I started David Novak's Natural Law in Judaism. I can't say that I understand it totally, but I do understand parts of it. Essentially, Novak is arguing that Judaism has a notion of natural law----the idea that there are morals that we can discern through reason, whether or not we have divine revelation. This is because God has created the world in a certain way. To support this thesis, Novak goes through the Hebrew Bible and rabbinic, medieval, and modern works by Jewish thinkers.
I won't go into every single argument that Novak discusses. I will, however, mention an argument that stood out to me. Novak goes through the Hebrew Bible and rabbinic interpretations of it, and he notices that there were moral expectations prior to the giving of the Torah. Cain was not supposed to kill Abel, Shechem was not supposed to rape Dinah, and Jethro was aware of laws even before Sinai occurred (though, as Novak notes, there is a view that not everything in the Torah is in chronological order). How were people aware of these laws, before the Torah was even given? According to Novak, it was through natural law: people could discern them through reason, apart from divine revelation.
I wonder, however, how the traditional Jewish view that the Torah predated Sinai fits into this scenario, or if it even does. There was a rabbinic notion that the world was made according to the Torah: that God looked into the Torah and used that as a model for creation. There was also the notion that we encounter in Jubilees and in rabbinic literature that the patriarchs observed the rituals of the Torah, such as the festivals. Don't these ideas equate the Torah with natural law, on some level? At least the first one does, for the second one may presume that God revealed certain rules to the patriarchs.
And then there's the consideration that, according to one rabbinic viewpoint, the Gentiles did not have to obey the entire Torah but were only bound to the seven Noachide commandments. Is this consistent with natural law, in that it implies that the Gentiles could be righteous apart from the Torah? Incidentally, the equation of the Torah with natural law often accompanied the notion that Gentiles should keep the Torah, too.
Why is Novak discussing this issue? He deems it to be relevant, and that seems to be because he believes that natural law is common ground between Jews and Gentiles.