Mishnayos Taharos Perek 3 Mishnah 6

בחזקת טהרה – for even though that regarding doubtful ritual impurity in the private domain is considered as doubtfully impure and this alleyway is the private domain, nevertheless since the doubt is in a matter where it the temperament/disposition [of the participants] is lacking to be able to be asked, as for example, a deaf-mute, imbecile and/or a minor child, he is considered pure in his status of doubt, for this we derive from Scriptural verses, as it is written (Leviticus 7:19): “Flesh that touches anything impure shall not be eaten.” This implies that something that is definitely impure shall not be eaten; if it is doubtfully impure it may be eaten, as it is written (Leviticus 7:19): “Only he who is pure may eat such flesh.” Someone who is definitely pure may eat it, but if he is doubtfully impure/doubtfully pure, he shall not eat it. But is it not that we learn from it here that when he has awareness/understanding to be interrogated, [as opposed to] when he doesn’t have awareness/understanding to be interrogated.

וכל הפקח – that he has awareness/understanding to be interrogated. If he does not know if he walked in a place of defilement or not, he is under the presumption of defilement. For someone of doubtful defilement in the private domain, his [presumption of] doubt is impure which we derive from [the case of] the Sotah/woman suspected by her husband of being unfaithful, and there is a doubt if she is defiled or not and this doubt is in a place of a married woman’s hiding with a man under suspicious circumstances, that is, in the private domain, and the All-Merciful stated (Numbers 5:13): “and she keeps secret the fat that she has defiled herself without being forced,” but a woman suspected by her husband of being unfaithful has awareness/understanding to be interrogated if she has been defiled or not.