Yevamos 38

A Yavum and the Sufaik kid are coming to inherit the grandfather. We say the Yavam collects the whole thing since he's definitely inheriting while the kid is only a Safeik (because if he's the Yuvam's kid he doesn't get an inheritance from the grandfather. Rashi seems to learn that since the Yavum gets at least half, so that's why he's considered a definite heir. Tosfos asks from many places that just because someone has half does not mean that he's entitled to all. One such place is the first Mishna in Bava Metzia. If two jointly hold onto a cloak. One claims he found it first and is completely his while the other claims they found it together and he has half. The Halacha is that the second half that is in dispute gets divided. We do not say just because he definitely is entitled to half the cloak he's a definite owner and is entitled for the whole thing. Rather, Tosfos explains that since the Yuvam is a definite heir of the grandfather while the kid is only a Safeik whether he is a heir, so we cannot take away from a definite heir to split it with someone who hasn't established himself as a heir. Rabbi Chaim Smulowitz If you ever though that Tosfos was beyond you. Think again. A few more free copies of Gemara and Tosfos: Sukka 2a-11a is still available at https://www.tosfos.ecwid.com