Sheviis, Chapter 9, Mishnah 4. In the previous section we learned about the mitzvah of biur, removal, which takes effect upon a person from the moment the produce is no longer to be found in the field. This raises the question: which field are we discussing, and what counts as available and accessible to the wild animals and the domestic animals in the field? On this point the Tannaim in our Mishnah disagree.
"Ochlin al hamufkar velo al hashamur":
The Mishnah opens with the words of the Tanna Kamma, that one may continue to eat, and there is no obligation of biur, as long as the produce is found in an ownerless place, in a manner that has no owner. But one may not eat on the basis of what is guarded: if the produce is found only within a guarded domain, for instance because people picked it and brought it into their homes for their private use, which is permitted in the Sabbatical year, it is not considered available. And once the produce is no longer found ownerless, but only in people's private possession, the time of biur has arrived and the obligation to fulfill the mitzvah takes effect.
The Tannaim disagree about this:
The Tanna Kamma: one may eat only on the basis of ownerless produce, and once the produce is found only in a guarded place, the time of biur has arrived.
Rabbi Yosei: "V'Rabbi Yosei matir af al hashamur" - one may continue to eat even if the only place where the produce still exists is a guarded domain.
"Ochlin al hatefichin":
One may eat, and there is as yet no obligation of biur, on account of the existence of the tefichin. What are the tefichin?
Some explain that the tefichin are hard kernels that grow in the field. They are still to be found, even though they are very hard, and the Tanna of the Mishnah teaches that this is enough to be considered available produce, even though this form of produce is very hard.
Others understand that the tefichin are earthenware vessels set in the holes of walls, which birds often use as nests. The birds leave kernels in them, and the Mishnah teaches that these kernels are enough to be considered produce that is still available.
"Ve'al hadufra":
A dufra is a tree that produces fruit twice a year, in the spring and again in the summer. Even if the fruit belongs to the second group, it too is considered available.
"Aval lo al hasitvaniyos":
The sitvaniyos are grapes that develop very late, even into the winter days. Since these grapes are of very poor quality and are not truly fit for eating, they are not something suitable for eating, and therefore they are not considered available.
Rabbi Yehudah disagrees and permits eating even on the basis of the sitvaniyos, on the basis of those inferior grapes, "kol zeman shebikru ad shelo yichleh hakayitz" - provided that they began to ripen before the end of the summer, even though they complete their ripening, or what counts as ripening in these grapes, during the winter season. This type of sitvaniyos is edible enough, and is therefore considered to be found in the field. Accordingly, as long as those sitvaniyos that began to ripen in the summer are still to be found, even though they only complete their ripening later, in the winter, the time of biur has not yet arrived.
In summary: in this Mishnah we learned what constitutes availability in the field that determines the time of biur. The Tanna Kamma and Rabbi Yosei disagree whether one may eat only on the basis of ownerless produce or even on the basis of guarded produce; we learned that one may eat on the basis of the tefichin, whether according to the explanation that they are hard kernels in the field or according to the explanation that they are the earthenware vessels in the walls in which birds leave kernels; and one may eat on the basis of the dufra, the fruit of the second yield of that same year. The sitvaniyos, the inferior winter grapes, are not considered available according to the Tanna Kamma, while Rabbi Yehudah permits them as long as they began to ripen before the summer ended.