5. Mose 21:15

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. Gen 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 Gen 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.

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 put an end to it and make them a law to themselves. Observe the supposition here: If a man have two wives, it is a thousand to one but one of them is beloved and the other hated (that is, manifestly loved less) as Leah was by Jacob, and the effect of this cannot but be strifes and jealousies, envy, confusion, and every evil work, which could not but create a constant uneasiness and vexation to the husband, and involve him both in sin and trouble. Those do much better consult their own ease and satisfaction who adhere to God's law than those who indulge their own lusts. 2. It shows how Providence commonly sides with the weakest, and gives more abundant honour to that part which lacked; for the first-born son is here supposed to be hers that was hated; it was so in Jacob's family: because the Lord saw that Leah was hated, Gen 29:31. The great householder wisely gives to each his dividend of comfort; if one had the honour to be the beloved wife, it often proved that the other had the honour to be the mother of the first-born. II. The law in this case is still binding on parents; they must give their children their right without partiality. In the case supposed, the eldest son, though the son of the less-beloved wife, must have his birthright privilege, which was a double portion of the father's estate, because he was the beginning of his strength that is, in him his family began to be strengthened and his qui…

21:15-17 The law required that a firstborn son receive the greater share of his father’s inheritance (see 21:15-16; 2 Kgs 2:9). The custom of giving the firstborn son a double portion is first recorded here but was implied in earlier practice (see Gen 25:31-34; 27:1-4; 48:8-22). • son of his father’s virility: The eldest male child carried this distinction because he proved that the man was capable of siring children, and a boy in particular.