Negaim, Chapter 13, Mishnah 11. The theme running through this chapter is the tumah of a bayis hamenuga, a house stricken with tzara'as, and the mishnah just before ours drew an important distinction. A person who walks into such a house is tamei from the moment his head and most of his body are inside, but the clothing on his back is not affected at that instant; his garments are caught only if he stays within for the shiur of achilas pras. Our mishnah carries that halacha to its outer limit and asks what the ruling is when a man is split between the two domains, part of him within the house and part of him beyond its wall.
Standing inside, hand stretched out
"Hoya omeid bifnim uposhat yodo lachutz vetaba'osov beyodov": he was standing inside a bayis hamenuga and stretched his hand out through the doorway, with his rings on his fingers. The mishnah is speaking of rings worn in the normal way, derech malbush, as an article of adornment on the finger, and not rings that he happens to be carrying in his fist.
"Im shoha kedei achilas pras": if he lingered inside for that measure of time, the rings on his fingers are tamei. Where they were positioned makes no difference; they hung outside the doorway the whole while, and still, since they sit upon a body that stands within the afflicted house, they are pulled along with their wearer. The Rav's expression captures the logic neatly: "deboser gufo geriri" - they go wherever the flesh they adorn goes.
The Rav brings a further point from the Tosefta. Imagine those same rings were not being worn as jewelry at all but were simply being held, lying on his open hand, derech masoy, in the manner of carried freight. In such a case they contract tumah at once. The dispensation of waiting out the shiur of achilas pras was granted to what a person wears, whether clothing or ornament, and it does not reach objects he is merely transporting in his grasp.
Since the shiur of kedei achilas pras governs this whole discussion, it is worth recalling how that measure is calculated. It is reckoned with wheat bread rather than barley bread, and with a relish or condiment accompanying the bread, both of which produce the shorter, stricter measure: wheat bread goes down more easily than barley bread, which is coarse and full of fiber, and a condiment helps the bread be swallowed faster.
Standing outside, hand stretched in
The mishnah now reverses the picture: "Hoya omeid bachutz uposhat yodo lifnim vetaba'osov beyodov." Here the man's place is outside the house, and only his arm reaches in through the opening, his rings on his fingers. Because his head and the bulk of him stayed outside, no tumah attaches to his person, and he touched nothing within; his hand is merely suspended in the airspace of the afflicted house.
"Rebbe Yehuda metamei miyad." In Rebbe Yehuda's view the rings are struck with tumah on the spot, with no waiting period whatsoever. "Vachachomim omrim": the Chachomim disagree. Even in this case, they hold, the usual measure governs, and the rings are tamei only if the hand stays within for as long as "kedei achilas pras."
The argument of the Chachomim
The Chachomim counter Rebbe Yehuda with a kal vachomer. "Mah im bizman" - consider the situation of "kol gufo tamei," where a man entered completely and his whole body drew tumah upon itself: even there we do not declare the clothing on him tamei before he has remained the shiur of achilas pras. Now weigh against that "bizman she'ein kol gufo tamei," where his body picked up no tumah at all because his head and most of him never crossed the threshold: "eino din" - is it not obvious that here too whatever he wears stays clean until that identical span has elapsed? If a full entrance does not render his garments tamei instantly, a hand pushed through the doorway certainly cannot.
Rebbe Yehuda's reasoning
Why then does Rebbe Yehuda rule the opposite way? The Rambam, whose comments on this mishnah are devoted entirely to explaining Rebbe Yehuda, builds his answer on a case everyone agrees about. If a non-Jew enters a bayis hamenuga, the garments he is wearing become tamei on the spot. The reason is that he himself is not susceptible to tumah at all, in accordance with the principle already set out earlier in the masechta. It emerges from this that a person who is himself capable of receiving tumah is the one who shields the garments upon him from immediate tumah, so that they wait out the time of achilas pras, whereas a person who cannot receive tumah shields nothing, and whatever is upon him is tamei at once.
From here Rebbe Yehuda argues about our case. One who merely inserts his hand into a bayis hamenuga does not become tamei himself, since his head and the greater part of him never entered. Having no tumah of his own, he has no protective power to lend to what is on his fingers, and so the rings upon his hand should become tamei immediately, exactly as if a non-Jew were wearing them. The Rambam concludes: "vehahalacha keChachomim," the halacha follows the Chachomim.
The Rav lays out Rebbe Yehuda's premise in its starkest form: "motzinu sheyafeh ko'ach hatomei lehatzil miko'ach hatahor." The ability to rescue what rests upon a person belongs more powerfully to one who is himself liable to tumah than to one who stands beyond its reach. A Yisroel, who does receive tumah, shields the keilim he is wearing inside a bayis hamenuga, and they remain clean until the shiur of achilas pras has run its course. A non-Jew, by contrast, or an animal draped in some sort of covering, cannot receive tumah at all and therefore shields nothing whatever: what lies upon them is caught instantly. Still, the Rav concludes, "Ve'ein halacha keRebbe Yehuda" - this is not the accepted ruling.
There is a broader idea worth noticing behind all of this. The more sanctity a thing carries, the more it is exposed to tumah, and the further something stands from kedushah, the less it is susceptible. That is precisely why Yisroel, who are susceptible to tumah, are the ones with the strength to hold back the tumah from the garments upon them, while one who stands outside the system of tumah entirely offers his garments no shelter at all.