Mishnah 3 in the fifth chapter of Maseches Temurah deals with a pregnant animal - a cow, a goat or a ewe - that a person comes to consecrate, and in the course of doing so wishes to give the fetus in its womb a status different from that of its mother with respect to the type of offering.
We learned earlier that a fetus in its mother's womb receives its status from the mother, and we examined two cases: an animal that became pregnant after it was consecrated, about which the Tannaim disagree and which does not concern us now; and an animal that was already pregnant at the time of consecration, about which they disagree whether the fetus is "the thigh of its mother," that is, like an additional limb of the mother, or whether it is a separate creature in its own right, so that we have before us two distinct entities. We will not resolve that dispute. In any event, all agree that a fetus in its mother's womb can receive a status from the mother, and therefore we will discuss one who attempts to consecrate the mother and the fetus with two different statuses - as two types of offerings.
The simple case - consecrating the offspring first:
One who says "V'ladah shel zu olah v'hi shelamim" - the fetus in her womb shall be an olah and the mother herself shall be shelamim - "devarav kayamin" - his words stand. There is no doubt that this is effective, and the mother and the fetus will have two different statuses. The reason: since he first designated the fetus, it emerges that the mother is still ordinary, unconsecrated property and only the fetus within her has become consecrated. And when he afterward consecrates the mother separately, her status no longer spreads to the offspring, since the offspring has already been designated in advance, and once one status has taken effect on something, it can no longer receive another status in matters of offerings.
He consecrated the mother first - the view of Rabbi Meir:
But what is the law regarding one who says of his pregnant cow, goat or ewe: "Harei zo shelamim u'ladah olah" - this one shall be shelamim and her offspring an olah, meaning that the mother shall be of one type and the fetus in her womb of another? On this the Mishnah teaches: "Harei zo v'lad shelamim, divrei Rabbi Meir" - both the mother, who was designated as shelamim, and the offspring in her womb shall be shelamim, while his second statement, that the fetus shall be an olah, has no force whatsoever.
Rabbi Meir's reasoning: he holds everywhere that we "seize the first expression." When a person makes a compound statement consisting of two parts, and the second part does not fit with the first or even contradicts it, we say that the first expression is the one that takes hold and is what he intended, while his later words have no force. Once he said that the animal before him is shelamim, that automatically took effect on the fetus in her womb as well, and when he adds a moment later that the offspring shall be an olah - he has already missed his opportunity.
Is there proof here for the question of "the thigh of its mother"?
This does not resolve the question of whether the fetus is like the thigh of its mother, an additional limb of hers, or a separate creature in its own right, for either way, when he consecrates the entire animal he consecrates both the mother and that which is in her womb. There is room to infer something from the wording of the Mishnah, "Harei zo v'lad shelamim" - this one is the offspring of shelamim - rather than simply "this one is shelamim": the phrase "offspring of shelamim" indicates that the offspring is subordinate to the mother, and this hints at the reasoning that it is the thigh of its mother. However, the Gemara raises this inference and rejects it: Rav checked and found a different version of the Mishnah in which the word "offspring" is omitted, and so no proof can be brought from there. The main novelty here, then, is Rabbi Meir's approach: seize the first expression, and the second statement does not nullify the first.
The view of Rabbi Yosi:
"Amar Rabbi Yosi: im lechach niskaven mitechilah, ho'il v'i efshar l'hotzi shnei sheimos k'echas - devarav kayamin" - Rabbi Yosi said: if this was his intention from the outset, since it is impossible to pronounce two designations at once, his words stand. If from the very beginning, at the moment he said that the mother would be of one type and the fetus of another, he had this entire compound intention in mind with both its parts, then since a person cannot pronounce two designations simultaneously, he was necessarily forced to state his words about the mother and about the offspring one after the other. Therefore both of his statements stand. And Rabbi Yosi's reasoning is that he holds everywhere that "a person is held to the conclusion of his words as well" - his final words also take hold.
It emerges in practice that if, at the moment he said of the mother that she is shelamim, he already had in mind: "and in a moment I will say that the offspring shall be an olah, and I do not wish to include it in this declaration" - the consecration takes effect as he stated it. For this reason he must actually be asked about his intention: when you said that the mother is shelamim and the offspring an olah, was the offspring already in your mind at that time, or did you first say that the mother is shelamim, and only afterward did a second thought arise in you that the offspring should be an olah? If he changed his mind afterward, his statement is ineffective. From this you learn that his words stand only if his intention regarding the second thought was already present at the time he made the first statement, as a single compound thought.
And so the Mishnah teaches explicitly: "Umi she'amar harei zu shelamim venimlach ve'amar velad olah - harei zeh velad shelamim" - one who said, "This is a shelamim offering," and then reconsidered and said, "Its offspring is an olah," the offspring is a shelamim. Someone who first consecrated an animal as a shelamim, and afterward changed his mind and designated the offspring as an olah - his later words do not take effect, and both the mother and the offspring are shelamim.
Why is the concluding clause needed?
At first glance this is puzzling: once Rabbi Yosei taught in the opening clause that his words stand specifically when he intended so from the outset, we automatically understand the converse - that if he did not intend so from the outset but rather changed his mind, his words do not stand. Why, then, did the Mishnah need to state this again explicitly in the concluding clause?
The answer is that Rabbi Yosei comes to teach us that the laws of "toch kedei dibbur" (within the span of a short utterance) do not apply in the context of consecrating animals, and as we will see in the next Mishnah - not to temurah (substitution) either.
The parameters of "toch kedei dibbur":
It is a general rule that sometimes a person's mouth, and for this matter even his body, gets slightly ahead of his mind, and immediately after he speaks his mind tells him: that is not what you meant, say something else. The halachah is that if he restated his intention correctly within a very brief span of time called "toch kedei dibbur," whose measure is the length of an utterance. There is a dispute about its measure: the time it takes to say "Shalom aleichem," or "Shalom aleichem rabbi," or "Shalom aleichem rabbi umori." The poskim tended toward the longer measure, "Shalom aleichem rabbi umori," which is about two seconds. If within that time he said something else, he generally has the power to nullify his first words.
For example: a person took a cup of water and recited the berachah "Baruch atah Hashem Elokeinu Melech ha'olam borei pri ha'eitz," and immediately remembered that this is not a fruit of a tree but rather requires the berachah of "shehakol," and he added "shehakol nihyeh bidvaro," correcting himself. Did the berachah count, and may he drink? The answer: if he said "shehakol nihyeh bidvaro" within toch kedei dibbur of the first berachah - the correction is effective.
The cases where "toch kedei dibbur" does not apply:
This rule does not apply everywhere. In four matters it does not apply at all:
Betrothal of a woman.
Divorce of a woman.
A blasphemer, one who curses Hashem's Name.
Idolatry.
The Rashbam explains the reason: it is true that sometimes the mouth gets ahead of the mind, but not in matters of such gravity as cursing Hashem's Name, Heaven forbid, or marrying a woman. Similarly here, the Mishnah adds consecration to this list, for the laws of toch kedei dibbur do not apply to it.
The halachah: The ruling follows Rabbi Yosei. Therefore, one who said two things that constitute a double intention, where the second does not sit well with the first or even contradicts it, we actually ask him what he intended and we believe what he says. And if he changed his mind after the first statement, even if he retracted and said something else within toch kedei dibbur, within those two seconds - it has no force in the context of consecrating animals. The first statement stands as it is, and there is no application of toch kedei dibbur here.