Ketubot 25 - Who is a Kohen?
The previous argument about collusion , where Kohanim were mentioned only incidentally, leads to a full-blown discussion of who is a Kohen.
Rabbi Yehudah, who required two witnesses, has a reason for this: if he sees a man who is eating the Kohen's portion (terumah), he will presume that this is a genealogically pure Kohen and permit him to marry without any further investigation. Thus, if we were to allow one witness to testify about a Kohen in regards to terumah, this would automatically count as evidence for family matters, and that cannot be, because in all family matters two witnesses are required. The other point of view is that just eating terumah does not mean that he is a pure Kohen; thus, we can accept one witness for this.
A certain man said to Resh Lakish about another man: "I know that he is a Kohen, because he was always called first to read from the Torah." Resh Lakish answered, "Did you also see him receive terumah?" Rabbi Elazar objected, "And if there were no granaries there, you would not have any Kohanim," but Resh Lakish did not accept it.
Later, Resh Lakish heard this from his own teacher, Rabbi Yochanan. He was cross with Rabbi Elazar: "Had you told me it was from Rabbi Yochanan, I would have believed you right away!"
Art: The White Granary by Edward Wilkins Waite