Taharos, Chapter 4, Mishnah 3. This chapter works through the many forms of safek tumah, the cases where we cannot establish whether contact with tumah actually occurred. Our mishnah continues a theme running through these mishnayos: tumah that is in motion. So long as the source of tumah is being carried about and never comes to rest, it has no fixed place, and a doubt concerning it is resolved leniently. The second half of the mishnah shows us where that leniency ends.
The mishnah begins: "hasheretz befi hachuldah, uneveilah befi hakelev, ve'avru bein hatehorim" - a weasel is gripping a sheretz between its teeth, and a dog is gripping neveilah, the carcass of an animal that died without proper shechitah. Both a sheretz carcass and neveilah transmit a severe grade of tumah. These two animals then walk through a group of people who were tahor. The mishnah presents the mirror image as well: "o she'avru tehorim beineihem", the tahor people were the ones who walked through where the animals were standing. Either way, no one can establish whether any contact with the tumah took place.
The ruling is "sfeikan tahor, mipnei she'ein latumah makom" - the doubt is decided in favor of purity, because the tumah has no place. The sheretz and the neveilah are in the mouths of animals that are moving, so the tumah never settles anywhere. Note how far this reaches: the parties involved here are people, and people certainly possess da'as lehisha'el, the awareness to be asked what happened. In the ordinary rules of safek tumah, that is exactly what would tip the case toward tumah; the leniency of a doubt is normally reserved for cases where there is no one who can be questioned. Here, though, it makes no difference. They could indeed be asked, yet the tumah still has no resting place, and a tumah that never rests does not generate a doubt strong enough to declare anyone tamei.
Now the mishnah changes one detail: "hayu menakrin bahen al ha'aretz" - the animals were picking and pecking at the sheretz or the neveilah on the ground. Picture the dog or the weasel having dropped its find and now poking at it, dragging it along the floor as it worries at it. And a person says: "halachti lamakom halaz, ve'eini yodea im nagati im lo nagati" - "I walked to that spot, and I do not know whether I touched it or not."
Every element of a classic safek is present. We have a human being who yesh bo da'as lehisha'el, someone with the awareness to be questioned, and he is genuinely unsure whether contact was made. The new variable is that the tumah is no longer riding along in an animal's mouth; it is on the ground, being dragged and picked at.
Here every ingredient of a standard doubt is in place. The one raising the question is a human being, and a person is by definition yesh bo da'as lehisha'el, capable of being questioned about what occurred, and his uncertainty is genuine. What has shifted is the position of the tumah itself: it is no longer traveling along in an animal's jaws but lying below, with the animal worrying at it there on the ground.
The two halves of the mishnah thus set the boundary precisely: tumah carried aloft in a moving mouth has no place and leaves doubts tahor, while tumah brought down to the ground acquires a place, and there a doubt is enough to render a person tamei.