We are in Maseches Menachos, chapter 12, mishnah 3. The mishnah comes to discuss a person who vowed to bring a minchah, except that the way he worded his vow obligates him - or at least sounds as though it obligates him - to bring a minchah in a manner contrary to the way the Torah established for bringing menachos. The question before us: does his vow take effect or not?
The basic rule in the laws of vows:
The rule regarding vows is that a person must say what he means and mean what he says - his mouth and his heart must be in agreement. Therefore, if he said one thing and intended something else, he is not bound by his vow. Here, however, the person said something that contradicts what the Torah permits.
For example, one who says "Hareini minchah min hase'orin" - I hereby take upon myself to bring a minchah from barley flour. Now, all voluntary menachos are always brought from wheat flour, and therefore the mishnah says: "Yavi min hachitin" - he must bring it from wheat flour, as the Torah commanded.
Let us clarify: the person took upon himself to bring a minchah from barley flour, and this is simply not possible. It is like someone who says, "I want to bring an olah from a zebra" - the Torah does not accept zebras as a korban, just as it does not accept voluntary offerings of barley for menachos. Therefore we turn to the one who vowed and ask him what he intended, and there are two possibilities before us:
His main intent was the minchah: He wants to bring a minchah, and it just happens that he has barley on hand, so he thought to bring it from that. If he is told that this is impossible and that it must be brought from wheat, he will answer: "Then I will bring it from wheat, as long as I bring a minchah." In this case he is obligated, since he intended to obligate himself in a minchah, and his mention of barley was not the main point.
His main intent was the barley: He has surplus barley and it is specifically from that barley that he wants to bring, and if he cannot bring it from barley, he does not want to bring it at all. In this case he never intended to obligate himself, and therefore he is not bound by his vow.
The mishnah's examples:
"Hareini minchah min hase'orin" - "Yavi min hachitin", assuming his intent was to bring a minchah, whatever it would be made from.
"Kemach" - one who says he wants to bring a minchah from ordinary flour, "Yavi soles", fine sifted flour.
"Belo shemen u'levonah" - one who says he will bring a minchah without olive oil and frankincense, "Yavi imah shemen u'levonah", as the halachah requires.
"Chatzi issaron" - one who says he will bring half an issaron of flour, "Yavi issaron shalem", since the recipe always requires at least one full issaron.
"Issaron u'machatzah" - "Yavi shnayim", two esronos of flour, because we do not bring partial measures for menachos, that is, partial esronos.
In all of these the rule is the same. We say to him: you said that you want to bring a minchah of half an issaron of soles - did you mean that if half an issaron is impossible you do not want to bring it at all? In such a case he is not obligated. Or perhaps you meant to bring a minchah, and you thought that half an issaron was a reasonable measure? And if you are told that the minimum measure is a full issaron, you would answer that this is acceptable to you and you would bring a full issaron - in that case he is bound by his vow.
The view of Rabbi Shimon:
Rabbi Shimon exempts him in all of these cases. In his view, it is not enough for a person to mean what he says; he must also say what he means, and we take into account not only the first part of his words but all of them. Since he explicitly said that it would be a minchah from barley, and no such thing exists, this is like someone saying, "I hereby bring an olah from a zebra," which carries no obligation at all. Therefore he is exempt, since his commitment does not fit the way Torah vows are meant to work.
In practice - the dispute between the Rambam and the Raavad:
The halachah follows the first Tanna, but the Rambam and the Raavad disagree about how exactly this works. The Rambam understood that according to the first Tanna we examine the opening words of his statement as an independent sentence: if it stands on its own as language that obligates him, then he said what he intended and he is obligated. But if there is no way to place a comma and say that the first half of the sentence stands on its own and obligates him, he is not obligated at all.
For example, one who says "I hereby undertake to bring a minchah of barley flour," and we ask him whether he meant barley specifically or whether wheat would also be acceptable to him, and he answers that wheat is acceptable as well - he did say "I hereby undertake to bring a minchah," and the continuation, "of barley flour," came afterward, so the opening stands on its own and he is therefore obligated. But if he used a construct form, such as "It is incumbent upon me to bring a barley minchah," there is no way to cut off the first half of the sentence and read it as an independent obligating statement, since one cannot say "It is incumbent upon me to bring a minchah of" - a minchah of what? (And the same applies to other formulations: one who said "I hereby undertake to bring a barley offering" - you cannot cut it off and say "I hereby undertake to bring barley," which is not an obligating statement.)
It emerges according to the Rambam that it is not enough for him to have intended what he said; he must also have said what he intended, at least in the opening words of his statement. If his words can be divided so that the opening stands on its own and obligates - "I hereby accept upon myself to bring a minchah," and only afterward does he mention the measure or the ingredient - he is obligated. And if that is not possible, he is not obligated.
The Raavad understood this in a far more flexible way, as was explained at the outset: he does not require that it be technically possible to cut off the first half of the sentence and let it stand on its own. Even if he said "Harei alai minchas se'orim" - "It is incumbent upon me to bring a barley minchah," we ask him: did you mean barley or nothing at all, or did you mean a minchah, and if barley is not possible you would be glad to bring it from wheat? And if he answered that he would be glad to bring it from wheat, he is obligated according to the Raavad. According to the Rambam, however, since it is impossible to cut off the opening and let it stand as an independent sentence, he is not obligated.
In summary: In this Mishnah we learned the law of one who vows to bring a minchah using a formulation that contradicts what the Torah established. According to the first Tanna we examine his intention: if his primary intention was the minchah, he brings it as required - from wheat, from fine flour, with oil and frankincense, and in a measure of a full issaron or two issronos; and if his primary intention was a condition that cannot be fulfilled, he is not obligated at all. Rabbi Shimon exempts him in every case, since no such thing exists in reality. The halachah follows the first Tanna, and there is a dispute about it between the Rambam, who requires that the opening words stand on their own as an obligating statement, and the Raavad, for whom it is enough to clarify the intention of the one who vowed.