Menahot, Chapter Five, Mishnah One
This chapter teaches general rules with regard to the minhah offering.
1) All minhahs must be offered unleavened, with the exception of the leavened cakes of the todah and the two loaves [of Shavuot] which are offered leavened.
2) Rabbi Meir says: the leaven must be taken from [the minhahs] themselves and with this they are leavened.
3) Rabbi Judah says: that is not the best way, rather [first of all] he brings leaven and puts into the measuring vessel and then he fills the measuring vessel up [with flour].
4) But they said to him: even so [it is not satisfactory], for it would be sometimes too little and sometimes too much.
Section one: There are only two minhah offerings that are offered as leavened bread and not unleavened matzotthe leavened cakes that accompany the todah (Leviticus 7:13; these will be explained below in mishnah 7:1) and the two loaves of bread brought on Shavuot (Leviticus 23:17).
Section two: The rabbis now debate where the leaven (sour dough) that leavens these loaves comes from. According to Rabbi Meir, the leaven is taken out of the minhah of the todah. In other words, he uses some of the flour to make leavened dough, and then he uses this to leaven the entire dough for the minhah. In this way, he can ensure that he doesnt use more flour than is called for.
Section three: Rabbi Judah says that the problem is that it wont be enough leaven to cause the dough to rise properly. Rather, he first brings some leaven and puts it into the measuring vessel. Then he fills up the rest with flour until he gets to a tenth. This is what he uses for dough.
Section four: The other sages (or Rabbi Meir) respond that this way of leavening the bread is also problematic. For if the leaven was good leaven, then because it was soft and swollen up, there would actually be too low of a measure, for if the space that the leaven occupied was taken up by flour, there wouldnt have been enough flour. The opposite is true if the leaven was low quality. If it was thick and dried up, it would take up too little space, and if there was flour in its place, there would be more flour than a tenth. Therefore, this method of leavening is also problematic.