For the source text click/tap here: Bechorot 9
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The Mishna on our daf discusses the law requiring someone whose donkey gives birth to a firstborn male to redeem the newborn donkey by giving a lamb to a kohen (see Shemot 13:13).
According to the Mishna, if twin males were born, since one of them is certainly the firstborn, the owner must give a lamb to the kohen; if twins were born – one male and one female – the owner should set aside a lamb, but he may keep it for himself. The reason he can keep it is because his obligation to the kohen is simply financial, and whenever we are faced with a situation of financial doubt, the ruling is ha-motzi me-ḥaveiro alav ha-ra’ayah – the one who wants to claim that his fellow owes him money, must prove his case. Here, since the kohen cannot prove that the firstborn was male, even though he sets aside a lamb in case the firstborn was male, the owner is under no obligation to give that animal to the kohen.
We explore David Avon’s analysis of the definition of birth which has a wide range of ramifications in halakhah. How birth is defined is significant for the laws of the firstborn of a kosher animal, the firstborn of a non-kosher animal, the human firstborn, the impurity of a woman after childbirth, circumcision, animals consecrated for the altar, corpse impurity, and more.
Anyone who tries to understand the definition of birth by examining each of these sugyot independently is likely to find himself thoroughly confused. At times the Gemara compares the various areas of law; at times it distinguishes between them; and at still other times it discusses the definition of birth required for one particular law without addressing the others at all.