Taharos, Chapter 4, Mishnah 8. Our chapter is occupied with cases of safeik, doubt, in matters of tumah and taharah. The chachamim drew up lists: one series of doubtful cases that they ruled tamei, and another series of doubtful cases that they ruled tahor. Our mishnah belongs to the second group, the doubts that are pronounced pure, and it presents one specific case out of that list.
The case is "safeik tumah tzafah al pnei hamayim", a doubt involving a source of tumah floating on the surface of water. Picture yourself looking into a puddle or a container of water and noticing an insect floating there, one of the creeping creatures that convey tumah. You are not certain whether it was there at all, or whether it made contact with you or with the object in question. That is the doubt our mishnah addresses.
The Tanna Kamma rules that this doubt is tahor, and he adds: "bein bekeilim bein bekarka", whether the water is inside a kli, such as a jar, or whether it is water gathered in the ground, a puddle. In either setting the doubt is resolved leniently.
"Rebbi Shimon omer: bekeilim tamei, uvakarka tahor." Rebbi Shimon narrows the ruling. He is not prepared to be lenient when the water sits in a kli, because a kli is a confined, fixed space, and within such tight quarters we cannot assume that the floating tumah stayed clear of what is inside. In water on the ground, however, there is room in every direction for the tumah to have drifted away, and there the doubt is judged tahor.
Rebbi Yehuda proposes a different criterion altogether. His words are "safeik yeridato tamei", and then "usafeik aliyato tahor": everything hangs on whether the man was going into the water or emerging from it. Picture someone immersing in a mikveh. Should the doubt have arisen while he was still going in, he is tamei, since a body entering water drags the water and whatever rides upon it inward toward itself, and we therefore reckon that the floating tumah was pulled along and made contact with him. Should the doubt have arisen while he was climbing out, he is tahor, since a body leaving the water pushes the water outward, away from itself, and anything afloat is carried off in that same direction rather than brought closer. With the current working against contact, such a doubt is settled in favor of purity.
Rebbi Yose goes further than all of them. Even a quantity of water so tight that it contains, in the mishnah's phrase, no more than "melo adam vetumah", exactly the space taken up by the man himself together with the source of tumah, with not a drop to spare, he still declares tahor. The word he uses is "afilu", even: even under those conditions, provided we are working inside the terms that Rebbi Yehuda laid down. To Rebbi Yose's mind, cramped quarters alone are no reason to conclude that contact must have occurred.
Such is the halachah of tumah that floats: a doubt arising in water, which the Tanna Kamma treats leniently wherever the water is found, and which Rebbi Shimon, Rebbi Yehuda and Rebbi Yose each refine along his own lines.