Jacob: "If thou art of opinion that there is no future world, and that the dead do not rise to new life, then why dost thou want thy birthright? Sell it to me, now, while it is yet possible to do so. Once the Torah is revealed, it cannot be done. Verily, there is a future world, in which the righteous receive their reward. I tell thee this, lest thou say later I deceived thee."
Jacob was little concerned about the double share of the inheritance that went with the birthright. What he thought of was the priestly service, which was the prerogative of the first-born in ancient times, and Jacob was loth to have his impious brother Esau play the priest, he who despised all Divine service.