Taharos, Chapter 5, Mishnah 2. Our chapter deals with cases of safek tumah, doubtful impurity, that arise in reshus harabbim, the public domain, where a person encounters two items standing side by side, one of which conveys tumah and one of which does not. The classic examples are a sheretz next to a frog, an olive's bulk from a corpse next to an olive's bulk of neveilah, or a clod of pure earth next to a clod from a beis haperas. In such cases the man is left unsure which of the two he came into contact with, and the previous mishnah recorded a dispute: Rabbi Akiva rules him tamei, while the Chachamim rule him tahor. Our mishnah restates that dispute and then introduces a third voice.
The Cases
"Echad she'amar nagati bezeh ve'eini yodea...": a man reports that his hand met one particular one of the two objects, yet he can no longer recall which status that object had. Was the thing he handled the source of defilement, or was it the harmless item lying beside it? That he cannot say.
"Nagati ve'eini yodea be'eizeh mishneihem nagati": all he knows is that contact was made with one member of the pair; identifying which member, the defiling object or the clean one, is beyond him.
These are essentially a review of the situations already presented, framed now as a report from the person himself.
The Three Opinions
"Rabbi Akiva metamei": Rabbi Akiva rules that he is tamei, consistent with his position that the leniency of reshus harabbim does not apply where the person has a means of repair available to him, namely tevilah and haza'ah.
"Vachachamim metaharin": the Chachamim rule that he is tahor, holding that the leniency of a safek tumah in reshus harabbim applies here as it does elsewhere. The halachah follows the Chachamim.
"Rabbi Yosei metamei bechulan umetaher bishvil." Rabbi Yosei is stringent across the board, with one exception. He draws the line in a different place than the other two: in all of these cases of doubt he rules the person tamei, but in the case of the two paths, where a man walked down one of them and does not know which, he rules him tahor.
Why Walking Is Different
What is his reasoning? The mishnah supplies it: "shederech bnei adam lehaleich ve'ein darkam liga", because it is the way of people to walk, but it is not their way to touch.
The Bartenura explains the logic. A person cannot simply refrain from walking. Walking is what human beings do; it is how they get from one place to another, and we cannot hold a man responsible for having traveled along a road. Touching, however, is entirely different. Nobody is compelled to handle every object he passes. Since he had no need to touch the item and touched it anyway, Rabbi Yosei will not extend the leniencies of reshus harabbim to him. Those leniencies are reserved for situations of walking, that is, for the specific cases involving the two paths.
So our mishnah leaves us with three distinct approaches: Rabbi Akiva, who is stringent in all these cases; the Chachamim, who are lenient in all of them and whose view is the halachah; and Rabbi Yosei, who is stringent in every case except the one where the man had no realistic way of avoiding the doubt.