Taharos, Chapter 5, Mishnah 1. With this mishnah we open a new chapter, and the shift is immediate: everything looks a little different once the case takes place in reshus harabbim, the public domain. The chapter continues the discussion of a safek tumah, a doubtful case of impurity, but now the question is what happens when the doubt involves two items standing side by side, one of which conveys tumah and one of which does not.
The mishnah opens by listing four pairs. Each pair consists of two things that a person could easily mix up, where one member of the pair carries tumah and the other does not, or where the two convey tumah in different ways.
The Four Pairs
"Hasheretz vehatzefardeia bireshus harabbim": a sheretz together with a frog. A sheretz is one of the eight creeping creatures enumerated in the Torah, and it transmits a severe form of tumah. A frog transmits nothing whatsoever. Yet among those eight sheratzim is the letaah, and a frog looks very much like it, so the two can readily be taken one for the other.
"Vechein kezayis min hameis vekezayis min haneveilah": an olive's bulk of flesh from a human corpse alongside an olive's bulk of neveilah, carrion. The kezayis from the meis is metamei through maga (touch), through masa (carrying) and through ohel (overshadowing). The kezayis of neveilah does not transmit tumah through ohel. Once again, two items presented side by side.
"Ve'etzem min hameis ve'etzem min haneveilah": a bone from a human corpse and a bone from a neveilah. The same juxtaposition, this time with bones.
"Vegush me'eretz tehorah vegush mibeis haperas": a clod of earth from ordinary pure ground next to a clod of earth from a beis haperas. A beis haperas is a field that was plowed over, where a grave may have been buried in it, so the plow may have scattered pieces of a corpse through the soil. Because of that concern the Chachamim declared such a field tamei; this is a tumah miderabbanan.
"Vegush me'eretz tehorah vegush me'eretz ha'amim": a clod of pure earth next to a clod of earth from outside Eretz Yisrael. The Chachamim declared the soil of the lands of the nations tamei as well, out of concern that it may have come from a burial ground.
These four pairs are lav davka, not an exhaustive list. They are simply illustrations of one situation: two items that could be mistaken for one another, where one is tahor and one is tamei, or where both convey tumah but not in the same manner.
The Three Cases of Doubt
"Shenei shevilin, echad tamei ve'echad tahor": there are two footpaths, of which one is tamei and the other tahor. "Halach be'echad meihen ve'eino yodea be'eizeh meihen halach": a person traveled along one of the two, and afterward he cannot tell whether the path he took was the impure one or the pure one.
"He'ehil al echad meihen ve'eino yodea al eizeh meihen he'ehil": he overshadowed one of the two items and does not know which one he overshadowed. This fits the case of the kezayis from a meis alongside the kezayis of neveilah, where only one of the two conveys tumah through ohel.
"Heisit es echad meihen ve'eino yodea eizeh meihen heisit": he moved one of them and does not know which one he moved. This refers to a form of moving that generates tumah in its own right.
In each of these three cases there are two possibilities. One would leave him tamei and one would leave him tahor, and he cannot determine which one occurred.
Rabbi Akiva and the Chachamim
"Rabbi Akiva metamei vachachamim metaharin." Even though the case takes place in reshus harabbim, Rabbi Akiva rules that he is tamei, while the Chachamim rule that he is tahor.
What stands behind each position? We already know the governing principle: a safek tumah in reshus harabbim is tahor. That rule has been at work throughout the previous chapters. So what moves Rabbi Akiva to rule strictly here?
The Bartenura explains that the leniency of reshus harabbim was extended only where there is no takanah, no way of repairing the situation. If a piece of terumah may or may not have been under an ohel, or may or may not have been touched by a sheretz, nothing can be done to remedy the doubt, and there we rule leniently. But a human being does have a remedy available to him: tevilah and haza'ah, immersion in a mikveh and sprinkling. Since a means of repair exists, Rabbi Akiva holds that it must be used, and the person is treated as tamei.
The Chachamim disagree with this entire line of reasoning. In their view, the availability or unavailability of a remedy is not the mechanism behind the leniencies of reshus harabbim at all. The distinction between reshus harabbim and reshus hayachid stands on its own grounds, for the reasons already laid out earlier in the masechta, and it applies here as well. That is the machlokes, and the halachah follows the Chachamim.