Ramban’s view of yibum has long fascinated me, because of the metaphysics it assumes about a seemingly natural event. He notes Yehudah tells Onan to perform yibum, meaning to marry his childless brother Er’s widow, Tamar. He clearly anticipates the Torah’s commandment, and Ramban considers what would lead Yehudah—and the wise men of his time, Ramban assumes—to value the idea.
Yehudah also makes clear he expects yibum le-hakim shem, to establish the name, of the deceased Er. Rashi to 38;8 thinks they will name their firstborn child after Er, a solution Ramban rejects as meaningless and inaccurate. First, what benefit does naming a child after a deceased relative achieve? More, when we see a semi-yibum in the book of Rut, she and Boaz do not name their first born for her deceased husband, Mahlon.
Ramban instead makes two remarkable claims: 1) yibum is an ancient secret, linked to the formation of children in the mother’s womb, known to the wise men of old, clear to anyone with eyes to see and ears to hear, and 2) while it is better with a brother, it can be performed with some efficacy with any relative, the closer the relative the better.
That’s all he gives us here (he also discusses it when the Torah obligates Jews to perform yibum or halitzah; there he comments in a way centuries of Jews have taken as a reference to reincarnation. I think it a misreading, but this is not the place to elaborate). Based on what he says, I think he is struggling to describe what we call genetics. The ancients recognized that a brother marrying the widow tended to be mekim shem, to establish the name, of the deceased.
For those to whom it’s not clear, I note one other point Ramban made, Onan’s refusal. The verse says he knew the child would not be his, but why wouldn’t it? If we stop our genetic investigation at the idea a brother’s genes are similar to those of the deceased, we still haven’t gotten far enough to explain Onan’s refusal to be part of this.
Again, Ramban does not say what I am about to suggest he meant, but I still think he meant it: with his inchoate sense of genetics, I believe Ramban thought God would make the child this couple produced actually be the genetic child the deceased would have had with this woman, had he lived long enough. Onan’s genes, left to their natural combination with Tamar, would have produced one kind of child. With yibum, God arranges the genes so the child who comes out is Er’s child, a fact Ramban thinks all who pay careful attention will see and hear.
Onan doesn’t want to do it because (like many men), he doesn’t want to raise another man’s son, and genetically, this son would be Er’s, not his. Obviously, I am not endorsing his position, I am only saying Ramban gives us a way to understand the issue.
For Ramban, the Torah tells us God does not always leave genetics to proceed “randomly” (if ever). For a man who passes away without children, for whatever reason, God provided a way to have one child. A willing brother—and, colloquially although not Biblically, another relative if no brother is available—can serve as the vehicle for God to arrange the genetics to allow the deceased a descendant to carry on his particular biological legacy.
It captures an element of Jewish thought I think even rationalists like Rambam accept in some places (although not necessarily yibum): the workings of “nature” are not as mechanistic as scientists of all eras want to believe. (I think, for example, the second paragraph of Shema inescapably links the Jewish people’s conduct in Israel with the weather patterns of rain and drought there; Rambam could and did argue God had embedded these linkages in the original Creation, but it still means spiritual issues have physical world impact).
Ramban thinks the same is true of genetics, metaphysical concerns will affect how the genes are arranged in creating a fetus. God pulls back the curtain, lets us see one of the factors shaping how God decides to form the babies we are granted to raise.