The Jewish people are chosen by God to perform his commandments, as listed in the Torah. Perhaps not all the listed commandments are divine, because some are clearly impractical. Presumably all violations must be punished, and no one should be singled out for prosecution, but how is it possible to uncover and prosecute all crimes of sex with menstruating women, something which would normally happen without witnesses? Some legal developments are clear: for example, the legist understood all the Ten Commandments to be capital crimes, but one of the Commandments mandates respect to parents; thus appeared an unrealistic law requiring the stoning of a child for reviling his parents.
If all of this sounds like a Reform apostasy, look at the rabbis who abrogated two-thirds of the commandments, including even such simple ones as blue thread in tzitzit. Or, in the case under discussion, look at how they have circumvented the unruly-son law by demanding that father and mother accuse him “in the same pitch of voice.” Rather, the rule is really simple: whatever Tanakhic law cannot—and could not—imaginably be practiced cannot be a divine commandment.
The Torah supports my criterion of practicality. A punishment for one sexual crime—sex with one’s daughter—is conspicuously absent from the exhaustive list in Leviticus 20. That relation is doubtlessly a crime both in a fortiori terms (since lesser-kin abominations are criminalized) and from Lev.18:6, which speaks of kin relationships broadly. A pervert is not executed for sex with his daughter for the simple reason that in the real world prosecution is impossible for the lack of legitimate witnesses. Also, execution of fathers even in the most abominable circumstances would endanger the societal fabric: we saw that in the Soviet Union, where the government encouraged children to report on their parents. It is inconceivable that crimes such as sex during menstruation—which are still less likely to be properly witnessed—are punishable by execution. Or that sex with one’s daughter is not a capital crime, but by reviling her parents the child incurs a death sentence.
This approach does not impinge on the Torah’s authority at all. Unlike the Reformists, who reject all Jewish law, and unlike the Conservatives, who rework every law to suit their understanding of it, I say that the burden of proof is on the challenger. I presume every word in the Torah and prophecies to be divine, but as a person with common sense, I’m prepared to accept that editorial alterations could creep in there. Again, the criterion is straightforward—a law which cannot possibly be enforced is not divine. This should not be confused with a law some people’s consciences argue against, such as executing perverts or killing enemy children. I’d rather accept that the prevailing romantic mores are wrong—as indeed every war suggests—than doubt the Torah. But some laws are physically impossible to enforce.