Parah, Chapter 8, Mishnah 2. The subject of this chapter is the protection of the mei chatas, the water mixed with the ashes of the parah adumah, and the unusual vigilance demanded of anyone who works with it. Our Mishnah records a restriction placed upon the mekadesh, the man who performs the kiddush by putting the ashes into the water. Behind the ruling stands a sequence of tumah that must be traced link by link: liquid is capable of contracting tumah; liquid that has contracted tumah can pass it on to a shoe or an item of clothing; and a shoe or garment in that state disqualifies the person wearing it from occupying himself with the chatas water.
Why no sandal
The Mishnah opens: "Hamkadesh mei chatas", "lo yin'al es hasandal." While a man is engaged in sanctifying the chatas water, he is not to have a sandal on his foot. The Mishnah supplies the reason at once: "she'im naflu mashkin", "al hasandal, nitma vetimahu." Were liquid to drip onto the sandal, the sandal would take on tumah, and the sandal in that condition would transmit tumah to its wearer, leaving him unfit for the task in his hands.
The strangeness of such a sequence is captured by the Mishnah in the form of words the man himself pronounces: "Harei hu omeir", "metamecha lo tim'uni", "ve'atah timtani." And whom is he addressing? The sandal he is wearing. His complaint runs like this: the very liquid responsible for your tumah could have landed on my body and left me entirely unaffected, and yet through you I am rendered tamei. A drop striking his skin does nothing to him; the same drop striking his footwear starts a chain that ends up reaching him.
Skin versus clothing
This distinction is spelled out by the Mishnah itself: "naflu mashkin al besaro, tahor." Liquid that lands on a man's body leaves him tahor, since flesh does not receive tumah in this fashion.
Clothing is not like that: "Naflu al kesuso", "nitmeis vetimasa tim'asu." Should that identical liquid land on what he is wearing, the garment contracts tumah and proceeds to pass it along to him. And so, once again, "Harei zeh omeir", "metamecha lo tim'uni", "ve'atah timtani." He addresses the garment in precisely the same terms: whatever gave you your tumah would have done nothing to me, and it is you who ends up making me tamei.
The question raised by the Ravad
So the Mishnah is read by the majority of the commentators. The Ravad, though, was bothered by its concluding clauses. Granted that a man stays tahor when liquid reaches his skin, while liquid reaching his clothing makes the clothing tamei and the clothing then makes him tamei: in that case, what is special about footwear? The Mishnah should have told him to work unclothed altogether, so that no liquid could settle on a garment, contract tumah there, and travel onward to him. Moreover, if the fear is tamei liquid arriving at him by way of his shoe, we ought to be no less afraid of the various other kinds of tumah that a sandal is capable of picking up and conveying to the one wearing it. And a further difficulty: why is the ruling attached to the man performing the sanctification in particular, and not to someone handling the mei chatas or someone sprinkling it?
The Ravad's resolution is that liquid in general was never the issue here. What the Mishnah has in mind is one specific liquid: the chatas water in front of him. In the course of the kiddush, a bit of that water is liable to fall onto his sandal, and a single such drop brings two losses with it. First, the water that spilled is thereby invalidated, because an accident of that sort shows that his mind was not properly fixed on his labor. Second, his sandal has acquired tumah, and a sandal in that state has consequences for the man on whose foot it sits. Here the principle is applied that someone carrying an object which bears midras tumah is still permitted to occupy himself with terumah, but not with kodesh, and all the more so not with the water of the chatas, whose requirements of taharah stand higher yet.
Garments follow the same logic. Once liquid has brought tumah upon a man's clothing, he slips down to the ordinary standing of those who handle sanctified food, notwithstanding that he had gone to the mikveh with the chatas specifically in mind, and he no longer occupies the higher rung that work with the chatas water demands of him.
Why, then, is he not instructed to strip off his clothing too? Because once we have determined that the only liquid under discussion is the sanctification water dripping in the middle of the kiddush, no prohibition on clothing is called for. A man is able to hold his garments clear of the water and see to it that nothing splashes onto them. Footwear presents the opposite situation: it sits down by the ground, exactly where falling drops land, and in that spot his carefulness cannot be counted on. Thus far the explanation of the Ravad.