But he shall acknowledge the son of the hated as the firstborn, by giving him a double portion of all that he has: for he is the beginning of his strength; the right of the firstborn is his.
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Ambrose of Milan
So the digression we made from one law to another was not pointless, so that we teach that the firstborn is not of that lovable wife, that is, the son of the wife who is indulgent and devoted to pleasure, although the verses before us express this thought. The words of Scripture say, “He shall not propose the son of his lovable wife as the firstborn when he knows that the son of his hateful wife is the firstborn.” Rather, he who is the holy offspring of a holy mother is genuinely the firstborn; true sons do not stray from their true mother though sinners do. Therefore, he is not the true firstborn who is not the son of the true mother, but like a firstborn he is helped by richers and honored lest he be in need. But the firstborn receives “a double portion of all that he has” in order that he may be rich, [just as] in Genesis you find that each patriarch received a gift of two cloaks from their brother Joseph when they were sent back to their father. [This signified] that their brother ...
Double portion. If a person left six children, his effects were divided into seven equal parts, and the eldest son received two of them, though others think that he was entitled to one-half of the whole, (Calmet) in order to enable him to support the dignity of the family, (Haydock) and the greater expenses which he had to incur for sacrifices and solemn feasts. (Grotius)
If he were dead, his children or heirs were entitled to his portion. This was the prerogative of the first-born, 1 Paralipomenon v. 2. (Selden)
The right to the priesthood, if they might have claimed it before the law, was now given to the family of Aaron. Females had no privilege above one another. They received equal shares, when there was no male issue, Numbers xxxvi. (Haydock)
First. Hebrew, "the beginning of his strength. "See Genesis xlix. 3.