1) THE HEIRS OF A MOTHER AND HER SON WHO DIED
QUESTION: The Mishnah discusses a case in which a house fell on a man and his mother and killed them both, but it is not known which of them died first. The son's heirs (such as his brothers from a different mother) claim that the mother died first and her son (their half-brother) inherited her property, and thus they are entitled to her property since they inherit their brother. The mother's heirs (such as her brothers) claim that the son died first, and the mother's property therefore was not inherited by her son, so they are entitled to it. The Mishnah teaches that Beis Shamai and Beis Hillel agree in such a case that the mother's property is divided between her heirs and her son's heirs.
The RASHBAM writes that the case of the Mishnah cannot be a case in which the mother had another son from a different father (and he is the mother's heir who claims the property). In such a case, Beis Shamai and Beis Hillel would not rule that the property of the mother is divided between the two sons; rather, they would rule that her living son inherits all of the property, and the heirs of the son who died with her (i.e. his brothers from the same father but from a different mother) receive nothing. The Rashbam explains that the living son is definitely (Vadai) entitled to inherit at least a share of his mother's property regardless of who died first. If the mother died first, then her living son would inherit half of the property, while her deceased son's heirs would inherit the other half; if the son died first, then her living son would inherit all of the property. Since the living son is definitely entitled to receive some of her property, he has a claim of "Vadai," while the other son's heirs have only a claim of "Safek" (for they would inherit the property only if the mother died first, whereas if her son died first they would not be entitled to any of the property), and there is a rule that "Ein Safek Motzi mi'Yedei Vadai" -- a claim of uncertainty cannot prevail over a claim of certainty.
TOSFOS rejects the Rashbam's explanation based on many sources that teach that a person who has a definite claim to only part of an object or property does not have a claim of certainty to the rest of the property. Furthermore, Tosfos argues that there seems to be no logical reason for a claim of certainty on one part of a property to strengthen a claim of uncertainty on another part. In the case of the Mishnah here, the living son's claim of certainty is only on half of the mother's property (for he would inherit that half regardless of whether she died first). His claim to the second half of the property is a claim of uncertainty, since he would inherit that half only if the mother died first. Thus, the living son's claim to that half of the property is no stronger or weaker than the claim of the dead son's heirs, and they therefore should divide that property, as the Mishnah states. Indeed, this is the way the RI cited by Tosfos explains the case of the Mishnah.
How can the view of the Rashbam be understood? On what basis does the Rashbam state that the living son's claim of certainty on half of his mother's property makes his claim on the other half also a claim of certainty?
ANSWER: The Acharonim (see CHIDUSHEI HA'GA'ON RAV NAFTALI TROP #192) explain that the dispute between the Rashbam and Tosfos revolves around the status of the estate before it is divided. The Rashbam and Tosfos are consistent with their views elsewhere (see Insights to Bava Basra 126:1).
An estate before it is divided is called, "Tefusas ha'Bayis." Tosfos maintains that the Tefusas ha'Bayis is considered like property jointly owned by partners; each of the brothers owns a part of the estate, but it has not yet been clarified which heir owns which part of the property. Tosfos understands that since each of the brothers owns only a portion of the estate, the living son has definite ownership of only part of the estate, and his claim to that part does not strengthen his claim to the other part.
The Rashbam disagrees and maintains that the heirs to an undivided estate are not comparable to partners. In a normal partnership, each partner owns only a portion of the property. In the case of the Tefusas ha'Bayis, each heir owns the entire estate jointly with all of the other heirs. Accordingly, in the case of the Mishnah here, the living son has a claim of certainty to the entire estate due to his definite ownership of part of it. Therefore, the principle of "Ein Safek Motzi mi'Yedei Vadai" applies. The Rashbam would agree, however, that in all of the cases that Tosfos cites (none of which are cases of Yerushah, inheritance), when a person has a claim of certainty to part of an object it does not strengthen his claim of Safek to the other part.