A practical halacha that arises three times a day is to make sure not to walk within four amot of someone davening shemona esrei. What if that person is davening in the aisle or near the door? Rav Shlomo Zalman Auerbach (Halichot Shlomo 8:36) rules that one may walk in front of that person. He explains that ein adam oser davar she’eino shelo, a person cannot forbid something that does not belong to him. Because the one davening does not own the aisle space, he cannot make it forbidden for others to walk there.

At first glance, this seems to be a questionable application of the rule of ein adam oser davar she’eino shelo. The Gemara seems to use this principle only in limited contexts. Rabbi Yochanan (Chullin 40a), for example, asserts that if a person bows down to his friend’s animal, it does not become forbidden as a idol because ein adam oser davar she’eino shelo. Although the words ein adam oser davar she’eino shelo imply that the rule encompasses all cases, how do we know that it should apply outside of the context of avoda zara? Additionally, there might be a difference between when the person has intent to make something asur, as in the case in the Gemara, and when he is trying to do a mitzvah and automatically, as an aside, causes something to be asur.

Rav Shlomo Zalman’s logic probably was based on Tosfot (Yevamot 83a). The Mishna (Kilayim 7:4) discusses a case in which person’s grapevine hang over his neighbor’s wheat, potentially involving the prohibition of kila’ei hakerem and making the wheat asur. Rabbi Meir holds that the wheat becomes asur and obligates the owner of the grapevine to pay his neighbor for the damages. Rabbi Yose and Rabbi Shimon, however, argue that one person’s grapevine cannot forbid his neighbor’s crop because ein adam oser davar she’eino shelo. The Yerushalmi explains that even Rabbi Meir agrees to the basic principle of ein adam oser davar she’eino shelo but disagrees concerning its application to kilayim.

Tosfot cite the Ri, who asks why the rule of ein adam oser davar she’eino shelo does not extend to all cases. If,
for example, a person throws pork into his friend’s kosher dish, why don’t we say that the dish remains kosher? The Ri takes it as a given that it would be preposterous to apply ein adam oser davar she’eino shelo to this scenario. Rather, says the Ri, the principle of ein adam oser davar she’eino shelo applies only to prohibitions that depend on having daat, but mixing kosher and non-kosher will make everything asur regardless of the mixer’s intention. Based on this Tosfot, we could explain the nature of ein adam oser davar she’eino shelo. If an action requires daat to make something asur, then it’s not really the action that has any effect; rather, the daat is the primary cause of the issur. It also is logical to assume that a person cannot express his daat about someone else’s items. Based on Tosfot’s extension of ein adam oser davar she’eino shelo to all areas of halacha where daat is a factor, if the prohibition to walk in front of someone davening depends on daat, it is logical that ein adam oser davar she’eino shelo should apply.

On the other hand, Rabbeinu Shimshon MeShantz argues that even things that depend on daat need not be subject to ein adam oser davar she’eino shelo. His proof is from a case where the Gemara (Bava Kama 56a) states that if a person does work while preparing his friend’s mei chatat, the water is pasul. Rabbeinu Shimshon claims that this depends on daat, because if the owner wasn’t pleased with the work that was done, the water would not become pasul. He therefore says that in most areas of Halacha, a person is able to forbid something that belongs to his friend. There are specific pesukim regarding avoda zara and kilayim that tell us that in these contexts ein adam oser davar she’eino shelo. If this is the case, then ein adam oser davar she’eino shelo in fact should apply regarding walking in front of someone davening.

Furthermore, even according to the Ri, perhaps the rule of walking in front of someone davening isn’t dependent on daat. There are two reasons given for why a person should not walk in front of someone davening: 1) it is not respectful to the shechina, which is resting in the four amot of the person davening shemona esrei, and 2) it interrupts the person’s kavana (see Bei’ur Halacha 102 s.v. Asur). According to the first reason, it seems that the area becomes forbidden for people to walk through as an automatic consequence of davening shemona esrei; the person’s daat to daven does not cause the prohibition, but rather his maaseh of davening causes the issur. According to the second reason, it is even more difficult to  understand Rav Shlomo Zalman’s application, because essentially the area isn’t forbidden; it’s the action of walking in front of the person and ruining his kavana that is forbidden. If so, what difference does it make where the person was davening?

Perhaps Rav Shlomo Zalman did not intend to literally apply this principle, but rather meant to say that Chazal did not impose a prohibition in circumstances where the person should not have davened in a manner inconvenient to passersby. Additionally, Rav Shlomo Zalman advised not to rely on his leniency so as not to make people degrade this Halacha of walking in front of someone else.