Our Gemara on amud beis discusses a paranormal method used to determine whether a child is the firstborn of the mother or the father. This has significant halachic implications, as only the natural firstborn of the father inherits a double portion, regardless of the mother's previous birthing history (Shulchan Aruch 277:8).
It is taught as a tradition that the saliva of a father's firstborn can heal a specific ailment, whereas the saliva of a mother's firstborn does not have this healing ability.
The Ben Ish Chai (Derashos Toldos) uses this concept to explain the divine justice behind Yitzchok losing his eyesight in old age. First, he quotes Harav Tzemach Dovid, who explains that Yitzchok’s blindness was providential, allowing Yaakov to take Esav’s place and receive the blessing without Yitzchok’s awareness. However, this raises the question: “Is the hand of the Lord limited (Bamidbar 11:23)?” Could God not have found another way for Yaakov to obtain the firstborn’s blessing?
The Ben Ish Chai answers based on a Midrash, which explains Yaakov’s claim to the firstborn status over Esav. This Midrash, also quoted by Rashi (Bereishis 25:26), explains:
“It was with justice that Yaakov was holding Esav back. Yaakov was conceived from the first drop and Esav from the second. Learn from a tube with a narrow opening: insert two stones, one after the other. The one that goes in first will come out last, and the one that goes in last will come out first. It turns out that Esav, who was conceived last, was born first, and Yaakov, who was conceived first, was born last. Thus, Yaakov held Esav back so that the first conceived would also be the first born, and he would open his mother's womb and rightfully claim the firstborn status" (Bereishis Rabbah 63:8).
Noam Elimelech offers an additional layer to this explanation. He suggests that Yaakov further validated his claim to being the rightful firstborn by the fact that Esav likely attempted many times to heal their father Yitzchok’s blindness with his saliva, yet it did not work. This failure indicated that Esav was not the authentic firstborn of their father. Thus, Yitzchok's blindness was not just incidental but served a divine purpose, further proving Yaakov’s rightful claim.
Ever since I first learned this Rashi (in third grade), I’ve been troubled by the apparent illogic of Yaakov’s argument. After all, if Yaakov's claim were valid in the legal sense, then by that reasoning, every firstborn twin should lose his rights to the second-born twin, but we know this isn’t the case. What kind of legal reasoning is this? As I grew older (I forget at what age), I came to the conclusion that the logic must be as follows: in truth, every firstborn twin achieves his rightful status by rotating in the womb; it was only by some twist of fate that Yaakov was unable to do so. Baruch Shekivanti, this is precisely the explanation given by the Beis Yaakov al HaTorah (Toldos), who offers compelling proof from the Biblical text. The Torah describes Yaakov as being born while grasping Esav’s heel (Bereishis 25:26), and similarly, we see this in the case of Peretz (Bereishis 38:29), who appears to have successfully executed this last-minute maneuver. From these examples, we learn that twins compete for the right to be born first and claim the status of the firstborn.
Another more mystical explanation relates to how halacha is understood in its potential versus its actualization, and how the Patriarchs may have fulfilled their moral obligations according to the intuitive sensitivities of the Torah prior to its formalization into a fixed set of laws. We will explore this concept further in tomorrow’s blog post, Psychology of the Daf, Bava Basra 127.
Translations Courtesy of Sefaria, except when, sometimes, I disagree with the translation 
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