Taharos, Chapter 7, Mishnah 8. This entire chapter is occupied with a single practical problem: how does a person who is careful about tumah and taharah conduct himself among people who are not careful at all? How do we relate to the am ha'aretz, and what do we assume happened when he was left alone near our food or near our body? Our mishnah takes that question in a slightly different direction and asks about the careful person himself: what happens to his taharah once he stops thinking about it?
The mishnah opens: "Mi shehayah tahor v'hesiach libo mile'echol," someone who was tahor, meaning a kohen who had been guarding his taharah in order to eat terumah, and then turned his attention away from eating. He decided that he was finished eating; the matter is off his mind. The question is whether his status of taharah survives that decision, or whether he now falls into the category of a person we can no longer rely upon.
Rebbi Yehudah's opinion comes first: "Rebbi Yehudah metaher," he holds that this man retains his taharah. His reasoning is stated as "shederech temei'im porshin mimenu," the normal pattern being that someone carrying tumah withdraws from his presence. Even an am ha'aretz is aware that a kohen lives by a stricter code, and he pulls back on his own initiative, either out of deference or out of a sense that such standards are beyond him. So the kohen stays protected without any effort on his part, since the people surrounding him are effectively doing the guarding.
Notice where Rebbi Yehudah places the responsibility. It does not rest on the kohen's own vigilance at all; it rests on the behavior of the others, the temei'im who stay away. That is a striking way to frame the halachah, and it becomes significant in a moment.
"Vachachamim metam'in," the Chachamim rule that he is tamei. In their view, once the kohen has no intention of eating, he simply is not paying attention any longer. A person who has closed the file on eating terumah is not monitoring what brushes against him and what he leans on, and a taharah that nobody is watching is not a taharah we can rely upon. The Chachamim, in other words, put the responsibility squarely on the kohen himself.
Now the mishnah applies the dispute to a narrower case, that of hands alone. "Hayu yadav tehoros v'hesiach libo mile'echol": his hands had the status of taharah, and then eating went out of his mind entirely. Suppose he then insists on their condition, "Af al pi she'amar," even though he says of himself, in the words of the mishnah, "yode'a ani shelo nitme'u yaday," that he is certain no tumah reached them. The ruling is nonetheless "yadav teme'os," his hands are treated as tamei. And on this point Rebbi Yehudah does not argue.
The reason is given in three words: "shehayadayim askaniyos hen," hands are busy. This is a fundamental principle in halachah. Hands move on their own; they reach, rub, scratch and handle without the owner's conscious involvement. His confident testimony about them is worth nothing, because the very nature of hands is that their owner does not track them. Rebbi Yehudah's logic cannot help here either: other people may keep their distance from a kohen, but a kohen's own hands are not going to keep their distance from him.
This principle stands behind a halachah we live with constantly. When we wash our hands before eating bread, we are not responding to a known contact with something tamei. We treat hands as presumptively unclean precisely because they are askaniyos: when a person is talking, thinking or otherwise occupied, his hands will inevitably have touched things he never registered. Our mishnah is the source of that assumption, and it teaches that a declaration of certainty about one's hands carries no halachic weight against it.