Demai, Chapter Four, Mishnah Two
This mishnah deals with a person, we’ll call him Reuven, who has a friend, whom we’ll call Shimon, who is trying to use a vow to get him to come eat with him. The way this works is that Reuven says that all of his possessions will become forbidden to Shimon unless Shimon eats with him. Shimon doesnt want to cause such a rupture in their relationship, but he doesnt think that Reuven tithes his produce. What is he to do?
1) One who vowed [that his friend could not benefit from him] unless he eats with him, and the friend does not trust him in respect of tithes, he may eat with him on the first Shabbat even though he does not trust him in respect of tithes, provided that his friend said to him that the food had been tithed.
2) But on the second week, even though he had vowed that he would not benefit from him, he may not eat with him unless he first tithed [the produce].
Section one: If Reuven invited Shimon to eat with him during the week, Shimon could just tithe the food that Reuven gave him to eat and there would be no problem. However, on Shabbat he cant tithe the produce, so the problem remains. The mishnah is lenient for the first Shabbat. If Reuven tells Shimon that the food has been tithed, Shimon can eat it, even though Reuven is not generally trusted. In this case we cannot explain the mishnahs leniency as being a result of the honor of Shabbat, because Shimon could eat at home instead of with Reuven. Rather the leniency is because of the ways of peace. In order to preserve the peace between Reuven and Shimon and to allow Shimon to eat at Reuvens home, Shimon can temporarily trust him that the food has been tithed.
Section two: The leniency in section one does not exist for the second Shabbat because by this point Shimon had time to go to Reuvens home and tithe his produce or make sure that Reuven had actually tithed. He need no longer rely on a leniency or on Reuvens word.
In looking at this mishnah and all of the other leniencies we have seen, it is worthwhile to make a general note about the function of demai in rabbinic society. The fact that rabbis did not trust others to tithe would have clearly separated them from the rest of the population. It seems to me that this is intentional. Rabbis wanted to draw some lines between themselves and others. However, these lines remain intentionally fuzzy and easily crossed. They are not the relatively clear lines that separate a Jew from a non-Jew, but rather more porous lines that permit a large degree of fraternization between rabbis and non-rabbis.