An argument for natural law over and against legal positivism?

Just curious what arguments y'all had for natural law (roughly the view that there exists a underlying moral law that explains or causes the positive/civil law, and that moral reasoning is necessary to determine the content of positive/civil).

Positivism about the law bt contrast is the view that if there is an underlying moral law, then it does not explain or cause the positive/civil law (perhaps there isn't even any necessary connection between the moral law and the civil law, however limited in scope and qualified) and moral reasoning is not necessary to determine the content of positive/civil.