Taharos, Chapter 7, Mishnah 2. The whole of this chapter is devoted to the am ha'aretz, or in common usage the amaratzim: those who take no care with the halachos of tumah and taharah. From the perspective of the person who does take care, and it is his world that the Mishnah is picturing, a practical issue arises: what status do we give such a person, and where does he fit into the system of purity and impurity?
Our mishnah puts the case in the most ordinary way imaginable: you have to step out, and an am ha'aretz is left alone inside your house. Keep in mind what that means. This man is himself tamei, and whatever he handles becomes tamei. He is something like King Midas, except that what his touch leaves behind is not gold but tumah. So the whole question becomes: while you were gone, what did he touch?
The Cases That Remain Tahor
"Hamani'ach am ha'aretz betoch beiso": one who leaves an am ha'aretz inside his house. The Mishnah then lists the situations one by one. "Er umetza'o er," you left him awake and you came back and found him awake. "Yashen umetza'o yashen," you left him asleep and found him asleep. "Er umetza'o yashen," you left him awake and came back to find that he had fallen asleep. In all three of these, "habayis tahor," the house remains tahor.
The reasoning follows the man's own awareness. If he slept through the entire time, wonderful: he touched nothing at all. If he was awake from beginning to end, he knew perfectly well that he had been left in someone else's house, and a person in that position keeps his hands to himself. And if he began awake and then drifted off, that is all the more reason for confidence, since he was conscious of his surroundings before sleep overtook him.
The Difficult Case: Asleep and Then Awake
And now the case over which the Tannaim divide. "Yashen umetza'o er": he was left sleeping, and upon your return you found him up and about. Here the Mishnah reports, "habayis tamei, divrei Rebbi Meir." In Rebbi Meir's opinion, the house has become tamei.
What sets this situation apart? We will follow the explanation of the Bartenura, which is the preferable reading here. Everything turns on what the man had a chance to observe. He entered while asleep, or was asleep from the start, and so he never absorbed where he was; it never registered with him that the utensils and the furniture belong to someone else. When his eyes open, the entire room is a fresh discovery, and he starts looking around and fingering whatever he finds, in the manner of a guest with a bit too much curiosity. Nosy Nancy, one might say. He picks things up without the slightest feeling that he is trespassing. That is how Rebbi Meir sizes up the situation, and consequently he rules the house tamei.
The View of the Chachamim
The Mishnah continues, "Vachachamim omrim," and the chachamim say: "ein tamei ela," nothing has contracted tumah, "ad makom shehu yachol," beyond the area he was able to reach, "lifshot es yado veliga," by extending his arm and making contact.
Notice that even the chachamim concede that something has become tamei. They simply refuse to assume that he did anything deliberate. In their view he would not go rummaging around and handling what is not his. The only tumah they are prepared to assume is the tumah of the moment of waking itself: a person who wakes up stretches and yawns and stretches again, and in the course of that stretch his arm may well have reached out and brushed against something. Whatever lies within that arm's reach is treated as tamei, and nothing beyond it.
Underlying the chachamim's ruling is a basic estimate of the am ha'aretz himself. He may be careless about tumah and taharah, but he is a decent and self-conscious human being who does not wander through a stranger's house touching whatever he pleases. We therefore have no grounds to assume that he touched anything, apart from what plain common sense about how people wake up would suggest. Accordingly, only what he might have made contact with accidentally is declared tamei, and that is how we pasken, like the chachamim.