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We are beginning siman 7. Continuing off the previous siman where he discussed trees, the Chayei Adam writes that if one finds fruit on the ground under a tree, there are two concerns to take into consideration. One is that the fruit may have fallen off the tree on Shabbos itself, and they are muktzah. The second concern is that should one be allowed to eat those fruits, they may end up climbing the tree to get more fruit and detaching fruits from it. Therefore, Chazal prohibited eating any fruits which have fallen off, out of a concern one will come to kotzair deoraysa of detaching fruit from the tree. Now that Chazal enacted an issur to eat the fruit, they have no use, so they are similar to raw meat in the freezer in that the fruit is muktzah. Once it is assur as muktzah, one cannot even move it.
The Mishnah Berurah adds that if the fruits were attached to the tree at the beginning of Shabbos, such that if one were to detach them, they would transgress the issur deoraysa of kotzair, the fruits receive the status of muktzah machmas issur. Once an item receives its status at the beginning of Shabbos, it retains that status throughout Shabbos, so the fruits remain muktzah machmas issur for the duration of Shabbos. This concept is known as migo d’iskatzai l’bein hashmashos, iskatzai lekulei yomah.
If so, we need to understand why Chazal enacted a specific issur to eat the fruits. One reason is that there are tannaim who disagree with the concept of migo d’iskatzai, and hold that once the fruit falls off the tree, its status will change. With the additional gezeira enacted over here, even those tannaim would agree that the fruit is assur.
Another reason is that, as the Chayei Adam implies, the concern is that “maybe” the fruits fell on Shabbos. In other words, a person may look at a fruit and say that they recognize the fruit and are confident that it fell before Shabbos. If so, it would not fall under the issurim above. If the person is not sure whether it fell on Shabbos or before Shabbos, they have a safeik. Since it is a safeik on an issur derabanan, we should apply the concept of safeik derabanan lekulah and it should be muttar to use.
The Gemara in Beitzah introduces a concept known as davar she’yeish lo matirim, that the concept of safek derabanan l’kula does not apply to something which will become muttar through waiting (assuming the status will change and that the item will remain extant until then). Thus, the issur of muktzah would not be sufficient to make the fruits assur, so Chazal had to enact a unique issur related to fruits which fall off a tree in order to protect a person from transgressing the issur deoraysa of kotzair.
Summary
It is assur miderabanan to take fruits which fell off a tree on Shabbos, unless one is certain that those specific fruits fell before Shabbos.