From where does Rabbi Eliezer derive his position that the only way to "protect" a shor muad from damaging others is to kill him? The Mishna in the beginning of the fifth chapter sets up two cases with a cow and her fetus - in the first case, the cow is gored by an ox (shor tam) and killed and the dead fetus is beside her also dead. However, it is unclear whether the death occurred before the goring or after and therefore unclear whether the owner of the ox owes damages for the fetus. In the second case, the cow gored and ox and is found with a newborn calf beside her and again it is unclear whether the calf was born before or after. If it was born after, the fetus is used to pay for damages as well. In both cases, the fetus is a case of doubt so a quarter damage is paid by/for the fetus instead of half. This is based on Sumchus' position that when in doubt regarding money, both parties split the amount. The rabbis disagree and hold the burden of proof lies on the one who is requesting money from the other party (hamotzi mechavero alav hareaya). They say that this is an important principle. Why was it necessary to say that? The Gemara brings two possible situations to answer that question. One is that even if the one who was damaged makes a definitive claim (bari) and the other side is not making a definitive claim (shema), we still hold by the principle (that the burden of proof is on the one who was damaged). The second is that even though in general we follow the majority, in cases of monetary this, this principle overrides that. What is the source of this principle? In the second case of the Mishna, the owner pays half the value of the cow and a quarter of the value of the newborn calf. The Gemara asks, why does the owner of the ox get three-quarters of the damage in this case when they should only be getting half? In response, Abaye understands the Mishna differently - 'half' means a quarter, 'a quarter' means an eighth, which amounts to three-eighths, as there were two animals responsible so each only covers half the amount that they should have had to pay if they acted by themselves. They explain Abaye's case only if the cow and its offspring had different owners. Rava rejects Abaye's explanation as it doesn't fit with the words of the Mishna and suggests an alternative explanation.