Toharot, Chapter Three, Mishnah Six
1) If a deaf-mute, a person not of sound senses, or a minor was found in an alley way that contained something that was unclean, he is presumed to be clean.
2) But any one of sound senses is presumed to be unclean.
3) And anyone/anything that lacks understanding to be inquired of is in a case of doubtful uncleanness presumed to be clean.
Section one: An alleyway is considered to be a private domain. In this alleyway there is something that is impure, but we are not sure if the person came into contact with it. The rule in section three will state that if someone or something cannot be asked if they came into contact with something impure, they are ruled to be pure. A deaf-mute, and a person not of sound senses and a minor are all considered to lack what we might call awareness or intelligence. One cannot legally ask them if they came into contact with the impure thing. Therefore, they are pure.
Section two: The person of sound senses can be asked, but in this case he doesn’t know if he had contact with the impure thing. Therefore, he is impure.
Section three: This is the general rule that explains section one.