Our Gemara on Amud Aleph discusses various opinions on how to evaluate which animal is the firstborn when they are twins and appear to be born simultaneously. According to Rabbi Tarfon, we give the healthier one to the Kohen. The presumption is that the healthier one, and therefore the more vigorous one, is the firstborn. We presume that the healthier one pushed itself out first, even if it was only quicker by a few moments.

There is a well-known Midrash quoted by Rashi (Bereishis 25:26; Bereishis Rabbah 63:8) that justifies Yaakov’s assertion that he ought to be the firstborn, even though Esav was chronologically born first. The argument is as follows: If you place two stones (or pearls, in the Midrash’s version) in a narrow tube, the first one in is the last one out.

On the surface, this is a difficult argument to understand. Why does this technicality matter, even if it was true? Are we supposed to think that every other firstborn in the history of the world was actually second-born, and only Yaakov was correct in asserting his rights? The simple answer is that Rabbi Tarfon was arguing a chazakah: firstborns tend to be healthier, and therefore stronger and more likely to push out first. Based on that, we assume the more fit-appearing animal is the firstborn and goes to the Kohen. We also then assume, because of its robust strength, that somewhere during the gestation process, or at least preceding birth, the firstborn strong-arms itself to leave the womb first. In Yaakov’s case, an exception to the rule, Esav shoved his way out first, but Yaakov was close behind, grabbing onto Esav’s heel (Bereishis 25:25–26). This answer is chosen by Beis Yaakov (Toldos 23:1).

Another approach is to consider that in the spiritual world, potentialities are viewed differently, and more concretely, than physicalities. Yaakov knew full well that in the material world Esav was indeed the firstborn. However, his point was that the kind of firstborn he was fighting for is of a spiritual nature, which exists on a higher plane. And this was the plane that Yaakov was expecting to manifest in this world. The message to Esav, and really all firstborns, is the world of changing and now we’re going to operate in a more spiritual fashion. We find this concept from time to time: there is an ideal spiritual world which does not often get manifested in practical reality. For example, we have a tradition that the world was to be created with strict divine justice, but in the end it was created with divine mercy (Rashi, Bereishis 1:1). Or the traditions about the Beis HaMikdash Shel Maaleh (a heavenly version of the Temple). All of these speak of a spiritual ideal state that is not always expressed on a physical plane. (By the way, some people say that this tradition was what inspired Plato in his assertion of the existence of “ideals,” and that the intellectual and cultural exchange in Alexandria in his time exposed him to Jewish theological ideas.)

Regarding Yaakov’s and Esav’s ideas about the privilege of the firstborn, the Maharal (Netzach Yisrael 15) develops this beautifully, fitting it into Esav’s response. Esav denied the significance of the firstborn from that perspective. He considered only this world to be the world that mattered. His argument was: This is a physical world, and that’s all there is to it, and we follow physicality, therefore, I am the firstborn and there’s nothing else to see here. This is the hidden meaning behind the words “behold I am doomed to die…” (ibid. 25:32). Furthermore, this corresponds with the tradition that at that moment of the sale of the first born, Esav also denied the existence of the World to Come, or the Resurrection of the Dead, has more context (Midrash Tehillim 9:8). Since he did not believe that there was a spiritual world beyond the physical, the rights of the firstborn naturally should follow the physical manifestation and not some deeper spiritual world.