The Mishnah (7:4) states:
If one allows his vine to grow over the grain crop of his neighbour, he forfeits it (it must be burnt) and he is responsible for it (i.e., liable for the damage caused). R’ Yosi and R’ Shimon say “one cannot render forfeit something that does not belong to him.”
The above debate requires further explanation. At first glance the opinion of R' Yosi and R' Shimon appears somewhat difficult, as in general one can make the property of his friend assur. The Rishonim therefore explain that R' Yosi and R' Shimon maintain that this case is an exception to the rule.
Some explain that the exception is built on a pasuk. The Tosfot Yeshanim (Yevamot 83b) uses the following pasuk to explain that one only has the ability to cause the prohibition to take affect on his own field:
"שדך לא תזרע כלאים"
The Bartenura argues similarly from the following pasuk (Devarim 24):
"לא תזרע כרמך כלאים"
The debate in the Mishnah is therefore understood as being connected to an understanding of the p'sukim.
The Tosfot (Yevamot 83a) however cite the following Mishnah (5:6):
If someone sees vegetables in his vineyard and says, “When I come to them I will remove them – this is allowed; “When I come back I will pluck them” – if they increased by one two-hundredth, it is forbidden.
They continue, explaining that R' Yosi and R' Shimon maintain that this prohibition is different, in that it depends on the thought of the field’s owner (machshava). Consequently without the consent of the owner, someone else cannot cause the prohibition to take effect on his field.
According the Tosfot, how would they understand the position of the Tana Kama? It is very difficult to say that this machshava can be provided by another source. One could suggest that in the case here, since the person is performing an action by bending his vine over the field of his neighbour, no machshava is required. Consequently the prohibition can take effect on someone else’s property like any other case. The Tana Kama may be arguing that machshava is only significant when there is no identifiable action (ma'aseh) like in the case of m'kayem cited by the Tosfot.
According to R' Yosi and R' Shimon, even though the friend's produce is not effected, what is law regarding the vine? The Yerushalmi (7:3) analyses this issue in further detail. R' Yochanan maintains that benefit from the vine is indeed prohibited, while R' Elazar argues that just as the object that prohibits (referring to the produce) does not become assur, so to the vine, which usually would become prohibited in regular case, does not become assur.
R' Elazar does not view the vine and produce as two equal ingredients that are prohibited to come together (like milk and meat). Rather he views the produce as the object that acts upon the vine prohibiting them both. Interestingly, the Gemarah continues explaining that if someone directed his neighbour’s vines over his own produce, then R' Elazar would agree with R' Yochanan that even though the neighbour's vine is unaffected, the produce becomes assur.
One should note from the above discussion that both agree that at some point, prohibition against deriving benefit from the produce (issur hana'ah) that has resulted from kil'ei kerem can be partially applied. A final open question may therefore be asked: Is this partial issur hana'ah resulting from a transgression of kil'ei kerem? Consequently, even though the issur hana'ah can only be partially applied, someone may still completely transgress kil'ei kerem. Alternatively, is kil'ei kerem inherently connected to a resulting complete issur hana'ah such that where the issur is only partially applied, the prohibition cannot be violated? Accordingly the partial prohibition would be the result of an externally applied rabbinic fine (knas).
These questions could underpin the debate between R' Yochanan and R' Elazar. R' Yochanan may also agree that the produce is defined as the object causing the prohibition and consequently no prohibition has been violated, yet the partial issur hana'ah is a rabbinic enactment. R' Elazar however argues that if the produce is unaffected, no prohibition is breached. Yet only once the produce becomes prohibited, even though the vine cannot be affected, the prohibition is still transgressed and as much that can be made assur (i.e., the produce) becomes assur.
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