Parah, Chapter 10, Mishnah 1. The final chapters of our tractate are devoted to the extraordinary care demanded of anyone occupied with the chatas water, the water mixed with the ashes of the parah adumah. Beyond the tumah that the Torah itself defines, the Sages erected additional fences around this water, and our mishnah introduces one of them: a light form of tumah called madaf.
The opening ruling is framed very broadly. "Kol haraui litamei midras": any article suited by its nature to receive midras tumah. Such an article, says the mishnah, has "madaf l'chatas", it bears the light tumah of madaf as far as the chatas water is concerned. And this applies "bein tamei bein tahor", that is, without regard to the article's present condition, whether it happens at this moment to be tamei or entirely tahor. The mishnah then adds a final clause, "v'adam kayotzei vo": a human being is included in precisely the same ruling.
The second half of the mishnah turns to a different category and records three views. "Kol haraui litamei tumas meis": whatever is capable of receiving the tumah of a corpse, and here too the question is asked "bein tamei bein tahor", in either condition. "Rabbi Eliezer omer, eino madaf": in his view such an article never carries madaf at all. "Rabbi Yehoshua omer, madaf": in his view it always does. The "Chachamim" distinguish between the two conditions: "hatamei madaf", an article that is actually tamei carries madaf, "v'hatahor eino madaf", while one that is tahor does not.
What midras tumah is
As we have learned in several places, furniture and utensils whose purpose is sitting or lying upon become an av hatumah when a zav sits, reclines or stands on them. Such an item transmits tumah to people and to vessels through contact, and it also renders a person tamei when he carries it or moves it.
What madaf is
The application of midras to kodesh and to chatas, however, is not Torah law but a rabbinic safeguard, a fence the Sages built to protect the heightened purity required for kodesh and for the chatas water. This lighter grade of tumah is called madaf, a word drawn from the phrase "kol aleh nidaf" (the sound of a driven leaf). Just as a leaf rustling in the wind is the slightest of sounds, so madaf is the slightest of tumos.
Our mishnah, then, is a list: which items bear this light madaf tumah with respect to chatas. Whatever is fit to receive midras carries madaf for chatas regardless of its present state, tamei or tahor, and the same holds for a human being.
The dispute of the second clause
The Rambam brings a concrete scene to bring the argument over items susceptible to corpse tumah into focus. Picture someone who is currently tahor for chatas shutting a door, and hanging on that door is a key, which is nudged by the door's motion. In Rabbi Eliezer's view, even a key bearing tumas meis leaves this person's status untouched: he continues to be fit for chatas and may fill the water, mix in the ashes and sprinkle just as he did a moment earlier. Rabbi Yehoshua takes the furthest opposite view: even a key in a state of complete taharah renders whoever shut that door unfit for chatas, since he set the key in motion. The Chachamim rule between them according to the key itself: a key that bore corpse tumah leaves him tamei for chatas, since what he stirred was a tamei object, whereas a key that was tahor leaves his fitness for chatas intact.
The lesson of the mishnah is how far the Sages were prepared to go in protecting the chatas water. An object that is entirely tahor, touched only indirectly and unintentionally, can already stand between a person and his fitness to handle the waters of purification.