among whom was his younger brother Judah ben Sheshet and his teacher Nissim ben Reuben, he was thrown into prison on a false accusation. After his acquittal he accepted the rabbinate of Saragossa. While Rabbi in Saragossa, there was conflict stirred up by the dayyan Joseph ben David. As a consequence he accepted the less important rabbinate of Calatayud; but when he was on the point of leaving Saragossa the leaders of that community induced him to stay. The peace, however, did not remain long undisturbed, and Isaac settled at Valencia, where he directed a Talmudical school.
In 1391 occurred the great persecutions of the Jews of Spain in consequence of the preaching of Fernandes Martinez. Isaac saved himself by flight. After sojourning a certain time at Miliana he settled at Algiers, where he was received with great honor.
A certain Spanish refugee who had settled at Algiers before him aspired to become the leader of the community, and, seeing in Isaac a rival, began to persecute him. To give to Isaac the power necessary to act against this man, Saul ha-Kohen Astrue persuaded the government to appoint Isaac rabbi of Algiers. But this won for him a still more powerful enemy in the person of Simeon ben Zemah Duran (Rashbaz), who disapproved of any intervention on the part of the government in the affairs of the rabbinate.
Notwithstanding these events, Isaac ben Sheshet was greatly venerated by the Algerian Jews, and pilgrimages to his tomb are still made on the anniversary of his death, which occurred in 1408. His tombstone was restored by the community of Algiers in 1862.
Wikipedia has the following to say about his writings. Isaac was the author of 518 responsa, to which great halakic value is attached by men like Joseph Caro, Jacob Berab, and many others. They are also of great historical importance as reflecting the conditions of Jewish life in the fourteenth century. In some of them are to be found details of the author's life; but unfortunately it is impossible to trace these chronologically, the original order of the responsa having been altered by the editors.
Although Isaac was very strict in his halakic decisions, he was far from being narrow-minded. He has nothing to say against secular knowledge; he disapproves the study of Aristotle only because the latter professed belief in the eternity of matter and denied God's providence. Isaac's responsa evidence a profound knowledge of the philosophical writings of his time. In one of them (No. 118) He explains the difference between the opinion of Levi ben Gershom (Ralbag) and that of Abraham ben David of Posquières (RABaD) on free will, and gives his own views on that complicated subject. His attitude to kabbalah was ambivalent. He advised against involving oneself with it unless one has received it from an adept; since he had not done so, he wrote, he could not address its claims.
Isaac's responsa were first published, under the title She'elot u-Teshubot, at Constantinople in 1546-47. A newer collection of the responsa was published under the title She'elot u-Teshubot ha-Ribash ha-Ḥadashot by David Frenkl at Muncas. In addition to these, he wrote novellæ on the Talmud which are no longer in existence. They are mentioned by him in his responsa (No. 106), and some of them, on the treatise Ketubot, are cited by Bezalel Ashkenazi in the Shiṭṭah Meḳubbeẓet. Azulai says that he has seen a manuscript containing a commentary on the Pentateuch by Isaac ben Sheshet.
May the merit of the tzadik Rabbi Isaac ben Sheshet Perfet protect us all. Amen