Keilim, Chapter 23, Mishnah 3. Our Mishnah takes up a distinction that belongs to the laws of a zav and the items he rests upon. When a zav sits or leans his weight on a utensil that was made for sitting or for riding, that item does not merely become tamei in the ordinary way; it becomes a source of tumah in its own right, and it can pass tumah on to a person and even to the garments that person is wearing. The Mishnah now asks how the two categories differ from one another, and then turns to a case of a riding accessory that does not enter these laws at all.
The Mishnah opens with a question: "Mah bein mirkav l'moshav?" What is the difference between an object that receives tumah as a mirkav, meaning something designed to be ridden upon, and an object that receives tumah as a moshav, meaning something designed to be sat upon? Both have been rendered temei'im by the zav through tum'at midras, the tumah that comes from the pressure of his body. So in what way are the two not identical?
The reply comes: "Mirkav chalak maga'o umasa'o." Where a mirkav is concerned, the halachah separates two forms of contact. One who merely lays a hand on the mirkav that the zav pressed upon contracts tumah in his own body, yet whatever he happens to be wearing remains tahor. Should he instead lift the object and bear its weight, both he and every garment on him are drawn into the tumah together.
Concerning the second category the Mishnah teaches: "U'moshav lo chalak maga'o mimasa'o." With a moshav, an item fashioned for sitting, no such line is drawn. Whether a person touched it or carried it, the outcome is the same in both cases: the person becomes tamei, and the garments he is wearing become temei'im as well.
Now that the two categories have been set side by side, the Mishnah brings an article of riding gear that belongs to neither of them and never acquires tum'at midras: "Tapich shel chamor", the pack frame strapped onto a donkey, "shehu yoshev aleha", on which a man does at times seat himself, is nonetheless "tehorah". Even when a zav leans his full weight upon it, no tum'at midras attaches to it, because the craftsman who built it intended it neither as a seat nor as a saddle.
Picture its form: it resembles a ladder, and it is laid flat across the animal's back so that heavy loads can be placed upon it. Its whole purpose is to bear the burdens that the donkey will transport. For that reason the rungs are not set close to one another; they are spaced widely apart, and a person cannot genuinely settle himself on such a surface. Since it was never made to be rested upon, it does not take on tum'at midras.
The final clause deals with an owner who remodels the frame. "Shinah bah et hanekavim": he repositions the sockets, boring fresh holes so that every pole is pulled in tightly against its neighbor and a man could genuinely seat himself on top. "O shepirtzan", or else he breaks the openings apart, "zeh letoch zeh", one hole into the next, leaving the poles standing shoulder to shoulder with nothing but a sliver of wood between the sockets that grip them, so that the sides of the frame are gone and the poles are gathered together in one place. In either event the halachah is "temei'ah": the frame does now receive tum'at midras. His handiwork has produced an object suitable for sitting, and whatever is fit to be sat upon falls under the laws of tum'at midras.