Isaac ben Sheshet Perfet (or Barfat) (1326–1408) (Hebrew: יצחק בן ששת) was a Spanish Talmudic authority, also known by his acronym, Rivash (ריב"ש).
Although Isaac, while still young, acquired a worldwide reputation as a Talmudic authority, and halakhic inquiries were addressed to him from all quarters, 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 rabbi.
To the grief caused by the death of his brother Judah and of his son-in-law was added that due to dissensions in the community, stirred up by the dayyan Joseph ben David.
Isaac in consequence accepted the less important rabbinate of Calatayud; but when he was on the point of leaving Zaragoza the leaders of that community induced him to stay.
But this won for him a still more powerful enemy in the person of Simeon ben Zemah Duran, who disapproved of any intervention on the part of the government in the affairs of the rabbinate.
Notwithstanding these events, Isaac 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.
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.
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.