Terumos, chapter 2, mishnah 2. Before us is the last in this series of mishnayos of this type, dealing with separating from two containers - A and B. For example: fifty units in container A and fifty in container B, and the person wishes to separate both units (the two percent) entirely from container B, for A and B together - may he do so. The case before us is identical to the case in the previous mishnah: one container holds tamei produce and one container holds tahor produce.
In the previous mishnah we dealt with separating from the tahor for the tamei. There we learned that even though by doing so the one separating is, as it were, doing the kohen a favor by giving him two units of tahor produce, nevertheless for incidental reasons - because of the concern of 'min hamukaf' - he would have had to bring the containers close to one another, and they are not brought close enough for fear that the tahor produce will become tamei, and therefore the Sages said not to do this.
The case in our mishnah - the opposite direction:
In mishnah 2 we go in the opposite direction: the person wishes to separate Terumah from tamei produce for tahor produce. By Torah law the separation takes effect, but rabbinically he is causing the kohen a loss and is really cheating him. For if he had fifty tamei and fifty tahor, and were he to give the kohen his fair share according to the proportion, he would give him one unit of each kind: the tahor one the kohen would eat, while the tamei one he cannot eat - he can burn it, and if it serves him as a source of fuel he does derive benefit from it, but clearly food to eat is far more convenient for him than figs to burn. It turns out that the one separating is causing him a loss. Therefore the Sages said: even though the separation is effective by Torah law, since in the end the kohen gets the short end of the stick, we do not allow him to do this - but with qualifications.
The wording of the mishnah:
"Ein tormin min hatamei al hatahor" - one may not separate Terumah from tamei produce for tahor produce. That is: A is tahor and B is tamei, and the one separating takes two units from the tamei produce in container B for A and B together - he may not do so. "V'im taram" - and if he did so, by Torah law the separation takes effect, and now we will examine whether rabbinically we allow it to take effect.
Unintentionally: He did not know that the produce in container B was tamei, but rather thought that both containers were equal in their purity, and after he separated two units from container B for B and A together he found out that what was in container B was tamei. In this case the mishnah says "terumaso terumah" - after the fact, even though he should not have done so at the outset, since he did not know and acted unintentionally, the separation takes effect. There are thus two units from container B, and they are tamei, and the kohen has to manage with them.
Intentionally: He knew full well that what was in pile A was tahor and what was in pile B was tamei, and even so he took from pile B for A and B - he is deliberately cheating the kohen. In this case it says "lo asah kelum": the Sages put their foot down and pulled the rug out from under him.
This is something exceptional: even though by Torah law the separation is effective, the Sages say that it is not effective. We see here that the Sages have the power to actually prevent something that would have taken effect by Torah law from taking effect. They do not empower the person to separate this Terumah, since we do not want to cause the kohen a loss, and therefore we invalidate the separation entirely.
And likewise with the Levi'im:
The mishnah continues that a similar principle applies with the Levi'im. A Levi has maaser rishon, and this maaser is tevel - maaser from which terumas maaser has not yet been separated. Let us present the matter simply (even more simply than the approach of the Bartenura here): he has maaser in hand, and he must give ten percent of it as terumas maaser. Let us assume he placed it in his cabinet and it became tamei.
What does he do? He goes around the city and collects maaser from his neighbors. Let us assume that what he received first and placed in his cabinet is ten liters of wine of maaser, which are tevel, since he did not separate terumas maaser from them, and he must take at least one of the ten and give it to the kohen. But he leaves the ten in his cabinet, and each time he collects another ten liters of wine from his neighbors he says: I owe a tenth of this as terumas maaser, and I am not giving it from the ten I have just collected, but rather I designate one additional liter and attribute it to the ten liters lying in the cabinet in my house. And this indeed is effective. It is worth noting that the laws of 'min hamukaf' by Torah law apply only to terumah gedolah and to challah, and do not apply to terumas maaser at all; but even if we set 'min hamukaf' aside, the point is that this is possible.
And our case is that the ten liters of wine in the cabinet at home are tamei. It turns out, exactly as in the first part of the mishnah, that the Levi is taking tamei terumas maaser for tahor maaser, and in the end he is cheating the kohen: when he eventually hands him the terumas maaser, the kohen will not be able to do anything with it and will not be able to drink it, since it is all tamei. And therefore the law is as stated: if he knew what he was doing - his separation is not effective; and if he did not know, and it was unintentional - his separation is effective.
The Bartenura's precise reading:
The Bartenura highlights an important point in the first case, that of separating the Terumah: what we said, that when it was done unintentionally what is done is done, applies only when the fruit in question had reached gemar melachah - that is, its processing had been completed, so that the time obligating one to separate Terumah from it had already arrived. But if the impure fruit had not yet reached gemar melachah, and there is as yet no absolute obligation to separate Terumah from it, we say that the Terumah was ineffective and he must separate again, even in a case where it was unintentional. It is hard to see how this ruling could apply in the case of the maaser before us, since the maaser has already been gathered and its processing has been completed.
The view of Rabbi Yehudah:
Rabbi Yehudah disagrees with all of this and says: "im hayah yodea bo bitchilah" - if he knew about it at the outset. In his opinion the matter is not so simple, and there is an intermediate case: what is the ruling if he knew that the produce was impure, since someone had told him that what was in the second vessel was impure, but he forgot the matter, and out of forgetfulness he designated what was in the second vessel as Terumah, and afterwards they reminded him that it had been impure? Is this considered unintentional, since at the moment of the act he did not know, or perhaps, since he had been told beforehand, he cannot forget and then claim that he acted unintentionally?
This is what is called an unintentional act that borders on a deliberate one: on the one hand it is unintentional, since he did not know at the moment of the act and it was done in error, but on the other hand there is a measure of negligence here, and so one cannot avoid calling his act close to deliberate. Rabbi Yehudah says: indeed it is close to deliberate, and the separation is ineffective - "lo asah kelum" - he has done nothing. Even though he acted unintentionally, and at the moment of the act he forgot and mistakenly designated the impure produce in the second vessel as Terumah, that is not enough, and in such a case the Sages say that the separation of Terumah does not take effect, just as in the case of one who acts deliberately.
The halachah, however, does not follow Rabbi Yehudah but rather the first Tanna: even one who knew and forgot is considered to have acted unintentionally, and his Terumah is valid Terumah.