Taharos, Chapter 4, Mishnah 12. Our chapter has been working through lists of sfeikos, doubts. First came the catalogue of doubtful cases in which we rule stringently and declare the item tamei, and after it a second catalogue of doubtful cases in which we rule leniently. Our mishnah is part of that second list, and it brings us close to its end: five more categories of safek in which the ruling is tahor, or permitted, or exempt.
Safek Chulin
The first case is safek chulin, a doubt involving ordinary, unconsecrated food eaten in a state of purity. To understand it, the Bartenura sketches out a hierarchy of three levels among Jews with respect to sensitivity to tumah.
At the top stand the ochlei terumah, the kohanim who eat terumah. Terumah demands a high standard of taharah, so they must be extremely careful. At the bottom is the am ha'aretz, the person who simply does not concern himself with questions of tumah and taharah at all. In between are the perushim: Jews who are not kohanim and do not eat terumah, yet who have taken it upon themselves to eat even their chulin, their ordinary food, b'taharah.
Since every group guards only the standard it has taken upon itself, whatever comes from a lower rung is treated as questionable by the rung above it. For a parush, a garment worn by someone who pays no attention to taharah carries midras tumah; for a kohen who eats terumah, a garment worn by a parush carries that very same midras tumah. Our case grows directly out of this: a parush may have stepped on a garment belonging to an am ha'aretz, or one who eats terumah may have stepped on a garment belonging to a parush, and there is no way to establish whether the treading ever occurred. Such a doubt is ruled tahor. The mishnah tags the case "zo taharas perishus": what is at stake here is only the added level of purity that the perushim accepted upon themselves, and a doubt arising inside a self-imposed standard is decided leniently.
Safek Sheratzim
Next is safek sheratzim. The eight sheratzim listed in the Torah, the creeping creatures whose carcasses transmit tumah, are an av hatumah. Picture the familiar scene of loaves set out, with these creatures scurrying among them. We did not see any contact, yet the odds are strong that one of the loaves was touched. Nevertheless, we apply the principle of k'sha'as metzi'asan: the status is fixed by the situation as we actually find it, not by what we calculate probably happened. Since we did not observe contact, and we judge only by where the creature is found at this moment, the loaves remain tahor.
Safek Nega'im
The third case is safek nega'im, where we cannot tell whether a person afflicted with tzara'as came into contact with a particular object. The mishnah supplies a rule in two stages, and everything turns on the kohen's ruling: "batchilah tahor", at the outset he is tahor, "ad shelo nizkak letumah", so long as he has not yet been bound over to tumah, and then "mishenizkak letumah sfeiko tamei", once he has been bound over to tumah his doubt is tamei. Practically speaking: before a kohen has pronounced him tamei he holds no formal status of tumah whatsoever, even when the blemish on his skin looks for all purposes destined to be ruled a nega and he is simply waiting to be examined; a doubt at that point comes out tahor. After the kohen's pronouncement the ruling runs the other way: even if the affliction seems to be receding, until a kohen actually comes and declares him tahor, a doubt regarding him is tamei.
Safek Nezirus
Now safek nezirus. How does a person land in a doubt about whether he is a nazir? It takes some doing. The Bartenura's example: a man declares that he will be a nazir if a certain silo contains a hundred measures of grain. Now he must go and establish the facts, and when he arrives he finds that the silo has been raided and emptied out, so there is no way to determine how much was inside at the moment he spoke. That is a safek nezirus, and the ruling is mutar: he is not bound by the restrictions of a nazir.
Safek Bechoros
The last of the five is safek bechoros: we cannot be sure that the one before us is genuinely a firstborn, a peter rechem, the first to break out of its mother's womb. The mishnah takes care to include both kinds, "echad bechorei adam", human firstborns, "ve'echad bechorei behemah", and animal firstborns as well. It may be that the mother, woman or beast, had already given birth before this, and there is no way to verify it. The mishnah goes on, "bein teme'ah bein tehorah", applying the identical ruling to an animal of either category.
The mishnah does not spell out the practical consequence, but it is understood: in all these cases the doubtful firstborn is not given the status of a bechor, and nothing is owed. What is at stake is a monetary claim by the kohen. With a human firstborn, the kohen comes to collect five sela'im from the family, as is still our practice at a pidyon haben today. With an animal, the kohen receives the peter rechem itself. In every one of these doubtful cases the answer to the kohen is "hamotzi meichaveiro alav hare'ayah": one who wishes to extract something from another bears the burden of proof. Until the kohen can establish that this is genuinely a firstborn and not a doubtful one, he has no means of collecting.
Five sfeikos, five lenient rulings, each resting on its own principle: the voluntary nature of the perushim's stringency, judging by what we find rather than what we surmise, the decisive word of the kohen in nega'im, the unverifiable condition of a vow, and the burden of proof in a monetary claim.