Genesis 30:3

Behold my maid Bilhàh. — Rachel had little excuse for this action; for there was no religious hope involved, as when Sarah gave Hagar to Abraham (Genesis 16:2), but solely vexation at her own barrenness, and envy of her sister. All that can be said in her defence is, that the custom existed, and, perhaps, because it was distasteful to the wife, was looked upon as meritorious (Genesis 30:18).

She shall bear upon my knees. — So in Genesis 1:23, it is said, in the Hebrew, that “the children of Machir were born upon Joseph’s knees,” not borne, as in our margin. It appears that there was a custom of placing the new-born child upon the knees, first of the father, who, by accepting it. acknowledged the infant as his own; and secondly, upon those of the mother. In this case, as Bilhah’s children were regarded as legally born of Rachel, they would be placed upon Rachel’s knees. Probably, too, the children of Machir, by being placed upon Joseph’s knees, were in some way adopted by him.

That I may also have children by her. — Heb., be built by her. (See Note on Genesis 16:2.)