Terumos chapter 4, mishnah 12 - a magnificent mishnah. The mishnah deals with two vessels, each containing fifty se'ah of chullin, and a se'ah of Terumah fell into one of them, but we do not know into which. All told we have here a hundred se'ah of chullin against one se'ah of Terumah, except that the Terumah fell into only one of them, and the two vessels do not combine - each vessel on its own contains only fifty. Is that enough?
The view of the Tanna Kamma:
Vessels whose contents are likely to become mixed together: When we are dealing with two vessels whose contents may well be joined in the future - a realistic and identifiable possibility - we are permitted to combine their contents, and fifty plus fifty add up to a hundred, which is the required measure. This is a leniency that was permitted because the prohibition of meduma is only rabbinic, and because the Sages were lenient in it with leniencies of this sort.
Vessels that are far apart from one another: But if the two vessels are located in two different cities, and their contents will certainly never become mixed together, we do not combine the contents of the two. Each one is considered as though it contains only fifty se'ah of chullin against the Terumah, and both of them are forbidden as a meduma mixture.
The wording of the mishnah:
The mishnah gives two examples. "Shtei kupos" - a kupah is a large vessel, something like a bucket, a chest or a container, holding fifty se'ah of grain. "Shtei meguros" - storage rooms, granaries for grain, as in the wording of the previous mishnah. One se'ah of Terumah fell into one of the two kupos or into one of the two meguros, and we do not know into which. If they are close enough to one another - in the same house or on the same floor of the storage rooms, or in any situation where there is a realistic possibility that their contents will become mixed together - each one counts toward nullifying the Terumah that fell into the other, and the contents may be combined together.
The view of Rabbi Shimon:
Rabbi Shimon holds that there is no need at all for the contents of the two vessels or the two storage rooms to be liable to become mixed together later on. In his view, even if they are in two different cities, they combine with one another to make a hundred. Since the entire scope of the doubt extends over both vessels, and the contents of the two together are a hundred se'ah of chullin, this is the required hundred, even though the Terumah fell into only one of them.
The halachah follows the Tanna Kamma: we combine the one against the other, provided that they are close enough to one another that there is a possibility that their contents will become mixed together.
Why do we not say here "a doubt in a rabbinic matter is resolved leniently"?
The issue of meduma - a mixture of Terumah in chullin - is only rabbinic. By Torah law a simple majority of chullin suffices, and nullification has already taken place, and the mixture may be eaten even by a non-kohen. The Sages raised the bar and required a hundred against one, a rule unique to Terumah, and if there is no hundred, this is a meduma mixture. But the requirement of a hundred is rabbinic by its very nature.
If so, in each and every vessel the worst case scenario is that a se'ah of Terumah fell into fifty se'ah of chullin, and we have before us a rabbinic prohibition of meduma. And since we do not know whether it fell into this one or that one, each of them is no more than a doubt, and at most a doubt regarding a rabbinic prohibition. And the rule is: a doubt in a rabbinic matter is resolved leniently. Seemingly, then, it should be permitted to eat from both vessels alike.
But this is not so: the rule that a doubt in a rabbinic matter is resolved leniently was not stated in a case where ruling consistently on both vessels would lead to permitting something that is certainly forbidden. Here there is definitely a prohibition before us, and we simply do not know in which of them it is. And if we look at the two vessels and say of this one "permitted" and of that one "permitted," we have certainly erred with regard to one of them - and that we will not permit. A doubt in a rabbinic matter is resolved leniently specifically where there is no certainty that a prohibition exists here or that a person has stumbled into something improper, and where our ruling does not amount to a determination that cannot possibly hold true.
To sharpen the point, consider a completely different case: we had before us two se'ah, one of Terumah and one of chullin. One of them was eaten by a dog, and the other fell into our container holding fifty se'ah. In such a case we would say: perhaps the one that fell was the Terumah and perhaps it was chullin - and we would permit the container, now holding fifty one se'ah, to be eaten by a non kohen, since by Torah law a majority nullifies, and the requirement of one hundred is only rabbinic, and what stands before us is no more than a doubt. There is no certainty here that a prohibition is present at all.
But in our Mishnah, since we know that the Terumah is present in one of the two storage bins or baskets, and that one is forbidden by rabbinic law, we do not permit both of them to be eaten, and we do not rely on the usual principle that a doubt in a rabbinic matter is treated leniently.