Following the theme of pure offspring not tainted from the pollution of the serpent, the Kuzari elaborates on the subject. Rav Yehuda Halevi said that every family was affected by this impurity even until Yakov Avinu. This explains why it was possible for Avraham Avinu to father a child like Yishmael, or for Yitzchak to father a child like Eisav. Yakov Avinu was the first person who was able to father an entire offspring of holy, pure souls that all came directly from the side of holiness. It was for this reason that Hashem chose Yakov to take this holy family and turn them into a nation called עם ישראל. From the twelve came seventy and from seventy came 600,000. It is significant that seventy is also the number of the nations of the world. Israel is meant to be unique and different and a "light unto the nations."