Deuteronomy 21:15

15.If a man have two wives. Inasmuch as it is here provided that a father should not unjustly transfer what belongs to one son to another, it is a part and supplement of the Eighth Commandment, the substance of which is, that every one’s rights should be preserved to him. For, if the father substituted another son in the place of his first-born, it was unquestionably a kind of theft. But, since it rarely happens that a father unnaturally degrades his first-born from his precedence, if all are born of the same mother, God reminds us that He did not enact this law without cause; for, where polygamy was allowed, the mind of the husband was generally most inclined to the second wife; because, if he had loved the first with true affection, he would have been contented with her as the companion of his life and bed, and would not have thought of a second. When, therefore, the husband grew tired of his first wife, and desired a second, he might be coaxed by her blandishments to leave away from the children of his first marriage what naturally belonged to them. Hence, therefore, the necessity of the remedy whereby the father’s power of altering the right of primogeniture is barred; for, although they might allege that they only gave what was their own, yet it was an act of ungodly arrogance to reject him whom God had deigned to honor. For he who arrogates such power to himself, or who assigns the birth-right to whom he will, almost arrogates to himself the ability to create. This right, as is stated in verse 17, was a double portion of the paternal inheritance. The reason which is added, is equivalent to saying, that the first-born is the principal honor and ornament of the father. Still, if there was a just cause for disinheriting the first-born, another successor might be substituted in his stead, as Jacob shewed in his case when he disinherited Reuben. (Genesis 49:4.) When it is said, “before the son of the hated,” some expound it to mean “during his lifetime;” others retain the Hebrew phrase, “before his face.” Their opinion, however, is probable, who take this particle comparatively, for “instead of her son.” The wife is called hated, not that her husband is positively her enemy, but because he loves her least; for contempt is considered as hatred, and he is called an enemy who does not render conjugal benevolence.