As naught was holy to Esau, Jacob made him swear, concerning the birthright, by the life of their father, for he knew Esau's love for Isaac, that it was strong. Nor did he fail to have a document made out, duly signed by witnesses, setting forth that Esau had sold him the birthright together with his claim upon a place in the Cave of Machpelah.
Though no blame can attach to Jacob for all this, yet he secured the birthright from him by cunning, and therefore the descendants of Jacob had to serve the descendants of Esau.