Instead, he must acknowledge the firstborn, the son of his unloved wife, by giving him a double portion of all that he has. For that son is the firstfruits of his father’s strength; the right of the firstborn belongs to him.
This law restrains men from disinheriting their eldest sons out of mere caprice, and without just provocation. I. The case here put (Deu 21:15) is very instructive. 1. It shows the great mischief of having more wives than one, which the law of Moses did not restrain, probably in hopes that men's own experience of the great inconvenience of it in families would at last...
Commenting on Deuteronomy 21:15-17
But he shall acknowledge the son of the hated for the firstborn,.... Own him and declare him to be so, both by his will and the division of goods by him; or he shall "separate" him, as Onkelos; distinguish him from all his other sons, and make known to all, as the Targum of Jonathan, that he is his firstborn: by giving him a double...
THE TREATMENT OF A CAPTIVE TAKEN TO WIFE. (Deu 21:10-23) When thou goest to war . . . and seest among the captives a beautiful woman . . . that thou wouldest have her to thy wife--According to the war customs of all ancient nations, a female captive became the slave of the victor, who had the sole and unchallengeable control of right to her person.
Commenting on Deuteronomy 21:10-23