Taharos, Chapter 7, Mishnah 5. The subject of this chapter is the am ha'aretz, and how someone who is careful about tumah and taharah is expected to conduct himself toward a person who is not. Our mishnah presents a perfectly everyday case: a man stations an am ha'aretz inside his home as a guard. Once he does so, what becomes of the taharah of the contents of that house?
When the owner can see who comes and goes
"Hamaniach am ha'aretz b'soch beiso l'shomro." One who leaves an am ha'aretz in his house to guard it. "Bizman shehu ro'eh es hanichnasin v'hayotzin," when the owner is positioned so that he can see who enters and who leaves, the halachah is as follows.
"Ha'ochlin v'hamashkin uchlei cheres hapesuchin tme'in." The food, the drinks, and any open earthenware vessels are tamei. This is unavoidable: the watchman has the run of the house, and there is nothing to stop him from handling any of these things.
"Aval hamishkavos v'hamoshavos uchlei cheres hamukafin tzamid pasil tehorin." But the places designated for lying and for sitting, the beds and couches and chairs, together with earthenware vessels that are sealed shut with a tight fitting cover, remain tahor. The reason is that rendering these items tamei requires a stronger form of tumah than we attribute to an am ha'aretz. As was explained earlier, we treat an am ha'aretz as though he carries midras impurity, but not as though he were a zav. A zav is an av hatumah: everything he sits or lies upon becomes tamei by the very act, and a sealed earthenware vessel that he moves becomes tamei even though it is closed. The am ha'aretz, however, is dealt with only on the level of tumas maga, contact. What his hands can reach is affected; a sealed vessel and a bed or chair are not.
When the owner cannot see
"Im eino ro'eh lo es hanichnasin v'lo es hayotzin," should the owner have no view of the entrances and departures, the entire ruling shifts. The Bartenura brings an alternate version of the wording: "v'lo es hayoshvin," nor those seated, according to which the worry is specifically over individuals who might have sat themselves down on the mishkavos and moshavos.
"Afilu muval, afilu kafus, hakol tamei." Even if the watchman himself is incapable of moving about, and even if he is literally bound in place, everything in the house is tamei. At this point the question no longer revolves around the watchman at all. Once the owner has no view of the traffic in and out, he simply does not know who has been in his house. It may have been a woman who is a niddah, it may have been a zav, it may have been any av hatumah. Having lost track of what entered and what left, he must reckon with the most severe level of tumah for everything inside.
The Rambam offers this explanation: from the moment the owner loses track of what goes on inside and has in practice turned the premises over to his guard, the house is viewed as the guard's own. That guard being an am ha'aretz, its entire contents are treated as tamei.
The mishnah thus draws a sharp line between supervision and abandonment. As long as the owner keeps his eyes on the doorway, only what the am ha'aretz could touch with his hands is affected. The moment the doorway is out of sight, the house has passed out of the owner's hands, and its taharah goes with it.