As we have learned, Torah law ( 21:15-17) makes clear that the firstborn will receive a double portion of the inheritance when his father dies, and that the father cannot show preference and choose another child – even one who is the firstborn to the preferred wife. One issue that is left unclear is how we determine which child is the firstborn.
Rabbi Abba teaches that if a father points out one of his children, we rely on the father’s testimony about his children. Rabbi Yoḥanan disagrees and says that the father cannot be relied upon. Rava explains this argument as referring to a case where the father points to one of his children and states that he was the firstborn – even if another one of the children had been assumed to be the firstborn, Rabbi Abba rules that we believe the father. Rabbi Yoḥanan rules that we must rely on what was generally known and accepted before the father made this statement. The Rashbam points out that even Rabbi Abba would not require a statement from the father. Under ordinary circumstances we would give the double portion to the child who was known to be the oldest. If, however, the father tells us that what was commonly assumed to be true was mistaken, we believe him.
The Ba’al Halakhot Gedolot understands from this that we will believe the father even if that will lead us to conclude that the other children who grew up in his house are mamzerim – the product of an adulterous relationship – since he denies that they are his children. According to most rishonim the father will be believed to say that any one of his children is illegitimate. The Ri”d argues with this ruling and suggests that the case in our Gemara is when one child appears to be smaller than his siblings and is therefore assumed to be younger. In such a case the father is believed to state that the smaller child is, in fact, the older one.