Taharos, Chapter 5, Mishnah 7. Our chapter has been working through cases of doubtful tumah, and this mishnah lines up several of them side by side: a person sitting or sleeping out in the open, exposed to whoever happens to pass by, who afterward has to figure out what his status is. The mishnah introduces three separate scenarios, and each one is decided by a different principle.
The opening case: "Mi sheyashav bireshus harabbim uva echad vedaras al begadav, o sherakak venaga beruko." A person is sitting in a public thoroughfare. Someone walks by and steps on his garments, or someone spits and he ends up touching the spittle. Two different exposures, and the mishnah rules on them differently.
About the spittle: "Al ruko sorfin es haterumah." Terumah that this person touched is burned. Even though we have no idea whose spittle it is or what that person's status was, spittle is treated strictly. Earlier in the previous chapter we encountered a list of doubtful cases in which the halachah rules stringently, and doubtful spittle found in a public place was one of them, since the spittle of a zav conveys tumah.
About the garment: "Al begadav holchin achar harov." Here we do not automatically assume the worst; we follow the majority. Who makes up the population of this town? If the majority are zavim and zavos, whoever stepped on the clothing is presumed to have been one of them, and the garment is tamei with midras. If the majority are tehorim, the garment stays tahor.
The second scenario: "Yashan bireshus harabbim ve'amad." A person took a nap in a public thoroughfare and then got up. He has no information at all: he does not know whether anyone stepped on him, leaned on him or passed over him while he slept. How does he assess himself? "Kelav temei'in midras, divrei Rabbi Meir." Rabbi Meir holds that his clothing is tamei with midras, since in a public place we may assume that somebody made contact with a sleeping man lying in the way. "Vachachamim metaharin." The Sages are lenient and declare him tahor, because this is a case of doubtful tumah that arose in the public domain, and the rule is that a doubt in the public domain is resolved leniently.
The third scenario reverses the two opinions, and that is what makes it so instructive. "Naga be'echad balailah ve'eino yadua im chai im meis." A person came upon someone lying in a public place at night and touched him, with no conversation and no way of knowing whether the man was alive and sleeping soundly or already dead. "Uvashachar amad umetza'o meis." When morning came, he found him dead. So he touched a body that may or may not have already been a corpse at the moment of contact. "Rabbi Meir metaher, vachachamim metamei'in."
Two principles that we have already learned are colliding here. Rabbi Meir rules that the man is tahor because the entire doubt took place in the public domain, and doubtful tumah in the public domain is resolved in favor of taharah. The Sages rule that he is tamei on the strength of a different principle: "shekol hatumos kesha'as metzi'asan." Every case of tumah is assessed according to the moment we discovered it. We do not reconstruct history backward and argue that things may well have been different earlier. It is the same principle we met repeatedly in the previous chapter: the jar is sealed now, so we do not speculate that it stood open before; the item was found here, so we do not imagine an earlier location. What we found is what we assume was the case, until evidence tells us otherwise. Here, the man was found dead in the morning, so we treat him as having been dead when he was touched.
There is one variation in which the Sages step back. Suppose the man had been seen alive when night fell, and only afterward, in the middle of the night, did someone touch him without knowing whether he was still living, and at dawn he was discovered dead. In that case the Sages concede to Rabbi Meir and rule the toucher tahor, since here we have an established prior status of a living person to rely upon.
And the halachah in our mishnah follows the Sages.