Parah, Chapter 1, Mishnah 1. This masechta is devoted throughout to the halachos of the Parah Adumah, the red cow, and it is from her that the tractate takes its name.
A word about terminology before we turn to the Mishnah itself. The Torah labels this cow a chatas, a sin offering, and Chazal adopted that label and kept it in use everywhere. You will therefore meet the chatas of purification, the paras chatas, or simply the bare word chatas, all referring to the same animal. Her ashes are spoken of as chatas ashes. Water that has been designated for sanctification with those ashes, and equally water in which they have already been placed, both go by the name chatas water. Those are the expressions our Mishnayos employ. When the ashes are combined with the water, the Mishnah's term for that act is kiddush, sanctification, and what results is called sanctified water.
The first Mishnah takes up a basic question: at what point in its life is the animal considered a parah, a cow, and no longer an eglah, a calf? The Torah requires a parah, so the definition matters.
The dispute over the years
Rabbi Eliezer's position is stated first: "eglah bas shnatah, ufarah bas shtayim." In his reckoning the animal is a calf during its first year, and from the age of two it bears the name parah.
"Vachachamim omrim," the sages rule otherwise, shifting each figure a year later. In their view "eglah bas shtayim": the term calf covers an animal through its second year. And "ufarah bas shalosh": a cow is one that has reached three. They add a further word, "uvas arba," teaching that an animal of four is equally fit for the mitzvah.
Rabbi Meir widens the range even beyond that. "Af bas chamesh," he says: a five year old too, "kesheirah hazekeinah," an animal already advanced in years, remains valid. He attaches a caution, though: "ela she'ein mamtinin lah," one does not hold off and let the animal reach such an age. His reason follows: "shema tashchir shelo tipasel." With the passing years the coat is liable to turn dark, and a single black hair invalidates the cow. Better, then, to use her while she is younger than to gamble the whole animal on a disqualification that may set in.
Rabbi Yehoshua and the word shalashis
Rabbi Yehoshua now speaks up. "Lo shamati ela shalashis": the only tradition that reached him used the term shalashis. The form of that word troubled the other sages, who asked him, "mah halashon shalashis?" Why say it that way rather than shlishis? His reply was simply "kach shamati, stam": this is the way it came down to him, with no explanation appended.
It was Ben Azzai who provided the reasoning. "Ani afaresh," he offered: allow me to clarify. Had the received wording been shlishis, third, one would understand the animal to be third "la'acheris baminyan," third in a sequence, the third born or the third counted. But the form shalashis, "ucheshe'atah omer shalashis," points unmistakably to age: "bas shalosh shanim," an animal of three years. The unfamiliar word was deliberately selected so that the intent could not be mistaken.
"Kayotzei vo amru," the Mishnah now brings a parallel from another area of halachah where the same distinction in wording appears. If you say revi'i, fourth, "la'acherim baminyan," you are speaking of the fourth in a count. But when you say a field is riva'i, you know you are speaking of a field "ben arba shanim," whose planting was four years ago. Again, the shifted vowel signals that years, not sequence, are being measured.
A third parallel: the pras eaten in a house with tzara'as
"Kayotzei vo amru," and a further case in which a precise formulation was preserved. Someone who is inside a house afflicted with tzara'as becomes impure once he has remained there long enough to eat a pras. A pras is a measure of bread, half a loaf, and the shiur here is the amount of time it takes to eat it. Rabbi Yehoshua's tradition described that loaf as "pras mishalosh lakav," from a batch that yields three loaves to a kav.
His colleagues objected: "Emor mishmonah asar lesa'ah," say rather from a batch of eighteen loaves to a se'ah. Rabbi Yehoshua answered as before, "kach shamati, stam," that is how he received it.
Here too Ben Azzai supplied the clarification. "Ani afaresh," he said: let me account for the difference. "Im atah omer mishalosh lakav, ein bo challah." A single kav of dough falls short of the amount that carries the obligation of challah, so each of the three loaves baked from it is complete. "Ucheshe'atah omer mishmonah asar lesa'ah, mi'atu chalato." A se'ah of dough, on the other hand, is subject to challah, and after the challah portion has been taken the remaining dough is diminished, leaving those eighteen loaves a little smaller than they would otherwise have been. The two formulations are thus not equivalent, and by holding to the wording he had received Rabbi Yehoshua preserved the exact measure that was taught to him.
We have learned, then, both a halachah and a lesson in transmission: the ages at which an animal is called an eglah and a parah, and how exact the sages were in guarding the very words of a tradition they had received.