Taharos, Chapter 5, Mishnah 3. The subject of this chapter is safek tumah that comes up in a reshus harabbim, a public domain, where the halachah rules leniently and the person involved keeps his status of tahor. Our mishnah returns to the well known case of the pair of paths, and pushes it one step further: what is the din when one and the same person travels each of the two paths on separate occasions, and after each trip he handles terumah?
The setting
The mishnah begins with "Shnei shevilin", two paths running through a public area. One of the two is tamei, perhaps a corpse lies buried under it, while the other is entirely free of tumah. The difficulty is simply that no one can tell which path is which.
The first trip
"Vehalach be'echad mehem ve'asah taharos vene'echlu." Our person traveled one of the two paths, with no way of knowing which of them he had chosen. When he arrived he prepared taharos, that is, terumah foods, and those foods were served and consumed. Nothing is left of them for us to inspect or dispose of.
"Hizah veshanah vetaval vetaher." Afterward he took himself through the full process of purification from tumas meis: sprinkling with the waters of the parah adumah on the third day, a repeat sprinkling on the seventh, tevilah in a mikveh, and the arrival of night, when his taharah is complete. This detail deserves a moment's thought. In the view of the Chachamim, and that is the view we follow, none of this was required of him at all, for a doubt that arose in a public domain leaves a man tahor. To follow the mishnah's case as it is presented, we have to imagine someone who purified himself voluntarily, going beyond what the halachah asked of him.
The second trip
"Vehalach basheini ve'asah taharos." Later on he walked the other path. He is still unable to identify which of the two carries the tumah, but he is certain that this time he did not use the path of his first trip. Here too he prepared terumah foods at journey's end. How are these foods to be treated?
"Harei eilu tehoros." The mishnah declares them tahor. Since the earlier batch was already eaten and no longer lies before us, we settle the doubt by supposing that the first trip was along the path of tumah, so that the second trip took place on the path where no tumah exists at all. The food actually in our hands may therefore be eaten as tahor.
If the earlier batch has not been eaten
"Im kayamos harishonos, eilu va'eilu teluyos." Should the first batch of terumah still be intact, then both batches are set aside in a suspended state. Nothing can be decided, since the two options push us in opposite directions. It may be that the first path was the tamei one, and then the first batch calls for burning while the second may be eaten. It may equally be that the first path was the clean one, and then the first batch is fit to eat and the second is what must be burned. Burning terumah that is genuinely tahor is prohibited every bit as much as eating terumah that is tamei. Both batches are therefore left in limbo, not eaten and not destroyed.
If no purification took place between the trips
"Im lo taher beintayim, harishonos teluyos vehasheniyos yisarefu." Alter a single detail: this time he did not purify himself at all between the two trips. The earlier batch remains merely suspended, since we truly have no idea whether that first path was the tamei one or not, and so those foods are neither consumed nor burned. With the later batch the picture is altogether different. By the end of the second trip he has walked over both paths without ever purifying himself, so he has certainly set foot on the path of tumah. His tumah is thus a certainty, the terumah he touched afterward is certainly tamei, and that later batch is burned.
Such is the teaching of our mishnah: the leniency of the public domain shields a person only for as long as the matter remains genuinely in doubt, and the moment the details combine into a certainty, that leniency is gone.