R. Isaac son of Rab Judah (or Isaac b. Judah or YIẒḤAḲ BAR JUDAH, Hebrew: רב יצחק בריה דרב יהודה) was a fourth generation Amora sage of Babylon. His father was Judah ben Ezekiel who was his Teacher par excellence, and he debated many Halakhaic issues with him. His father regarded him highly, and would submit statements in his son's name, which was considered unusual, since usually an Amora does not submit statements in the name of their students (but rather, the other way around). At times he would commentate on his father's work.
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Biography
Rab Isaac was born to Rabbi Judah probably at an old age. In his childhood already he was showing signs of wisdom and his wit was more than the rest of his peers. For example, it is storied that when the sages of Pumbedita Academy, debated the law stipulating that a purchase from young children who sold movables to make a living is a valid due to a Takkanah, stipulating if only they have sufficient understanding of the meaning of their actions, then Judah ben Ezekiel pointed out to his son, Isaac, who was about six or seven at the time, as one who may sell movables to make a living. When another opinion stipulating the age nine or ten was brought before him, he replied that each child varies according to his intelligence at certain age, like his son who at the age of six had great intelligence.
He did not marry until a very late age, because his father did not know where to find a family with no Marriage restrictions, until Ulla once came to visit his house and guided him with the signs needed to locate those with no Marriage restrictions. He studied also under Rav Huna, as well as under Rabbah bar Nahmani along with Rab Samuel, the son of Rabbah bar bar Hana, and Aha bar Hana.
After their death he became a student of Rami bar Hama, who was considered an Oker Harim scholar, and would reply to each question addressed to him with reason rather than one relying on a teaching (from the Baraitas) that had preceded him, contarary to Rav Sheshet who would reply based on previous sources. This thinking of Rami bar Hama made Rab Isaac leave his lectures, and go study under Rav Sheshet.
Along with brother, Huna b. Judah, he debated the Halacha with Abaye and Rava.
Family
His Granddaughter Homa, the daughter of his son Issi, was termed Isha katlanit ("lethal/deadly woman"), since she married three men, all of whom died: Rehaba of Pumbeditha, Rab Isaac, the son of Rabbah bar bar Hana, and Abaye.
Halakhic technical expertise
Rab Isaac developed a special method of testing Tzitzit of Tekhelet, whether they are painted the right material of a certain Gastropoda's blood