The debate between legal positivists and natural lawyers quickly become semantic. If you are so inclined, you can stipulate that "law" means "the occasions on which coercion is morally permissible/obligatory." Or you can say it refers to the social fact of how specialists in social practices like litigation and staute passing think. Each are perfectly sensible things to think about, and there is really no point in arguing about it: chacun à son gout. Study what you want.
So let's take the positivists preferred course, and study how legal specialists think. There's no doubt that they often refer to constitutive proclaimations of legal authorities. The law is X because Parliament or the Supreme Court said so. I can agree that it is X, even if I think it should be Y because the said bodies, however legally authoritative, are being morons.
But then you notice that Parliament, the Supreme Court, and everybody else will state the law in such a way that it needs everyday moral intuitions if we are going to apply it to physical events going on in the world. Contracts in Quebec and the US need to be interpreted in good faith. Statutes in Canada need to be seen as means to address some social mischief. Liability in negligence depends on whether the defendant acted reasonably.
And incorporating these moral intuitions into legal decision-making sometimes makes the law more, rather than less, predictible. The hardest thing to give an opinion about is how non-morally-inflected legal language will be interpreted. Most of us have a better idea what a jury or gut-based judge will think. So even on the dimension of certainty and respect for the will of the parties/legislature, which are the selling-points of positivism, we might be better off just incorporating moral intuitions into law.
Of course, we aren't always better off. People disagree about moral intuitions (although the extent to which this is true is probably exaggerated), and a big part of the point of law is to have a predictible way of resolving those differences. A natural lawyer would say that this fact is a major justification for authority. So just as looking at the actual practices of lawyers (as positivists recommend) provides partial support for natural law, looking at the moral justifications for coercion provides a basis for positive authority.
The difference is that the natural lawyer would say that authority is always limited by the reasons for it, and has less incentive to exaggerate the extent to which the actual legal system depends on command, rather than moral intuitions.
Anyway, if we actually concentrate on the way that a legal system does (or should) rely on each is a more interesting question than "what is law?"