Demai, Chapter Three, Mishnah Four
This mishnah discusses situations in which a Jew who has tithed his produce gives over the produce to a Samaritan, an am haaretz or a Gentile for them to either mill the wheat or guard it. The question is: do we fear that the Samaritan, am haaretz or Gentile switched the produce with their own? If we do have such a fear, then when the Jew gets his produce back he must tithe it again.
1) One who takes his wheat to a Samaritan miller or to an am haaretz miller, [the wheat when ground] retains its former status in respect of tithes and the law of seventh year produce.
a) [But if he carried it] to a Gentile miller, [the wheat when ground has the status of] demai.
2) One who deposited his produce with a Samaritan or am haaretz, [the produce] retains its former status in respect of tithes and the law of seventh year produce.
a) [But if he left it] with a Gentile, it is like the produce of the Gentile.
i) Rabbi Shimon says: [it becomes] demai.
Section one: The mishnah assumes that the Samaritan or am haaretz miller did not switch the tithed produce that was given to him by the Jew, and therefore we need not fear lest the grain is untithed or that it is grain that comes from the Sabbatical year. Produce from the Sabbatical year (shmita) has certain restrictions, which we will learn about in greater length in Tractate Sheviit.
However, if he gives the grain to a Gentile miller, he must fear that the Gentile mixed it up with his own produce, and hence it has the status of demai. The difference between giving the produce to a Gentile and giving it to a Samaritan or an am haaretz seems to be that we know that the Gentiles produce is untithed whereas the Samaritan or am haaretz might have tithed their produce. Therefore in the case of the Gentile there is only one doubtdid he switch it with his own grain. In the case of the Samaritan or am haaretz there are two doubts. Did he switch it with his own grain, and if he did, was his grain tithed? Since there are two doubts, the law is lenient.
Section two: The first part of this section is the same as the first part of section one.
However, the rule with the Gentile is slightly different. Here, according to the first opinion, we must treat the produce as if it was certainly switched with the Gentiles own produce. When he gets his produce back, the Jew will be obligated to separate terumah and all tithes, not just those which one is obligated to separate from demai.
The difference between this scenario and the case of the Gentile miller above seems to be that this is just a friend with whom one has left something to guard. A person who is doing a favor for a friend might not care so much if he switches the produce with his own. However, the miller might be more cautious in switching the grain he was given with other grain because as a professional, he doesnt want to upset his clients. Since it is more doubtful that he switched it around, the status of the grain is only demai.
Rabbi Shimon says that in the case of the deposit the returned produce is only demai, because it is still only doubtful whether the Gentile switched the Jews produce with his own. Since it is only doubtful whether the Gentile switched it, the Jew takes out tithes and terumah, but he doesnt have to give the terumah to a priest, because the priest cant prove that the Jew owes him this terumah. The Jew may sell the terumah to a priest and keep the proceeds for himself but he may not eat it.