Two Wives, One Beloved and One Hated

On the verse "If a man has two wives, one beloved and the other hated" (Devarim 21:15), the Rama explains that there are two kinds of souls. The beloved wife is the soul without a yetzer hara, born a tzaddik. The hated wife is the soul born with a yetzer hara, who has battles to fight and passes through many struggles. The verse continues that the firstborn son belongs to the hated wife. It is precisely the son of the hated wife who is the firstborn and who merits the double portion, because a person who is born with an evil inclination and fights it is preferable to one who is born without it.