Taharos, Chapter 3, Mishnah 6. The chapter has moved into the subject of doubt, safek, in the laws of tumah and taharah. Here the Mishnah narrows the subject further and turns to doubts that revolve around people: who was where, who touched what, and what we assume when nobody can tell us. This mishnah, and the ones that follow it, are built around that question.
The Mishnah teaches: "Cheresh, shoteh, ve'katan shenimtze'u bemavoi sheyesh bo tumah." A deaf-mute, a person who is not of sound mind, or a minor who is found in an alleyway in which there is tumah, "harei eilu bechezkas taharah," these are presumed to be tahor. "Vechol pikeiach bechezkas tumah," but any person of full understanding found in that very same alleyway is presumed to be tamei. And the Mishnah closes with a rule that governs far more than this one case: "Vechol she'ein bo da'as lehisha'el, sefeiko tahor." Anyone who does not have the awareness to be questioned about his own status, his doubt is resolved toward taharah.
Two principles at work
Let us begin with the second half of the Mishnah, the case of the pikeiach, the person of sound mind, and only afterward return to the cheresh, shoteh and katan.
A mavoi, an alleyway, is a reshus hayachid, a private domain. That detail is the entire engine of the ruling. There is a foundational rule in the laws of tumah: a safek tumah that arises in a reshus hayachid is resolved stringently, and we treat the person or object as tamei; a safek tumah that arises in a reshus harabim, a public domain, is resolved leniently, and we treat it as tahor. So the pikeiach who is discovered standing in an alleyway where tumah is present is in a private domain with a genuine doubt hanging over him, and we assume he became tamei.
But what would lead the Torah to draw a line between a doubt in a private setting and a doubt in the street? The derivation comes from the sotah. Strip that case to its essentials and it is precisely a safek tumah occurring in a reshus hayachid: nobody knows what happened behind closed doors, whether the woman actually was with that man or not, and the whole suspicion grew out of a private seclusion. Yet the Torah refuses to wave the uncertainty away, and instead handles it with all the weight of something known for certain. That is the basis for the broad principle that uncertainty in a private place is judged strictly. (Some of the commentators follow a different route, anchoring these halachos in the verses of Vayikra chapter 7, which discuss korban meat whose permissibility to be eaten is in question. On either approach, the halachah that emerges is identical.)
Why the cheresh, shoteh and katan are different
Now we can understand the first clause of the Mishnah. What is it about a deaf-mute, one who is not of sound mind, and a minor that removes them from the ordinary stringency of a reshus hayachid?
Look closely at the model from which the whole law was derived. The sotah is an adult who can be questioned, who understands the choice before her, and who bears moral responsibility for what she did. Every element of the derivation assumes a person of that kind. The three individuals listed in our Mishnah are not of that kind. They cannot be meaningfully asked whether they became tamei, and they could not give a coherent answer if they were asked. The comparison to the sotah simply does not reach them, and so the stringency built on that comparison does not either.
That is the meaning of the rule with which the Mishnah concludes: "Kol she'ein bo da'as lehisha'el, sefeiko tahor." Where there is no one who can be questioned about the doubt, the doubt is settled toward taharah. This is a principle of wide application, and it will govern the cases that follow as well. A doubt is treated harshly only when it belongs to someone who could have told us the answer.