Keilim, Chapter 26, Mishnah 9. In the previous chapter we established a basic principle: once a kli has become tamei, it does not simply shed its tumah on its own. The tumah remains until the object itself undergoes a real physical change. Our mishnah applies that principle to a leather mat that carries tumat midras, the tumah of something sat or lain upon by a person who is tamei. The question is one of measurement, in a sense: how much of a change must the mat undergo before it counts as a different object and the tumah departs?
Rabbi Yehuda: the first cut of the knife
Our mishnah opens with the case: "Or shehu tamei midras", a hide that has contracted midras tumah, "vechishev alav lirtzuot velisandalin", whose owner has now decided to turn it into straps or into sandals for walking. "Keivan shenatan bo et ha'izmel, tahor": from the moment he applied the cutting blade and made a start, the hide is pure. "Divrei Rabbi Yehuda", so rules Rabbi Yehuda. In his view the faintest alteration of the material already suffices to remove the tumah.
Why does the thought alone not accomplish this, and why does the smallest cut accomplish everything? Because a person's plans are not reliable on their own. Today he means to make straps, tomorrow he may spread the mat out again and lie on it. Intention by itself can always be retracted. But once he has actually begun to reshape the leather for a completely new use, we can be confident that the plan will stand. The two elements work together: the intention becomes real only when the blade enters the hide, and once he cuts, he has revealed what he truly means to do, and the kli loses its status of tumah.
The Chachamim: the shiur of five by five tefachim
"VaChachamim omrim: ad sheyema'ateno pachot michamishah tefachim." The Chachamim disagree. In their ruling the hide holds onto its tumah until its dimensions have been brought below five tefachim in each direction, and that is so even after the cutting has begun. So long as a section of the qualifying size is still intact, the tumah rests upon that section.
Their reasoning is drawn from the minimum measure that governs midras tumah in the first place. Flat leather or cloth, having no receptacle, is not susceptible to midras at all unless it covers a certain area, and in the case of leather that area is five tefachim by five. Consequently, undoing the tumah demands that the piece fall short of the very dimension that rendered it susceptible. Purity arrives only once the leather no longer holds that measure.
Rabbi Elazar bar Tzadok: what counts as a real change
"Rabbi Elazar bar Tzadok omer: af ha'oseh mitpachat min ha'or, temei'ah." Rabbi Elazar the son of Rabbi Tzadok teaches that should someone slice a section out of the leather mat and shape it into a wiping cloth, that cloth is still tamei. Cloth and mat belong to a single category of kelim: both lie flat, and neither holds anything inside it. Reworking the one into the other, then, is no genuine transformation at all, only the exchange of one flat function for another, and so the midras tumah of the mat passes straight into the new cloth.
"Umin hakeset, tehorah." But if a person cuts a napkin out of a mattress that bore tumat midras, the napkin is tahor. Here the original object was a mattress, and a mattress is classified as a receptacle, since it holds stuffing inside it. When he converts a receptacle into a flat object that holds nothing, that is a fundamental change. The original kli is regarded as having been broken, and with it the tumah is gone.
On the main issue, then, Rabbi Elazar bar Tzadok stands with the Chachamim and against Rabbi Yehuda: applying the blade to the leather achieves nothing at all. The act of cutting on its own is of no consequence; the object has to be altered in a substantial way before its status of tumah is touched. And he presses the point further still: even where someone has genuinely reworked the hide into a fresh kli whose size falls short of the required five tefachim, that too is insufficient, and the midras tumah stays in place. What the halachah demands is a shift of category, from an item possessing a receptacle, such as a mattress, to an item possessing none, such as a wiping cloth. Only a transformation of that order brings the tumah to an end.
The mishnah, then, gives us three levels of change: the first stroke of the knife, according to Rabbi Yehuda; reduction below the shiur of five by five tefachim, according to the Chachamim; and a shift from a receptacle to a flat object, according to Rabbi Elazar bar Tzadok. In each view, the same principle from the previous chapter is at work: tumah does not fade away with a change of mind, only with a change in the kli itself.