Ohalos, Chapter 18, Mishnah 6. This chapter deals with the halachos of a Beis HaPeras, a field in which a kever was plowed over so that fragments of a meis may now be scattered through its soil. Because of that concern, the Chachamim declared such a field tamei, and the mishnayos of the chapter map out exactly when a person who passes through it becomes tamei and when he remains tahor. Our mishnah teaches two separate halachos: first, ways of crossing a Beis HaPeras without contact with its earth, and then a parallel din regarding one who travels in eretz ha'amim, the lands outside Eretz Yisrael.
Walking on stones that do not budge
The mishnah begins with the words "Hamahalech beBeis HaPeras al avanim she'eina yachol lehassisan": someone who makes his way across such a field by treading upon rocks that he has no ability to dislodge. They are wedged in so tightly that his footsteps leave them exactly where they were. He remains tahor.
Here a question presents itself. The previous mishnah already taught that when a Beis HaPeras is covered over with stones that do not move, it is tahor. What, then, is our mishnah adding?
The meforshim explain that the earlier case was one where the owner took the whole field and paved it entirely, fixing the stones solidly in place. Once that is done, the place is no longer a Beis HaPeras at all. Our mishnah is speaking of something else: he laid only a path of stones through the field. The rest of the Beis HaPeras remains tamei as before, and nevertheless one who walked along that path of firmly set stones is tahor.
A practical question grows out of this. Suppose a man comes before a rav and describes his situation: a Beis HaPeras lies in his path, but a paved walkway of rocks runs across it, and he wants to know whether he may use it. Would the rav grant him permission in advance? Opinions divide here, some allowing it and some refusing, and lechatchilah a person should keep away. If, however, the crossing has already taken place, and he kept himself the whole way on rocks that were immovably lodged in the ground, our mishnah is the source teaching that he stays tahor. All of this applies, of course, only where those rocks genuinely stayed put under his weight.
Riding on a person or an animal
The mishnah continues with another way of crossing: "al ha'adam ve'al habeheimah shekochan yafeh," if he rides across the Beis HaPeras on a person or on an animal whose strength is good. This bearer is not wobbling and staggering under the load; he carries the rider steadily. Since the rider has no connection to the ground at all, and it is the man or the animal standing on the stones and bearing his weight independently, the rider is tahor.
The mishnah then turns to the opposite situations. Should he make his way "al avanim sheyachol lehassisan," upon rocks that he is capable of dislodging, or "al ha'adam ve'al habeheimah shekochan ra," upon a man or a beast of poor strength, one who can barely keep him aloft, he becomes tamei. Why should that be, seeing that he never made contact with the afar of the Beis HaPeras at all? Because a bearer who is straining does not truly support the body of the one he carries; the rider's weight ends up pressing down through him and onto the rocks underneath. And once those rocks are being pressed and jostled, an etzem kaso'orah, a bone the size of a barley grain, may be lying directly beneath them.
How do we measure strength?
One naturally looks for a shiur here: where exactly is the line between a bearer strong enough to carry and one who is straining? The Bartenura takes this up in his closing comment on the mishnah, asking how we identify a man of poor strength. The answer is read off his legs. If the carrier's legs shake while the rider is upon him, his knees knocking one against the other beneath the burden, that is kocho ra. Legs that hold steady show kocho yafeh.
For an animal the sign is different. A beheimah that is finding its load too heavy will be matil gelalim, it will drop its waste from the strain. That is the indication that the animal is struggling under the weight it carries.
Traveling in eretz ha'amim
A second halachah now opens: "Hamahalech be'eretz ha'amim," one who travels in the territory of the nations, meaning chutz la'aretz. Here too the Chachamim imposed tumah. The reason, which the coming mishnayos develop at greater length, is that among the ovdei kochavim there was no practice of marking off a burial place, so a traveler had no way of knowing where a grave lay. The worry was real, and it was everywhere.
The decree stretched far enough that the mishnah rules tamei even "beharim uvasela'im," where his route took him only over mountains and rocky ground, terrain in which burial was not the custom. What is the concern in such a place? The Bartenura explains that the dirt of those lands travels: it is blown and rolled up into these spots from ground where people genuinely were buried.
"Bayam uvashehunis," however, is tahor. Bayam means he traveled by sea outside Eretz Yisrael; shehunis means he walked along the seashore of a place outside Eretz Yisrael. And how far inland still counts as the shore? The mishnah defines it: "kol makom shehayam oleh beza'afo," every place that the sea reaches when it rises in its rage. Is the sea truly angry? It looks angry in a storm, when the waves surge high and far inland. As far as those storm waters reach, that is the shehunis, and there the din is tahor, since whatever soil arrives there is washed away by the water.
A yellow light on the word tahor
The Bartenura warns us not to read this "tahor" as a complete taharah, not even for the man out at sea or standing on the shore. As is set out in Maseches Shabbos, the decree upon eretz ha'amim was enacted in two stages. Chazal first imposed tumah on the gush, the body of the earth itself: whoever treads upon the soil of those lands, or touches it, or carries it, becomes tamei. Afterward they broadened the decree to take in the avir as well, the air of eretz ha'amim, so that simply entering that airspace brings tumah.
The two decrees were not issued with equal severity. For the gush, Chazal imposed a far heavier tumah, treating the man as though he had come into contact with a meis: his tumah lasts seven days, and his taharah requires mei chatas, the purifying water containing Parah Adumah ashes, to be sprinkled upon him on the third day and again on the seventh. Terumah or kodshim rendered tamei through the gush are burned.
Where there was no contact with the gush at all, and the man was only within the airspace, out upon the water, for instance, or standing on the strip of shoreline that the sea rinses free of any soil, they were not nearly so strict. He is tamei, but for that day alone, and he needs no sprinkling of mei chatas. As for terumah or kodshim that pick up this tumah from the air of eretz ha'amim, they may not be eaten, and yet they are not burned either; their standing is that of safeik tamei.
So when our mishnah says "bayam uvashehunis tahor," it does not mean fully tahor. It means he is clear of the severe seven day tumah of the gush, but he still carries the tumah of the avir of eretz ha'amim, and he is tamei for the day.
Summing up the mishnah
We have learned that a person crossing a Beis HaPeras who steps only upon rocks lodged solidly in the ground, rocks that will not budge, remains tahor, and this holds even where nothing more than a walkway was laid down and the remainder of the field keeps its tumah; the same is true of one carried across upon a man or upon an animal of good strength. As for being carried by a person, it is worth noting that several shitos restrict this to an oved kochavim, who does not become tamei from a Beis HaPeras, or else to someone bundled in many layers of begadim.
But if he walked on stones that can be shifted, he is tamei even if he protests that he was careful and no stone actually moved under him; the fact that these stones are movable is enough. So too if he rode on a person of poor strength, whose knees knock together beneath him, or on an animal of poor strength, whose sign is that it drops its waste from the strain of the load, he is tamei, since it is treated as though he walked there on his own feet.
And we have learned that a traveler in eretz ha'amim contracts a seven day tumah even upon mountains and crags, where nobody buries the dead, since the wind may have deposited grave soil in such spots. Out on the ocean, however, or on the band of shoreline that the storm waves reach, where the sea rinses off whatever soil arrives, he is tahor, and the Bartenura sharpened the point for us: tahor from the tumah of touching the earth of those lands, so that no seven day tumah falls upon him, while he still carries the tumah of that day, which comes from the separate decree concerning the air of that region.