All the sages who were mentioned were the leaders of the generations. Among them were heads of academies, heads of the exile, and members of the great Sanhedrin. Together with them in each generation, there were thousands and myriads that heard their [teachings].
Ravina and Rav Ashi were the final generation of the Sages of the Talmud. Rav Ashi composed the Babylonian Talmud in Shin'ar approximately one hundred years after Rabbi Yochanan composed the Jerusalem Talmud. The intent of both the Talmuds is to elucidate the words of the Mishnah, to explain its deeper points, and [to relate] the new matters that were developed by each court from the era of Rabbenu Hakadosh until the composition of the Talmud.
From the entire [body of knowledge stemming from] the two Talmuds, the Tosefta, the Sifra, and the Sifre, can be derived the forbidden and the permitted, the impure and the pure, the liable and those who are free of liability, the invalid and the valid as was received [in tradition], one person from another, [in a chain extending back] to Moses at Mount Sinai.
Also, [the sources mentioned above] relate those matters which were decreed by the sages and prophets in each generation in order to "build a fence around the Torah." We were explicitly taught about [this practice] by Moses, as [implied by Leviticus 18:30]: "And you shall observe My precepts," [which can be interpreted to mean]: "Make safeguards for My precepts."
Similarly, it includes the customs and ordinances that were ordained or practiced in each generation according to [the judgment of] the governing court of that generation. It is forbidden to deviate from [these decisions], as [implied by Deuteronomy 17:11]: "Do not deviate from the instructions that they will give you, left or right."
It also includes marvelous judgments and laws which were not received from Moses, but rather were derived by the courts of the [later] generations based on the principles of Biblical exegesis. The elders of those generations made these decisions and concluded that this was the law. Rav Ashi included in the Talmud this entire [body of knowledge, stemming] from the era of Moses, our teacher, until his [own] era.
The Sages of the Mishnah also composed other texts to explain the words of the Torah. Rabbi Hoshaia, the disciple of Rabbenu Hakadosh, composed an explanation of the book of Genesis. Rabbi Yishmael [composed] an explanation beginning at "These are the names" [the beginning of the book of Exodus,] until the conclusion of the Torah. This is called the Mechilta. Rabbi Akiva also composed a Mechilta. Other Sages of the following generations composed other [collections of the] interpretations [of verses] (Medrashim). All of these works were composed before the Babylonian Talmud.
Thus, Ravina, Rav Ashi, and their colleagues represent the final era of the great Sages of Israel who transmitted the Oral Law. They passed decrees, ordained practices, and put into effect customs. These decrees, ordinances, and customs spread out among the entire Jewish people in all the places where they lived.
After the court of Rav Ashi composed the Talmud and completed it in the time of his son, the Jewish people became further dispersed throughout all the lands, reaching the distant extremes and the far removed islands. Strife sprung up throughout the world, and the paths of travel became endangered by troops. Torah study decreased and the Jews ceased entering their yeshivot in the thousands and myriads, as was customary previously.
Instead, individuals, the remnants whom God called, would gather in each city and country, occupy themselves in Torah study, and [devote themselves] to understanding the texts of the Sages and learning the path of judgment from them.
Every court that was established after the conclusion of the Talmud, regardless of the country in which it was established, issued decrees, enacted ordinances, and established customs for the people of that country - or those of several countries. These practices, however, were not accepted throughout the Jewish people, because of the distance between [their different] settlements and the disruption of communication [between them].
Since each of these courts were considered to be individuals - and the High Court of 71 judges had been defunct for many years before the composition of the Talmud - people in one country could not be compelled to follow the practices of another country, nor is one court required to sanction decrees which another court had declared in its locale. Similarly, if one of the Geonim interpreted the path of judgment in a certain way, while the court which arose afterward interpreted the proper approach to the matter in a different way, the [opinion of the] first [need] not be adhered to [absolutely]. Rather, whichever [position] appears to be correct - whether the first or the last - is accepted.