Eruvin, Chapter 8, Mishnah 2. This mishnah discusses the measure, the quantity that must be placed for an eruv techumin, and in it we will learn the various opinions as to what that measure is.
The mishnah asks: "Kamah hu shiuro?" - how much is its measure? And the answer: "Mazon shtei seudos l'chol echad" - food for two meals for each person. The idea underlying this is that by placing two meals in a particular spot, a person reveals his intent that this is where he is establishing his resting place for Shabbos, since that is where his meals are located.
What are the two meals - the dispute between Rabbi Meir and Rabbi Yehudah:
Rabbi Meir: "Mezono l'chol v'lo l'Shabbos" - the measure is determined by the amount of food a person eats at a weekday meal, and not by his Shabbos meal.
Rabbi Yehudah: "L'Shabbos v'lo l'chol" - the measure is determined by the amount of food a person eats on Shabbos, and not by the weekday.
The mishnah comments on both opinions: "Zeh v'zeh miskavnim l'hakel" - both this one and that one intend to be lenient. Both Rabbi Meir and Rabbi Yehudah seek to adopt the smaller of the two measures, and their dispute is only about when a person's meal is smaller:
According to Rabbi Meir the weekday meal is smaller, since on Shabbos the tasty foods stimulate a person's appetite and he eats more, whereas during the week his meal is not so large.
According to Rabbi Yehudah it is the opposite: on Shabbos a person eats three meals, and therefore he does not eat much at each one, so his Shabbos meals turn out to be smaller.
Rashi's approach to understanding the dispute:
Some understand that the dispute concerns the amount of bread a person actually eats. Rabbi Meir holds that because the foods on Shabbos are tasty, a person eats more bread on Shabbos, and therefore the measure should be set according to weekday eating, which is the lenient one. Rabbi Yehudah holds that precisely because a person has additional foods on Shabbos, he eats less bread, and therefore the measure should be set according to Shabbos.
The measure of Rabbi Yochanan ben Berokah:
Rabbi Yochanan ben Berokah says: "Mikikar b'pundyon me'arba se'in b'sela" - the two meals are measured by a loaf of bread purchased for a pundyon (a small coin), at a time when four se'ah of grain sell for a sela (a large coin). This is the calculation:
Four se'ah are twenty-four kav, since each se'ah consists of six kav, and six times four is twenty-four.
A sela consists of forty-eight pundyonim.
It emerges that each kav sells for two pundyonim, and therefore a loaf purchased for a pundyon is made from half a kav of wheat.
However, the Gemara explains that one must subtract half of that and set the measure at a quarter of a kav, because of the expenses incurred by the shopkeeper - the grinding, the baking, the wood and the like. It emerges that the loaf purchased for a pundyon is a quarter of a kav, which is the measure of six eggs, and this is the quantity that constitutes food for two meals according to Rabbi Yochanan ben Berokah.
The measure of Rabbi Shimon:
Rabbi Shimon says: "Shtei yados lakikar mishalosh lakav" - he sets the loaf at three loaves per kav. Since there are twenty-four eggs in a kav, a third of a kav - which is the loaf under discussion - is the measure of eight eggs. But Rabbi Shimon says "two hands," meaning two thirds of that loaf, which is five and a third eggs, and this is the food for two meals for an eruv. Rabbi Shimon is lenient in this, for although a full meal is eight eggs - a complete third of a kav - in the case of an eruv one may be lenient and reduce the measure by a third.
Additional measures learned from this loaf:
The mishnah goes on to list additional halachic measures based on that same loaf that constitutes two meals, each according to its own opinion:
"Chetzyah l'veis hamenuga" - one who enters a house that has the plague of tzaraas in it becomes tamei himself immediately, but the clothes he is wearing become tamei only if he remained there for the amount of time it takes to eat half of the aforementioned loaf.
"Vachatzi chetzyah lifsol es hageviyah" - a quarter of the original measure is the amount whose consumption disqualifies a person's body from eating Terumah. One who eats tamei food in the measure of half of the half, which is a quarter of the aforementioned loaf, is disqualified from eating Terumah.
In summary: In this mishnah we learned that the measure for an eruv techumin is food for two meals, which reveals a person's intent as to where his resting place is. Rabbi Meir and Rabbi Yehudah disagree as to whether the measure is determined by the weekday meal or by the Shabbos meal, and both intend to be lenient; Rabbi Yochanan ben Berokah set the measure at a loaf purchased for a pundyon - a quarter of a kav, six eggs; and Rabbi Shimon set it at two thirds of a loaf of a third of a kav - five and a third eggs. Finally, we examined the measures for the house with tzaraas (half of it) and for disqualifying the body (half of the half), both measured by that same loaf.