Demai, Chapter 5, Mishnah 3. In this mishnah we will examine a fundamental principle in the separation of Terumah and tithes, and its implications for the laws of demai.
The principle: one separates only from produce that is itself obligated:
It is a fundamental principle that Terumah and tithes may be taken only from produce that is itself still obligated in Terumah and tithes. Even if a person has before him a large quantity of produce requiring separation, he may separate on its behalf only from other produce whose obligation is still intact. Produce from which Terumah and tithes have already been separated cannot serve as Terumah and tithes for a different batch of produce, because in order to serve as a separation for something else, that item must itself be obligated.
Accordingly, when we come to separate tithes and terumas maaser for demai, we must make sure that the separation is being made from something that likewise has not had tithes separated from it. It must be at least on the same level of doubt and suspicion regarding non-separation of tithes as the item on whose behalf we are separating.
The wording of the mishnah - the view of Rabbi Meir:
The mishnah states: "HaLokeach min hanachtom" - a person who bought bread from the baker may "le'aser min hachamah al hatzonenes" - tithe from the warm loaf on behalf of the cold one. That is, if he has before him two loaves bought from the same baker, one of them warm - bread baked today, fresh bread - and the other cold, yesterday's bread, he may tithe from today's bread on behalf of yesterday's bread. And likewise the reverse: "min hatzonenes al hachamah" - even from yesterday's bread on behalf of today's. Moreover, "afilu mitipus harbeh" - even if the loaves come from different molds, from different shapes that the baker baked. "Divrei Rabbi Meir" - this is the view of Rabbi Meir.
The reason: since we are dealing with a single baker, we assume that the grain he grinds into flour and bakes comes from a single source - either from someone who already separated tithes, or from someone who did not. We do not assume that he bought part of it from a person who separated tithes and the rest from someone who did not. Therefore there is no concern that one is separating from exempt produce on behalf of obligated produce, or from obligated produce on behalf of exempt produce. Even if the bread is from two different days, since the baker is one, we assume he bought everything from a single person, and it is all either obligated in tithes by law or exempt from it. For this reason it is permitted to separate demai from one on behalf of the other, and there is no need for concern.
Separating from the less choice produce:
As an aside: although as a rule Terumah and tithes must be separated from the choicest produce, and yesterday's bread is not as fine as fresh bread, here, since we are dealing with demai, they permitted separating even from the less choice.
The view of Rabbi Yehudah - he forbids:
"Rabbi Yehudah oser" - in his view one may not take from one day's bread on behalf of another's, even with a single baker. His reason: "She'ani omer, chitim shel emesh hayu mishel echad, veshel hayom hayu mishel acher" - the wheat the baker received yesterday belonged to one person, and the wheat he baked with today belonged to someone else. There is therefore reason to be concerned, even with a single baker, that he received from different sources, and one of them may be obligated in Terumah and tithes while the other is exempt.
The view of Rabbi Shimon:
"Rabbi Shimon oser biterumas maaser" - regarding the separation of terumas maaser, which is ultimately the primary separation that must be made for demai, Rabbi Shimon agrees with Rabbi Yehudah that there is reason for concern: if the bread comes from two different days, this may be a problem. Not only that, but the commentators understand that Rabbi Shimon goes a step further - that even if the bread is from the same day, if it is from different shapes there is reason for concern, since different shapes may come from two different people. And all of this applies specifically to terumas maaser.
"Umatir bechallah" - but regarding the obligation of separating challah there is no need to be concerned that the dough came from different people, and the reason is simple: the obligation of challah takes effect only once the dough is in the baker's hands, from the moment it became dough. Until that moment no one ever separated challah from it, and therefore there is no concern that it came from two different people - and even if it did come from two, it makes no difference, since neither of them separated challah before it became dough in the baker's hands.
In summary: in this mishnah we learned the principle that one separates only from produce that is itself obligated, and its application to demai - that the separation must be made from something whose level of doubt is equal to that of the item on whose behalf one is separating. According to Rabbi Meir, one who buys from a single baker may tithe from the warm loaf on behalf of the cold one and from the cold on behalf of the warm, and even from many molds, because we assume the grain came from a single source; and even though the cold bread is not the choicest, this was permitted in the case of demai. Rabbi Yehudah forbids it, lest yesterday's wheat belonged to one person and today's wheat to another. Rabbi Shimon forbids it regarding terumas maaser - and according to the commentators even with different shapes from the same day - but permits it regarding challah, whose obligation begins only once the dough is made in the baker's hands.
This principle will be illustrated further and further throughout the mishnayos that follow, until the end of the fifth chapter.