We continue with Maseches Shabbos, chapter 13, mishnah 6, as part of our study of the melachah of trapping.
"Tzvi shenichnas l'vayis":
The mishnah discusses a deer that entered a house on its own, and was not trapped inside by a person. As long as the door is open there is no trapping here, and therefore the trapping is accomplished by closing the door in front of the animal: closing the door is the act that is considered trapping the deer.
The three cases in the mishnah:
"V'na'al echad bifnav" - a single person who shut the door in front of the deer, whether by closing the door or by blocking the opening with his body, is liable for this act.
"Na'alu shnayim" - two people who shut the door together, in a case where each one of them could have shut it by himself (such as closing an ordinary door), are exempt. This follows the rule of 'shnayim she'asa'uhah': when two people together perform a melachah that either one of them could have performed alone, both are exempt.
"Lo yachol echad lin'ol v'na'alu shnayim - chayavin" - with an unusually heavy door that requires two people to close it, where one person could not have shut it by himself, and two shut it together successfully - both are liable.
The reason for the distinction: the exemption of 'shnayim she'asa'uhah' applies specifically when they did not need one another and each one could have acted separately. But a melachah that by its very nature requires two people to carry it out - those who do it together are liable.
"V'Rabbi Shimon poter":
Rabbi Shimon disagrees and exempts even in this case. His approach throughout the laws of Shabbos is that two people who performed a melachah together are exempt, even when neither one could have done it separately; the very fact that the melachah was done by two people brings about the exemption.
In summary: in this mishnah we learned that a deer that entered a house on its own is trapped by shutting the door in front of it: a single person who shuts it is liable; two who shut a door that either one could have shut alone are exempt because of 'shnayim she'asa'uhah'; but with a door that one person cannot shut, two who shut it are liable - and according to Rabbi Shimon they are exempt even in this case.