Here, Peter Leithart points out the curious language surrounding Sarai’s plan to have Hagar bear a son for Abraham.

Sarai’s goal is to obtain children, but the Hebrew of Genesis 16:2 says literally “perhaps I can build [myself] from her.” For Sarai, having a son is a construction project, which builds her.

The phrasing goes back to Genesis 2, where Yahweh constructs (banah, build) Even from Adam’s rib. Sarai wants to be the new Eve, the built woman, not by being formed from her husband’s rib but by having, through her maid, her husband’s son.

In Sarai (later Sarah’s) eyes, what’s important is that the son be of the flesh of Abraham. Whether she is the mother or not is incidental. She is determined and not without resources. What she does with her slave Hagar is done by her own concerted will. This brings to mind a phrase from the New Testament.

But as many as received Him, to them He gave the right to become children of God, to those who believe in His name: 13 who were born, not of blood, nor of the will of the flesh, nor of the will of man, but of God.
-John 1:12-13

“The will of the flesh”, and “the will of man” – I’ve often thought the presence of both phrases was redundant, but now it seems not. It was Sarai’s will to “build” a son in some fashion, even if not with her own flesh. The widespread use of in vetro fertilization and surrogate mothers today, along with the “light” eugenics that goes along with the practice (scrutinizing eggs for possible disease) seems also to be a sort of building. This would definitely fall under the category of “will of man” rather than “will of the flesh”, hence the differentiation. The having of children is a much richer topic than sex.

Our salvation on the other hand comes from neither power, but from God whose love for us is not so much engineered from what was lying around, but rather inherit in the architecture from before the earth itself was formed.