"First, as in that which measures and rules: and since this is proper to reason, it follows that, in this way, law is in the reason alone.
"Secondly, as in that which is measured and ruled. In this way, law is in all those things that are inclined to something by reason of some law: so that any inclination arising from a law, may be called a law, not essentially but by participation as it were."
I think nowadays we've largely lost this second sense of law. We speak of scientific laws in the same terms we speak of political laws; moving objects "obey" the former, good citizens "obey" the latter. We enumerate the laws of thermodynamics just as we enumerate the laws of traffic.
All that's fine, I suppose, until we come to moral laws governing the free actions of humans. No one (I trust) really thinks balls on a pool table "obey" the laws of mechanics in the same sense humans "obey" the laws of driving. But in what sense do humans "obey" the law, "You shall not commit adultery"? Put another way, in what sense is, "You shall not commit adultery," a law?
If our only choices are, "That which measures and rules physical actions," and, "That which measures and rules human conduct," we have to go with the latter. And if these measures and rules are seen as purely external -- if, so to say, there is nothing in us except our free will that participates in the law -- then the Divine Law becomes merely an external imposition, instead of, as St. Thomas saw it, the means to direct us to our end of eternal happiness with God.
It seems to me that if we've lost that second sense of law, we are headed for one of two places with respect to the moral law: either it's a set of more or less arbitrary rules against which we're measured to obtain temporal and eternal rewards; or it's a set of more or less arbitrary rules against which we're measured to no purpose at all. To accept the moral law would be to live according to rules that have no meaning in and of themselves; to reject the moral law would be to refuse to play along with a pointless game.
I think we should hold out for Door Number Three: That the moral law represents something that is true about ourselves whether we will it or not.