I said yesterday that I would return to the passage of Augustine that Aquinas quotes, so let me do that here.
In De libero arbitrio (“On the Free Choice of the Will”) 1.5, Augustine says, ironically:
Non ergo lex iusta est, quae dat potestatem vel viatori ut latronem, ne ab eo ipse occidatur, occidat; vel cuipiam viro aut feminae ut violenter sibi stupratorem irruentem ante illatum stuprum, si possit, interimat. Nam militi etiam iubetur lege, ut hostem necet: a qua caede si temperaverit, ab imperatore poenas luit.
Then the law is unjust that permits a traveler to kill a highway robber in order to keep from being killed himself, or that permits anyone who can, man or woman, to kill a sexual assailant, before he or she is harmed. The law also commands a soldier to kill the enemy; and if he refuses, he is subject to penalties from his commander. (tr. Thomas Williams)
He continues ironically for one more sentence, but at the end slips in a serious point which is confirmed in the final sentence:
Num istas leges iniustas, vel potius nullas dicere audebimus? Nam mihi lex esse non videtur, quae iusta non fuerit.
Surely we will not dare to say that these laws are unjust, or rather, that they are not laws at all. For it seems to me than an unjust law is no law at all.
Augustine writes in the Ciceronian tradition here (not surprising given how much Cicero he had internalized): for laws to be laws in fact, and not in name only, they must be consistent with some higher standard of justice, and not simply with the will of the lawgiver. Otherwise, say what we will, we are talking nonsense. For “an unjust law is no law at all.”
