We begin the thirteenth and final chapter of Maseches Menachos with Mishnah 1. The subject of the chapter is vows: a person who takes a vow upon himself, a personal obligation to bring something to the Beis HaMikdash, except that there is uncertainty about the nature of the vow he made.
The uncertainty can arise in one of two ways:
The one vowing worded his statement vaguely, and even he himself does not know exactly what he meant.
The one vowing knew perfectly well what he meant at the time of the vow, but afterward he forgot the details of his obligation.
In all of these cases we are dealing with voluntary obligations - no one is commanded to make a vow. But once he has vowed, a new obligation is created for him, as it were, a sort of "mitzvah six hundred and fourteen": from now on there rests upon him a Torah obligation to fulfill what he said. Hence the need to understand what he must do in order to discharge his obligation.
The governing rule in the two cases:
In a vague vow: he must bring something that meets two conditions, which are really two sides of the same coin: first, that it fit the words that came out of his mouth, since a vow always requires a formal verbal commitment; and second, that it be reasonable to say that this is what he meant - that is, that he can say, "Indeed, I did not have anything specific in mind, but I could have meant this." The smallest measure that meets these conditions is enough.
In a forgotten vow: he must bring the amount needed in order to be completely certain that he has fulfilled his obligation, and no more than that. This measure varies according to what he himself testifies was possible in his mind at that moment.
The cases in the Mishnah:
"Harei alai issaron" - a person who obligates himself to bring an issaron of fine flour. Since the basic format of a minchah is one issaron, the Mishnah says: "yavi echad" - he brings one issaron of fine flour as a single minchah and has discharged his obligation, for what else could he have meant?
"Esronim" - he obligated himself using the plural, and did not have a specific number in mind. The word "esronim" indicates a plurality, and can include two, three, or even more. Since the number of esronim in a minchah is always a whole number, and a plural cannot be understood as one, while two is the most reasonable number he could have intended - "yavi shnayim" - he brings two and has discharged his obligation.
"Peirashti, ve'eini yodea mah peirashti" - the one vowing testifies that when he obligated himself to bring esronim he had a specific number in mind, but he does not remember what it was. In this case, "yavi shishim issaron" - he brings sixty esronim. As we learned in the previous chapter, the largest possible minchah consists of sixty esronim of fine flour; since he certainly did not intend more than that, he brings sixty and thereby discharges his obligation with certainty.
It is worth noting that since his commitment concerned the quantity of fine flour and not the number of menachos, he has the option of fulfilling it in one of two ways: to bring sixty separate menachos of one issaron each, or to bring one large minchah of sixty esronim. We may need to qualify this somewhat in the next Mishnah, but the point is that the commitment is not for sixty menachos but for sixty esronim of fine flour.
"Harei alai minchah" - here the discussion is not about quantity but about type, since the standard minchah is one issaron. There are five voluntary menachos that a person can bring:
Minchas HaSoles - plain flour that is not baked.
Minchas Machavas and Minchas Marcheshes - which are prepared in a vessel.
Minchas Chalos and Minchas Rekikin - which are baked in an oven.
Since he simply said "a minchah" without specifying which type, "yavi eizo sheyirtzeh" - he may bring whichever one he wishes, any of the five menachos. The ingredients of all of them are essentially identical: a tenth of an ephah of flour, and usually a bit of oil and frankincense upon it, and therefore each one of them fulfills the wording of his vow.
Rabbi Yehudah disagrees and says: "yavi minchas hasoles" - specifically the unbaked minchas hasoles, the raw flour, which is the most distinguished of the menachos and stands on its own. The reason is this: when the verse uses the word "minchah" without qualification, with no accompanying term and no description - not minchas machavas and not minchas marcheshes - it means specifically minchas hasoles.
For it says in Vayikra (chapter 2, verse 1): "venefesh ki takriv korban minchah laHashem soles yihyeh korbano" - and when a person brings a minchah offering to Hashem, his offering shall be fine flour. We see that the unqualified minchah of the Torah is minchas hasoles. Therefore, one who says "I take upon myself a minchah" - we assume he intended the meaning that the verse uses, and he must bring a simple minchas soles.
However, the halachah does not follow Rabbi Yehudah but rather the Tanna Kamma: one who says "I take upon myself a minchah" may bring whichever he wishes of the five types of voluntary menachos.