Machshirin, Chapter 1, Mishnah 4. Masechta Machshirin teaches the laws of hechsher, the way liquid renders food susceptible to tumah. Two things are needed. The food must become wet, and the wetting must match the pasuk "vekhi yutan mayim al zera," meaning that the water came onto the food in a manner the owner desired. Rain that falls on produce against the owner's wishes is not bekhi yutan and creates no hechsher. Our mishnah asks what happens when a person later moves that same rainwater around by his own deliberate act.
The Case
The mishnah's case: "Hano'er agudah shel yerek," one who shakes out a tied bunch of greens, "veyatztzu mitzad ha'elyon latachton," and the moisture is flung off the upper portion down onto the portion lying beneath it. Picture a bunch of greens that was left outdoors when rain came down. The owner had no interest in that rainfall, so at the moment it landed there was no bekhi yutan and nothing became susceptible to tumah. He then picks the bunch up, gives it a shake, and the rainwater sitting on top travels down onto the greens below.
The Machlokes
"Beis Shammai omrim": "bekhi yutan." Their position is that hechsher takes effect here. The situation resembles the earlier cases in the perek involving a shaken tree, where water knocked off the tree onto something below produces hechsher. True, the rain first arrived without the owner wanting it, but he himself is now setting that water in motion, driving it from one section of the bunch onto another. Such a transfer is treated as intentional, and the lower greens therefore become mukhshar lekabel tumah.
"Beis Hillel omrim": "einan bekhi yutan." Their reasoning is that the bunch is one object, and we refuse to split it into an upper item and a lower item. Being a single unit, water sliding around inside it means nothing. It is still the identical rainwater, and the halachah looks at it as if the rain had landed on the lower greens from the very start. Since it carried no status of bekhi yutan when it fell, no such status attaches to it now, the deliberate shaking notwithstanding.
Beis Hillel's First Argument
Beis Hillel press their case with a question: "Vahalo no'er es hakelach": consider someone shaking off one lone stalk. Do we say "chosheshin anu shema yatztzu," that we must be concerned the water sprayed, "me'aleh le'aleh," from one of its leaves onto another? No one would advance such a claim about a single rained-on vegetable. We would never rule that shaking the upper leaves sends moisture downward and thereby makes the stalk's lower leaves susceptible to tumah. The suggestion is untenable: it is one stalk, and the particular spot where the water happens to settle is irrelevant. Whatever moisture is on it lacks the status of bekhi yutan altogether. Beis Hillel argue that a tied bunch stands in precisely the same position.
Beis Shammai's Reply
"Amru lahen Beis Shammai: shekelach echad aval kelachim harbeh." Beis Shammai answer simply. A stalk is a single item, but a bundle is a collection of many stalks. We do not view it as one unit; we view it as many separate things. Consequently, when the water moves from the upper part down to the lower part, it is like water traveling from a branch of a tree onto something lying beneath it, exactly as in the cases taught earlier in the perek.
Beis Hillel's Second Argument
Beis Hillel offer a second illustration: "hama'aleh sak malei peiros," one who lifts a sack packed with produce, "venasno al gav hanahar," having set it down at the river, where it grew soaked. When the sack is raised or turned, liquid inevitably drains off the higher side toward the lower one. Do we then say "chosheshin anu shema yardu," "mitzado ha'elyon latachton," and call it bekhi yutan? Surely not, because the sack is a single object. Many separate pieces of fruit sit inside it, yet even Beis Shammai must admit that no water has passed from one thing onto a different thing. There is one sack of produce, and the same liquid stays right where it was. Beis Hillel maintain that a bunch of greens deserves identical treatment, since the water throughout is one single body of water.
Where Beis Hillel Concede
Beis Hillel mark the boundary themselves: "aval shnayim," but where there are two such sacks, "zeh al gav zeh," one resting upon the other, and moisture from the upper one drips down onto the one below, then "hatachton bekhi yutan." Beis Hillel grant that the bottom sack has now been mukhshar lekabel tumah. In that arrangement the two sacks are obviously not a single object, and the water genuinely crossed from one item onto a separate one.
Rabi Yosi's Position
"Rabi Yosi omer": "hatachton tahor." Even in that scenario he differs. As happens frequently throughout these mishnayos, the word tahor is not a statement of absolute purity; it means only that no hechsher lekabel tumah has taken place. In his understanding, a pair of stacked sacks still forms one unit despite one of them sitting above the other, because the water in both derives from a single source. Beis Shammai as well as Beis Hillel reject this and rule that the bottom sack does become mukhshar.