Keilim, Chapter 21, Mishnah 2. The subject here is a plow, and the question is which of its many pieces count as one unit together with the iron blade that cuts into the ground. A plow is not a single carved object. It begins at the animal, with the equipment that harnesses it, and runs back through a series of wooden and metal pieces until it reaches the blade itself. So when the blade is tamei and a person lays his hand on one of the other pieces, has he touched the plow or has he touched something else? The Mishnah sorts the parts into those that are loosely and temporarily joined and those that form one entity with the blade.
The Mishnah begins with the pieces that belong to the animal's end of the plow. "Hanogea ba'ol": one who touches the yoke, the piece laid across the backs of the animals so that they can draw the plow. "Uvakartzov": or the crossbar that passes beneath the animals' necks and holds the yoke in place. "Ba'ayin": or the collar, the padding placed around the animal's neck so that the yoke should not injure it. "Uva'avot": or the long beam that runs from the yoke back to the plow itself. And the Mishnah adds a striking detail: "afilu bish'at melachah", even if he touched one of these while the plowing was actually going on, with everything harnessed and in motion, he is "tahor".
Why so? Because all of these items are attached in a slack and passing way. They get fastened when the plowing is about to start and are unfastened the moment the day's labor ends. Such an arrangement never fuses them into a single article with the blade. And so the plowshare at the rear may carry tumah, yet whoever lays a hand on the yoke, the crossbar, the collar or the beam remains unaffected by it.
Now the Mishnah turns to the pieces that are genuinely part of the plow. "Bacherev": one who touches the handle by which the plowman grips and guides the implement. "Uvaborech": the curved piece of wood fixed to the rear of the plow. "Uvayatzul": the piece that serves to steer it. In all of these, "tamei", the one who touches becomes tamei.
Here the ruling is the reverse, because these components are not tied on for the day's work and removed afterward. They are all seen as one whole with the blade, a single implement made of several pieces. Whoever touches any one of them has touched the plowshare itself, and he becomes tamei along with it.
Three further components of this second category follow. "Ba'ayin shel matechet": the metal loop by which the plow is secured to the long beam. "Balechayayim": lengths of wood positioned forward, in front of the animals, that dip down and start crumbling the soil so that the iron blade behind them meets ground already softened. "Uva'ara'in": timbers that shatter whatever lumps of earth remain standing once the plow has gone by. Each of these is ruled "tamei". Since they are affixed to the plow on a permanent basis, they count as connected to it regarding tumah, and should the plowshare be tamei, one who handles them contracts tumah too.
The Mishnah closes with a dissenting opinion. "Rabbi Yehudah metaher balechayayim." Rabbi Yehudah holds that one who touches the lechayayim, those forward pieces that soften the soil, remains tahor. His reason: "she'einan asuyin ela lerabot et he'afar", they were made for nothing more than to increase the amount of loose earth that the plow will turn over. They are an aid to the work, not a limb of the implement. Since they are not needed on every occasion, they are not kept permanently fixed in place, and a piece that comes and goes in that way is not considered connected to the plowshare for the purposes of tumah.
The Mishnah thus draws a clear line down the length of the plow. Whatever is bound on for the duration of the work and untied when the work ends stays separate, even in the middle of plowing. Whatever is fastened permanently is one implement with the blade, and touching it is touching the plow.