Taharos, Chapter 3, Mishnah 5. With this mishnah the tractate shifts its focus. Over the next several mishnayos we will be dealing with situations of uncertainty and how such doubts are resolved in matters of tumah and taharah. Our mishnah introduces one of the great principles of this subject.
The principle is stated in these words: "kol hatumos kish'as metzi'asan." Every case of tumah is judged according to the moment at which it was discovered. We do not reconstruct the past and we do not extrapolate backwards. We take the situation exactly as it presents itself to us and rule accordingly.
The Mishnah spells this out: "im temei'os temei'os, im tehoros tehoros." If the thing is found in a state of tumah, we treat it as having been tamei; if it is found in a state of taharah, we treat it as having been tahor.
The Bartenura offers a case that brings this to life. A person went over to someone during the night and shook him. At the time he had no knowledge and no reason to suspect that the man was anything other than alive. When morning came, he saw that the man was in fact dead. The question now presses itself: did he handle a living person, in which case nothing happened, or a corpse, in which case he contracted tumas mes? He had no way of knowing at the time. The answer follows our principle: kol hatumos kish'as metzi'asan. Since the man is found dead, and we have nothing pointing in the other direction, we treat him as having already been dead when he was touched.
The Mishnah then applies the same rule to another kind of question: "im mechusos mechusos, im megulos megulos." If the vessels are found covered, we consider them to have been covered; if they are found uncovered, we consider them to have been uncovered.
Here the setting is a house in which there is a corpse. A corpse imparts tumah throughout the entire ohel, the whole space of the dwelling, and everything in that space becomes tamei unless it is sealed off. Suppose there was a jar in the room whose contents we very much want to remain tahor. The corpse is removed, we come back in, and we find the jar closed. May we conclude that it was closed all along, and that its contents were protected? The principle answers yes: since it is covered now, we treat it as having been covered then. And the rule cuts both ways. If we find the jar open, we may not comfort ourselves with the possibility that it had been covered earlier and was opened afterward. We assume it was open the whole time, and the contents are tamei.
A third application closes the mishnah: "machat shenimtze'ah mele'ah chalu-dah o shevurah": a needle that is found completely covered with rust, or found broken. A needle in either of these conditions is no longer usable, and a utensil that cannot serve its function is not a keli at all, and a non-keli is not capable of receiving tumah. Therefore the Mishnah rules "tehorah," it is tahor, "shekol hatumos kish'as metzi'asan": because all cases of tumah are judged by the moment of discovery. We do not speculate that the needle was whole and serviceable when it lay next to the source of tumah and only later rusted through or snapped. We take it as we found it, and as we found it, it was never a vessel that could become tamei.
One principle, three very different situations: a person touched in the dark, a jar in a house of mourning, and a rusted needle. In each of them, halachah refuses to build a ruling on a reconstructed past. What we see before us is what we rule on.