Parah, Chapter 11, Mishnah 2. Our chapter deals with the care demanded by mei chatas, the water mixed with the ashes of the parah adumah that is sprinkled to bring a person from tumah to taharah. This Mishnah compares the rules of doubtful purity as they apply to chatas with the rules that apply to terumah, and then adds a halachah about lattice work.
Doubt in chatas follows doubt in terumah. "Kol hasafeik tahor literumah, tahor lechatas." Any case of doubt that the halachah resolves in favor of purity when it comes to terumah is likewise treated as pure when it comes to chatas. Chatas is not made stricter than terumah in these safeik cases.
The middle category does not exist for chatas. "Kol hatalui literumah, nishpach lechatas." Anything that would be left in suspension for terumah is simply poured out when it is chatas. To appreciate this, consider the three possible outcomes for terumah in a case of doubt. First, the doubt may be resolved in favor of purity, and the terumah is used. Second, the doubt may remain unresolved: since it is terumah we are not permitted to destroy it, and since we cannot establish that it is tahor we are not permitted to use it, so it is set aside in suspension, neither eaten nor burned. Third, the terumah may be definitely impure, in which case it is destroyed and not eaten.
Chatas has no such middle category. If the doubt is resolved in favor of purity, the water is used. If it is definitely impure, it is destroyed. And if we simply cannot determine whether it is tahor or not, we may not use it, but here, unlike terumah, there is no prohibition against getting rid of it. So instead of being placed in suspension, it is poured out.
The Mishnah adds: "Im asu al gabav teharos, teluyos": if pure items were handled over such water while it stood in this uncertain state, those items are themselves left in suspension.
Gratings. The Mishnah now moves to a new case: "Harefafos tehoros lekodesh uleterumah ulechatas." A refafah is a grating, planks fastened together into one framework. Such an object holds its taharah across the board, whether we are dealing with kodesh, with terumah, or with the chatas water.
"Rabbi Eliezer omer: harados temei'os lechatas." Rabbi Eliezer rules that planks merely resting alongside one another are susceptible to tumah where chatas is concerned. The Rambam explains the difference this way: when the boards are tightly bound into a single grating, the structure withstands tumah, and its taharah is relied upon even for kodesh, even for terumah, even for the sprinkling water. Boards that are simply set down loosely, however, can be moved from place to place and used as a seat, so that a zav or another impure person sitting on them conveys tumah to them. A fastened grating has the status of midras, a permanent installation whose very character keeps it beyond the reach of this tumah, whereas loose planks do not.
The Gra explains the case differently, turning on whether the boards can be lifted out and carried around. Nevertheless, our ruling follows the Rambam.
The Mishnah has given us both a principle and an application: in matters of doubt, chatas is not treated more severely than terumah, though it lacks terumah's category of suspension, and in the case of boards, only a firm, fixed structure carries the presumption of purity.