Keilim, Chapter 17, Mishnah 6. This section of the perek is occupied with a basic question about the measurements the Chachamim established throughout the halachot of tumah and taharah. When they name a fruit or an object as a shiur, which specimen of that fruit or object did they have in mind? The preceding mishnah dealt with the pomegranate: when a hole the size of a pomegranate renders a large wooden vessel unfit for use, and therefore tahor, Rabbi Meir taught that the pomegranate intended is neither an unusually large one nor an unusually small one, but an average one. The mishnayot that follow apply that same principle to other shiurim the Chachamim gave, and our mishnah turns to one of the most frequently used measurements of all.
Not Large and Not Small
The mishnah begins by pinning down the egg itself: "Kabeitzah she'amru" - the egg-volume of which the Chachamim spoke. What is it? "Lo gedolah", not an oversized specimen; "velo ketanah", and not an undersized one either; "ela beinonit", but rather one of middling size. Wherever a kabeitzah is named as a shiur, the reference is to an ordinary, ordinary-sized egg.
This shiur appears in many places. To take one familiar example, food that has become tamei transmits tumah to other foods or to liquids only when there is at least a kabeitzah of it. So the question of which egg serves as the standard is not academic; a great deal of practical halachah rests on it.
Rabbi Yehudah's Method
Two roads to that middling size are now presented. The first belongs to Rabbi Yehudah: "Rabbi Yehudah omer". His instruction is "meivi gedolah shebagedolot", one brings the biggest egg among the big ones, "uketanah shebaketanot", together with the tiniest among the small ones. Then "venoten letoch hamayim": each is lowered in turn into a container filled with water up to its rim, and the water pushed out over the edge is collected and measured. Finally "vecholek et hamayim", he divides that water in two. Half the total overflow of the two extremes gives him the amount a beinonit egg would displace, and that quantity is his shiur.
In other words, Rabbi Yehudah wants the shiur established by water displacement: the average of the two extremes, arrived at by measurement rather than by impression.
Rabbi Yose's Objection
Rabbi Yose is not satisfied. He asks: "Vechi mi modi'eni", and who is going to tell me "eizo hi gedolah", which egg deserves to be called the large one, "ve'eizo hi ketanah", and which the small one? The experiment has the appearance of exactness, yet its foundation cannot be checked. One has selected whatever oversized egg and whatever undersized egg came to hand and crowned them the two poles of the range. But who says no bigger egg exists anywhere, or no smaller one? If such eggs do exist, then the water that was so carefully halved represents nothing at all. Exact arithmetic performed on unverified data yields an error dressed up as a measurement.
Rabbi Yose therefore rules: "Ela hakol", rather the entire matter depends "lefi da'ato shel ro'eh", on the assessment of whoever is examining the egg before him. Should it strike him as an ordinary, unremarkable egg, that egg supplies the shiur.
The Underlying Assumption
Rabbi Yose's position is not a retreat into vagueness. We may assume that the Chachamim never intended a single exact volume when they said "medium." They intended a range: any egg falling within the normal middle range qualifies as beinonit. Once that is so, a person who examines an egg and judges it to be of average size has genuinely fulfilled the intent of the Chachamim. His estimation is not an approximation of the halachah; it is the halachah as the Chachamim framed it.
So the mishnah leaves us with two approaches to the same goal. Rabbi Yehudah seeks the medium egg through the physical evidence of displaced water, while Rabbi Yose maintains that the honest eye of the observer is the standard the Chachamim themselves had in mind.