Limaskono, the Gemerah comes out that it's talking about where it's a strong wall, if so what's the chiddush, of course he's Patur it's an Ones. Thank you
Asher Moskovits, Brooklyn, USA
Tosfos on 55b cites the Gemara in the first Perek that says that the Mishnah was written to teach us the Chidush that during the day he is Chayav. Tosfos also says that even if he knows at night that it broke loose, he is not responsible to look for it in the dark.
As for the primary Halachah that the owner is Patur when he watched it, which is discussed in the Reisha -- "Na'al b'Faneha k'Ra'uy," the Mishnah needs to tell us that doing a proper Shemirah works to exempt him from, for he might have thought that he is still Chayav since although he did guard it, it is his animal which did the damage. The fact that Shemirah helps must be written in the Mishnah.
D. Zupnik