Parah, Chapter 10, Mishnah 5. This chapter is occupied with the extra stringencies that govern anything pure for chatas, meaning the ashes of the parah adumah and the water sanctified with them. Whatever is not itself pure for chatas conveys impurity to a person or a vessel that is, and even indirect contact can be enough. Our mishnah records a dispute about a case where nothing touched anything at all: a vessel was simply carried through the air over a place that is impure for chatas.
The Case
"Haya omed chutz la-tanur", he was standing on the outer side of the oven, "u-pashat yado la-chalon", and he reached his arm toward the window, "v-natal es ha-lagin", and lifted up the lagin from there, "v-he'eviro al gabei ha-tanur", carrying it across the top of the oven. "Rebbi Akiva metamei", Rebbi Akiva rules that it has become tamei, "v-Chachamim metaharin", while the Chachamim rule that it remains tahor.
Envision the scene. A man who has been guarded in taharah for chatas is positioned on the far side of an oven, with the oven standing between him and a window. He extends his arm across it, takes hold of a lagin, a flagon of chatas water resting on the sill, and draws it back toward himself. In the course of that motion the flagon travels through the air immediately above the oven.
The Dispute
Rebbi Akiva holds that the chatas water has contracted impurity. He is applying a principle of his that appears elsewhere in this tractate: an object in motion through the air is judged as though it had already come down and settled beneath. Since the flagon's path took it above an oven, a location that lacks purity for chatas, we view it as having rested there, and both the flagon and its water are therefore tamei.
The Chachamim disagree. They do not accept that travelling through the air is equivalent to resting, and so nothing here has touched anything impure. The flagon stays pure, and the halacha follows the Chachamim.
What May Be Held Over the Oven
The mishnah continues: "Aval omed hu", but he is permitted to stand, "ha-tahor l'chatas", this man who has been kept in taharah for chatas, "al gabei tanur", directly atop the oven, "u-v'yado kli reikan tahor l'chatas", while grasping an unfilled utensil that has been guarded in taharah for chatas, "u-mayim she-einan mekudashin", or water into which the ashes have not yet been placed.
The stringency of the opening case is limited to the ashes of the cow themselves and to water already sanctified with them; those alone must be shielded from a spot that lacks purity for chatas. A utensil that was designated and guarded in taharah for chatas but happens to be empty, and likewise water drawn but not yet mixed with the ashes, fall under no such restriction. Hence the ruling here: the man carrying them may position himself squarely upon the oven, and they may even be set down in a place where taharah does not obtain.