Moses did not originate the rights of primogeniture (compare Genesis 25:31), but recognized them, since he found them pre-existing in the general social system of the East. Paternal authority could set aside these rights on just grounds Genesis 27:33, but it is forbidden here to do so from mere partiality.

One beloved, and another hated - That is, one loved less than the other. This is the true notion of the word hate in Scripture. So Jacob Hated Leah, that is, he loved her less than he did Rachel; and Jacob have I loved, but Esau have I Hated, that is, I have shown a more particular affection to the posterity of Jacob than I have to the posterity of Esau. See the note on Genesis 29:31. From this verse we see that polygamy did exist under the Mosaic laws, and that it was put under certain regulations; but it was not enjoined, Moses merely suffered it, because of the hardness of their hearts, as our Lord justly remarks Matthew 19:8.

If a man have two wives,.... Which is supposed, but not approved of, though permitted because of the hardness of men's hearts; for it was not so from the beginning, when only one man and one woman were created, and joined together in marriage; but as it was connived at, and become customary, a law is made to prevent confusion, and preserve order in families:

one beloved and another hated; or less loved, yet continued his wife, and not divorced. Aben Ezra observes, this follows upon the former, because it is there said, that though first he had a desire to her (the captive beautiful woman), yet afterwards had no delight in her:

and they have borne him children both, the beloved and the hated; as Rachel and Leah did to Jacob, who were, the one very much beloved by him, and the other less:

and if the firstborn son be hers that was hated; or not so much beloved as the other, as was the case in the above instance.

The Right of the first-born. - Whilst the previous law was intended to protect the slave taken in war against the caprice of her Israelitish master, the law which follows is directed against the abuse of paternal authority in favour of a favourite wife. If a man had two wives, of whom one was beloved and the other hated, - as was the case, for example, with Jacob, - and had sons by both his wives, but the first-born by the wife he hated, he was not, when dividing his property as their inheritance, to make the son of the wife he loved the first-born, i.e., was not to give him the inheritance of the first-born, but was to treat the son of the hated wife, who was really the first-born son, as such, and to give him a double share of all his possession. בּכּר, to make or institute as first-born. וגו בּן על־פּני, over (by) the face of, i.e., opposite to the first-born son of the hated, when he was present; in other words, "during his lifetime" (cf. Genesis 11:28). יכּיר, to regard as that which he is, the rightful first-born. The inheritance of the first-born consisted in "a mouth of two" (i.e., a mouthful, portion, share of two) of all that was by him, all that he possessed. Consequently the first-born inherited twice as much as nay of the other sons. "Beginning of his strength" (as in Genesis 49:3). This right of primogeniture did not originate with Moses, but was simply secured by him against arbitrary invasion. It was founded, no doubt, upon hereditary tradition; just as we find in many other nations, that certain privileges are secured to the first-born sons above those born afterwards.

If a man have two wives, one beloved, and another {g} hated, and they have born him children, both the beloved and the hated; and if the firstborn son be hers that was hated:

(g) This declares that the plurality of wives came from a corrupt affection.

21:15 Two wives - This practice, though tolerated, is not hereby made lawful; but only provision is made for the children in this case. Hated - Comparatively, that is, less loved.

15-17. If a man have two wives, one beloved, and another hated-In the original and all other translations, the words are rendered "have had," referring to events that have already taken place; and that the "had" has, by some mistake, been omitted in our version, seems highly probable from the other verbs being in the past tense-"hers that was hated," not "hers that is hated"; evidently intimating that she (the first wife) was dead at the time referred to. Moses, therefore, does not here legislate upon the case of a man who has two wives at the same time, but on that of a man who has married twice in succession, the second wife after the decease of the first; and there was an obvious necessity for legislation in these circumstances; for the first wife, who was hated, was dead, and the second wife, the favorite, was alive; and with the feelings of a stepmother, she would urge her husband to make her own son the heir. This case has no bearing upon polygamy, which there is no evidence that the Mosaic code legalized.

21:15-17 This law restrains men from disinheriting their eldest sons without just cause. The principle in this case as to children, is still binding to parents; they must give children their right without partiality.