Taharos, Chapter 9, Mishnah 1. With this mishnah we open a new chapter, and a new subject: olives. Anyone who has followed this masechta will end it knowing considerably more about olive production than a city dweller ever expected to know, and this is where that education begins.
The Background
Before we can appreciate what is being asked here, we have to review how any food item first becomes capable of taking on tumah at all. These halachos do not behave like ordinary physics, where touching one thing to another produces an automatic result. A food has to be readied for impurity, and part of that readiness lies in the mind of its owner: there must be machshavah, a human attitude toward what happened.
The readying begins with moisture, and not just any moisture. Chazal list seven fluids that have this power: wine, honey, milk, blood, oil, dew and water. Yet contact with such a fluid is only half the story. A drashah teaches that the wetting has to be something the owner welcomes. We ask not merely whether liquid reached the food, but whether its arrival suits the owner's purposes. Should he regard the dampness as damage and wish it had never happened, it has come upon the food against his will, and the food remains unable to contract tumah.
Nowhere does this principle show itself as sharply as with olives. An olive counts as a dry item right up until it starts releasing its oil, and oil appears on that list of seven fluids. Here, then, is a food that carries a qualifying liquid within itself and lets it out with no outside help at all. Everything therefore hinges on a single question: when does that seeping oil represent progress the owner is happy about, and when does it represent olive slipping away from him?
The Mishnah's Question
"Zeisim mei'eimasai mekabelin tumah?" From what point do olives become susceptible to tumah?
"Misheyazi'u zei'as hama'atan, aval lo zei'as hakuppah, divrei Beis Shammai." Beis Shammai says: from the moment they give off the sweat of the ma'atan, but not the sweat of the kuppah. The ma'atan is the vat or pile where olives are heaped so that they will warm, soften and moisten in preparation for crushing them into oil. When olives in the vat begin to exude liquid, that is precisely what the owner wants: the process he set in motion is under way, and he is delighted. The kuppah is the basket, where olives are gathered to be kept and sold as olives rather than pressed. There, every drop of oil that seeps out is olive he no longer has. He is unhappy about that moisture, and so it does not prepare the olives to contract impurity.
"Rebbi Shimon omer: shiur zei'ah sheloshah yamim." Rebbi Shimon fixes the standard by the calendar rather than by the setting: the sweating carries weight only once the olives have lain in place for three days, and from that point onward the liquid they give off is what readies them for impurity.
"Beis Hillel omrim: misheyischabru sheloshah zeh lazeh." Beis Hillel proposes a different sort of test, measuring not time but quantity of liquid: once three olives have grown soft and moist enough to stick to one another, enough oil has come out for the olives to be susceptible.
"Rabban Gamliel omer: mishetigamer melachtan." Rabban Gamliel says: from the moment the work is finished. As long as the harvest is still going on and more olives are being brought in, oil seeping out is an unwelcome loss in the middle of an unfinished job. Once the labor is complete and the olives begin to give off their liquid, that is a good thing, and that is his threshold.
"Vachachamim omrim kidvarav." The Chachamim agree with Rabban Gamliel. This is the Mishnah's way of granting his view its stamp of approval and telling us that this is how we rule.