Genesis 25:6

6.But unto the sons of the concubines. Moses relates, that when Abraham was about to die, he formed the design of removing all cause of strife among his sons after his death, by constituting Isaac his sole heir, and dismissing the rest with suitable gifts. This dismissal was, indeed, apparently harsh and cruel; but it was agreeable to the appointment and decree of God, in order that the entire possession of the land might remain for the posterity of Isaac. For it was not lawful for Abraham to divide, at his own pleasure, that inheritance which had been granted entire to Isaac. Wherefore, no course was left to him but to provide for the rest of his sons in the manner here described. If any person should now select one of his sons as his heir, to the exclusion of the others, he would do them an injury; and, by applying the torch of injustice, in disinheriting a part of his children, he would light up the flame of pernicious strifes in his family. Wherefore, we must note the special reason by which Abraham was not only induced, but compelled, to deprive his sons of the inheritance, and to remove them to a distance; namely, lest by their intervention, the grant which had been divinely made to Isaac should, of necessity, be disturbed. We have elsewhere said that, among the Hebrews, she who is a partaker of the bed, but not of all the goods, is styled a concubine. The same distinction has been adopted into the customs, and sanctioned by the laws of all nations. So, we shall afterwards see, that Leah and Rachel were principal wives, but that Bilhah and Zilpah were in the second rank; so that their condition remained servile, although they were admitted to the conjugal bed. Since Abraham had made Hagar and Keturah his wives on this condition, it seems that he might lawfully bestow on their sons, only a small portion of his goods; to have transferred, however, from his only heir to them, equal portions of his property, would have been neither just nor right. It is probable that no subsequent strife or contention took place respecting the succession; but by sending the sons of the concubines far away, he provides against the danger of which I have spoken, lest they should occupy a part of the land which God had assigned to the posterity of Isaac alone.