Chapter 6, Mishnah 4. In the previous mishnah we dealt with the cases of the heads of rows and of a sheaf that a person took hold of in order to carry it and then forgot. Our mishnah, according to the Rav's explanation, comes to expand our understanding of both of these cases.
"V'eilu hen rashei shuros":
The mishnah defines what those "heads of rows" are, concerning which the previous mishnah taught that they are not considered forgotten: "Shenayim shehis'chilu me'emtza hashurah" - two people who begin gathering the sheaves of grain from the middle of the row, one turning north and the other turning south, for in a field there are rows running from north to south and rows running from east to west. "V'shachechu lifneihem ul'achareihem" - they forgot sheaves both in the direction in which they were walking and at the point from which they set out, which is behind both of them.
"Shelifneihem - shichechah": a sheaf that was forgotten in the direction in which each of them was walking, after he passed it as he went along, is considered forgotten.
"V'shela'achareihem - eino shichechah": the sheaf that was left behind both of them, at the place where they began, is not considered forgotten.
The Rav and Tosafos explain the reason: since that sheaf lies between the two of them, each one relies on his fellow to take it. The law of forgotten sheaves applies only when a person forgets on his own account; but when the forgetting stems from his partnership with another, it is not considered forgotten. Therefore the sheaf behind both of them is not forgotten, while those that they passed as they walked are considered forgotten.
An individual who began from the head of the row:
According to the Rav's understanding, the continuation of the mishnah expands the law of "a sheaf that he took hold of in order to carry it" that was taught in the previous mishnah. Now the case is of an individual who begins gathering the sheaves from the head of the row, "v'shachach lefanav ul'acharav" - "behind him" means sheaves that he has already passed, and "before him" means sheaves that he has not yet reached, and he walks on and leaves them there.
"Shelefanav - eino shichechah": the sheaf that he has not yet reached is not considered forgotten, since he did not pass it.
"V'shela'acharav - shichechah": the sheaf that he already passed is considered forgotten, "mipnei shehu b'bal tashuv" - the Torah's prohibition applies to it, not to go back and take what he forgot.
There is a further reason why the sheaf before him is not forgotten: it meets the criterion of "a sheaf that he took hold of in order to carry it" in the previous mishnah. As was explained, since in a field there are rows running from north to south and rows running from east to west, someone walking from north to south who leaves aside a sheaf that is still ahead of him may well intend to gather it afterward when he walks from the other direction, from east to west. Therefore that sheaf is not subject to the law of forgetting, since it is destined to be gathered.
"Zeh haklal":
The mishnah concludes with a general rule: "Kol shehu b'bal tashuv - shichechah" - any sheaf that was forgotten and to which the verse "you shall not go back to take it" can apply is considered forgotten. "V'she'eino b'bal tashuv - eino shichechah" - a sheaf to which "you shall not go back" does not apply, because he has not yet reached it and one can say that he will gather it from another direction, is not considered forgotten.