Yevamot 3: Why These Exceptions?

A strong start: why isn't rape of one's daughter mentioned specifically?  The rabbis want to understand why certain relationships are mentioned before others.  In asking this question, they note that some of the relationships mentioned are discussed in Torah and some are only referred to homiletically.   Rape of one's daughter is one of the latter.  Thus the rabbis find one more clue regarding the order of the exceptions to yibum.

With regard to the tzarot, the rival wives,  the Gemara questions why the Mishna states that they are exempt from yibum  rather than being prohibited from yibum when the first wives are exempt.  The rabbis decide that tzarot are not in fact prohibited from yibum but are only exempt in these circumstances.  Following this, the Gemara considers yibum and chalitza.  The rabbis determine that any woman who is eligible for yibum by Torah law is eligible for chalitza.  Any woman who is ineligible for yibum according to Torah law is not eligible for chalitza.

The Gemara wonders why fifteen women were excluded from yibum; whom is not on this list?  The tzara of a sota, an aylonit, a minor who did not refuse her marriage.  The rabbis discuss some of the implications of tzara.  One interesting note suggests that yibum allows a brother to build the house of his deceased brother (Deuteronomy 25:9).  He is unable to fully 'build' when the yevama is forbidden to him by Torah law and when one of her tzarot is forbidden to him.

An interesting game of logic: why not mention the six categories of relationships that are prohibited most severely (marrying one's mother, etc.).  The Gemara walks us through the rabbis considerations: one's mother can never marry one's brother, for she is either also the brother's mother or she is his father's wife.  Thus she will never be eligible for yibum.  Her tzara, however will be permitted to this brother for yibum.  When a first wife is not even remotely eligible for yibum according to Torah law, her tzara is in fact permitted.

This brings our rabbis to an overarching question: can a positive mitzva override a prohibition?  Yes, say the rabbis, but not when the prohibition is punishable by karet.  But how can we justify yibum at all?  A man is not to marry his brother's wife.  Ah, say the rabbis, but yibum only occurs when the brother and his wife have no children.  

This argument might suggest that the important part of marriage is procreation.  Ultimately, these laws - as much as they seem to address social and familial relationship issues - are based upon the imperative of procreation and the patrilineal chain of inheritance.