(Mishnah): If he had 10 rows of 10 jugs of wine and he said, "One outer row is Maaser'' and it is not known which (one he meant), he should take two barrels on a diagonal (to be the Terumas Maaser). (In all of the cases of the Mishnah, he is taking Terumas Maaser in a way that ensures that he will certainly take it from the Maaser Rishon that he separated. But since he is taking more Terumas Maaser than is necessary, he can sell to the Kohen the portion that he took, minus the value of the actual Terumas Maaser that he was required to give.)

If he said, "Half the outer row is Maaser'' and it is not known which, he should take four barrels from the four corners.

If he said, "One row is Maaser'' and it is not known which, he should take a diagonal row.

If he said, "Half a row is Maaser'' and it is not known which, he should take two diagonal rows.

If he said, "Half a barrel is Maaser'' and it is not known which, he should take a part of each and every barrel.

(Gemara) Question: (In the case of (b) in the Mishnah) Why doesn't he take only two - since his Maaser only fixed up half of the jugs?

Answer (R. Kohen citing Rabbanan of Kisarin): He had already separated half from elsewhere.

(Tosefta) (Rebbi): If there were two barrels in front of him, one of Maaser Tevel Tahor (Tevel meaning that their Terumas Maaser hadn't yet been separated) and one of Maaser Tevel Tamei - he should bring two jugs and take from the Tahor barrel enough Terumas Maaser for both barrels and do the same thing with the other barrel. Then he should take one jug and say, "If this contains the Tahor Maaser, it should be the Terumas Maaser of both barrels; and if it the Tamei, it should not.'' He should then do the same with the other jug. He may then immerse and drink the first jug immediately and then immerse and drink the second.

(R. Eliezer ben R. Shimon): He must only immerse at the end (by which time he is certainly Tamei).

Question: What is Rebbe's reasoning? Perhaps he already drank Tamei liquid.

Question: What is R. Eliezer ben R. Shimon's reasoning? Perhaps he drank Tamei liquids (only) at the end (since until then we can rely on his Chazakah of being Tahor).

All agree that if a person drank Tamei liquids, he may not eat even doubtful Terumah. All agree that if he drank doubtfully Tamei liquids, he may then eat even certain Terumah. So where is the dispute? Rebbi's reasoning is that since one is certainly Tamei, he will certainly at some point become Tamei - so after he drank the first, he must already immerse.

And R. Eliezer bei R. Shimon's reasoning is that he drank less than a Revi'is (which is the minimum amount to bring Tumah; and then waited a significant period of time before drinking again less than a Revi'is, so he doesn't become Tamei from the first drinking).

And for what is Rebbi concerned? That he would perhaps drink a Revi'is (and become Tamei, so he required immersion even after the first).

Question: Does R. Eliezer bei R. Shimon not agree with that concern?

Answer (R. Yosi bei R. Bun): Rather, it must be that Rebbi was concerned that perhaps he drank Tamei liquids at the beginning and R. Eliezer bei R. Shimon was concerned that he drank them at the end.