Taharos, Chapter 3, Mishnah 4. This chapter has been occupied with the minimum quantities, the shiurim, that give a substance its halachic standing, and with a question that follows naturally from them: what happens when the item in question changes size? A substance is tamei, or forbidden, only when there is enough of it. So what do we say about a piece of food or flesh that was once the right size and is no longer, or that was once too small and has since grown?
The Mishnah opens with the case of food: "kebeitzah ochlin shehinichan bachamah venisma'atu." An egg's volume of food, the shiur by which food becomes capable of receiving tumah, was left out in the sun, and it dried out and contracted, so that what remains is less than a kebeitzah.
The Mishnah then lists a series of parallel cases, each one a substance whose significance depends on reaching a fixed measure:
- "kezayis min hames": an olive's volume of flesh from a human corpse.
- "kezayis min haneveilah": an olive's volume of neveilah, the carcass of an animal that was not properly slaughtered.
- "ke'adashah min hasheretz": a lentil's volume from one of the eight sheratzim, which is the shiur for the tumah of a sheretz.
- "kezayis pigul": an olive's volume of pigul.
- "vekezayis nosar": an olive's volume of nosar.
- "kezayis cheilev": an olive's volume of cheilev.
These last three belong to the world of the Mikdash and the issurei achilah connected with it. Pigul is a korban that was invalidated because of an improper intention at the time of its service. Nosar is the flesh of a korban left over past the time in which it was permitted to be eaten. Cheilev is the forbidden fat of an animal, which the Torah tells us not to eat. Each of these carries a punishment only when a kezayis is eaten.
Now to the ruling. Each of these items shrank in the sun until it fell below its shiur, and the Mishnah says: "harei eilu tehorin." They are all tahor. The flesh of the corpse, the neveilah, the sheretz, once they no longer hold the measure the Torah assigned to them, no longer convey tumah.
And the Mishnah continues: "ve'ein chayavin aleihen mishum pigul, nosar, vetamei." One does not become liable for them on account of pigul, on account of nosar, or on account of eating consecrated food while tamei. Just as with any forbidden food, liability attaches only to a certain minimum amount. If a piece that once exceeded the measure has dried down below it, one who eats it is exempt.
The Mishnah then presents the reverse situation: "hinichan bigshamim venispachu." These same items were left out in the rain rather than in the sun, and instead of contracting they absorbed moisture and swelled, so that a piece which had been under the shiur now reaches it. Here the ruling is: "temei'in vechayavin aleihen mishum pigul, nosar, vetamei." They are tamei, and one is indeed liable for them, whether the issur is pigul, or nosar, or eating kodashim in a state of tumah.
The principle is a single one, applied in both directions. Halachic status here is not fixed by the item's history but by its present dimensions. A kezayis that has become less than a kezayis is treated as the small piece it now is; a piece that has grown into a kezayis is treated as the full measure it now holds.