Talmudic chronology is an uncertain science. Aside from stray data here or there in the Talmud, there are only two sources: Seder Tannaءim ve-ءAmoraim and ءIgeret Rav Sherira Gaon. Both are Geonic, separated by centuries from the Amoraic period. Moreover, they deal mainly with heads of yeshivot (or perhaps master–disciple circles), not with individual Amoraim. Avinoam Cohen's revision of the dating of the sages named Ravina and of a few other Babylonian Amoraim who functioned during Rav Ashi's time (d. 427) and afterwards is a closely reasoned study and, as such, a paradigmatic work of talmudic historiography. Through painstaking readings of Talmudic passages and medieval commentators, Cohen takes issue with the regnant dating of those Amoraim and, in arguing for his revisions, explains how to identify historically relevant information in talmudic sugyot and how to utilize it.