Menachos Chapter 5, Mishnah 1. From here on the Mishnah turns to the practical side of preparing the menachos. The first two mishnayos deal mainly with the flour component, which is common to all menachos and is what defines them as such. Usually this is wheat flour, and sometimes barley flour, and generally it is soles, finely ground flour.
Two rules regarding the soles:
The measure of the soles: The required amount is generally one issaron, that is, a tenth of an eifah. The eifah is four hundred thirty two eggs in volume, so a tenth of an eifah is about forty three eggs (43.2), roughly the measure that we today call a gallon. The Mishnah will discuss how this measure is taken.
The prohibition of leavening: Most voluntary menachos are baked or fried, and for that the flour has to be turned into a dough or a batter by adding water. As a rule, menachos may not be chametz; and those leavened menachos that the Mishnah will specify are not offered on the mizbeach but are eaten. This raises the need to deal with the practical side of leavening the dough.
What is se'or?
In the time of the Mishnah, bread was leavened by means of se'or: dough left over from the previous day that had fermented well, serving as a powerful leavening agent that is worked into today's dough to make it rise. Nowadays we use yeast for this, but in the time of the Mishnah there was no factory produced yeast, and so dough from yesterday's batch served as the starting point of the leavening. The problem our Mishnah deals with is how to handle the se'or, which may undermine the precision of the required measure.
The Mishnah opens: "Kol hamenachos ba'os matzah" - every minchah that is offered on the mizbeach, and likewise one that is made for its own sake, comes as matzah. This does not mean matzah as we know it today, but rather dough that has not been leavened.
The Mishnah immediately specifies the two exceptions: "Chutz mechametz shebatodah ushtei halechem, shehen ba'os chametz":
The chametz of the todah: The todah offering is accompanied by four kinds of menachos, one of which is leavened loaves. These too are eaten: one goes to the kohen, and the remaining nine to the person bringing the todah and his guests. They do not go up on the mizbeach.
Shtei halechem: The two loaves of bread that are waved on Shavuos together with the lambs of the festival. They too are eaten and do not go up on the mizbeach, but they are fully leavened.
The view of Rabbi Meir:
According to Rabbi Meir, "se'or bodeh lahen mitocham umechamtzan" - one produces the leaven for them from within them and leavens them with it. In his view, the measure of an issaron is a measure of genuinely dry flour, and nothing external may be introduced into the mixture. Therefore one pours the full measure of an issaron of flour into the vessel, takes a bit of that flour, makes it into a dough, lets it ferment a little, returns it to the flour, and with it leavens the whole.
An explanation of the wording: "bodeh" - an uncommon word, meaning something like extracting; "mitocham" - from within the dry flour mixture itself; "umechamtzan" - he returns it to the dough in order to leaven it.
This method is not ideal, nor is it the usual way of baking. Ordinarily one would take a solid, well fermented leaven, which gives the bread a far better flavor, whereas here the resulting dough is of lesser quality. But Rabbi Meir holds that external leaven invalidates the measurement: there is no way to know how many particles of flour it contains. If it was dense, it will contain more flour than that same volume would otherwise contain; and if it was airy, it will contain less. Since Rabbi Meir wants to measure the quantity of dry flour alone, he will not arrive at the precise measure.
Why not simply take some of the flour, let it ferment for a few days until it becomes fine leaven, and then use it the way any ordinary recipe would, knowing exactly how much flour went into the mixture? The Tosefta answers that they were concerned about leaving the leaven sitting for a long time, since as a minchah it must remain in its state of purity, and therefore they hurried to return the dough to the flour as early as possible.
The view of Rabbi Yehudah:
Rabbi Yehudah disagrees and holds that there is no need for this strange method, which produces bread of inferior taste, because the measure of an issaron is the measure of whatever is in the vessel, and not specifically of dry flour. Therefore: one brings leaven from elsewhere, just as is done for any tasty loaf of bread, places it in the vessel, fills the measuring vessel to the top with dry flour, mixes, adds water and lets it rise. The total volume of the dough is an issaron, and since the leaven is included in the measure, it is valid.
To this Rabbi Meir responds: the dough prepared for the minchah will not come to the precise measure of an issaron, but will be "o cheseirah o yeseirah" - either less or more. If the leaven was puffed up, the amount of flour in the mixture will be less than required; and if it was dense, it will be more.
The dispute can be formulated this way: Rabbi Meir measures by the weight of the dry flour, while Rabbi Yehudah measures by the volume that results afterward. Both want a volume of an issaron, but when the weight is not exact, because the mixture contains more particles of flour or fewer of them, for Rabbi Meir this is a problem and for Rabbi Yehudah it is no problem at all. Rabbi Yehudah does not accept this argument, and prefers over it the ideal taste and the finest bread.
The halachah: The ruling follows Rabbi Yehudah. It turns out that in practice the minchah is prepared from the finest type of bread: one takes leaven from elsewhere, places it in the mixture, and measures the total volume of the flour and the leaven together as one issaron, and it is valid even lechatchilah.