Summa Theologiae 2-2.57.2, my rough translation. The Dominican Fathers translation is here, the Latin is here. The Isaiah quote is interesting; leges iniquas would usually be translated as 'wicked laws', but iniquas literally means 'unequal', i.e. preventing things from being even or level. Thus the laws in question are those that are not adequated to other so as to be equal, and therefore require a work inconsistent with the just or right. The reply to the third objection seems to me to be probably influenced by the Jewish dispute over reasons for the precepts of Torah, filtered through Maimonides, with Aquinas taking a position intermediate between Maimonides and his opponents; but there's not enough here to be definitely sure that this is what Aquinas specifically has in view.
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It seems that the right [ius] is not appropriately divided into the naturally right [ius naturale] and the positively right [ius positivum]. For that is natural that is immutable and the same for all. But such a thing is not found in human matters [in rebus humanis], because every rule for the human right in some cases falls short [deficiunt], nor does it have the same force everywhere. Therefore there is not anything that is the naturally right.
Further, that is said to be 'positive' that proceeds from human will. But something is not the just [iustum] because it proceeds from human will, because otherwise human will would not be able to be unjust. Therefore, since the just is the same as the right, it seems that nothing is the positively right.
Further, the divinely right [ius divinum] is not the naturally right, because it exceeds human nature. Likewise, it is also not the positively right, because it is not originated by human authority, but divine authority. Therefore it is inappropriate to divide the right into the naturally so and the positively so.
But contrariwise is what the Philosopher says, in V Ethic., that the politically just is either natural or legal, that is by positive law.
I reply that it must be said that, as was said, the right, or the just, is some work adequated to the other according to some way of being equal. But there are two ways a man can be adequated to something. In one way from the nature of the thing, as when someone gives so much so as to receive so much. And this is called the naturally right. In another way, something is adequated to or commensurated with the other from agreement or from common consent, as when someone counts himself satisfied if he receives so much. And this is able to be done in two ways. In one way, by some private agreement, as when it is established by some contract between private persons. In another way, from public agreement, as when the whole people consent to something being as if adequated or commensurated to the other, or when the prince who has the care for the people and governs on their behalf [eius personam gerit] ordains it. And thi sis said to be the positively right.
To the first, therefore, it must be said that that which is natural to one who has an immutable nature, ought to be so always and everywhere. But human nature is mutable. And so that which is natural to man can in some way fall short, as when natural equality involves the deposit being returned to the depositor, so that if human nature were always right [recta], this would always have to be done. But because it can happen that the human will is corrupted, there are cases in whcih the deposit is not to be returned, so that it is not used for evil by a man having a perverse will, as when a madman or enemy of the common good [reipublicae] demands his deposited weapons [arma].
To the second it must be said that human will from common agreement is able to make something the just insofar as it does not involve something repugnant to natural justice. And in this is the place for the positively right. Wherefore the Philosopher says, in V Ethic., that the legally just is that in which originally there is nothing that makes a difference one way or another, but when it is put forward, makes a difference. But if something from itself involves repugnance to the naturally right, human will cannot make it to be the just, as when statute permits the committing of theft or adultery. Wherefore it is said in Isaiah X, Woe to those who make unequal laws [leges iniquas].
To the third it must be said that the divinely right is said of that which divinity promulgates. And such things are partly those that are naturally the just, but the justice of them is hidden to human beings, and partly those that are made the just by divine institution. Wherefore the divinely right can be distinguished according to these two, just as the humanly right. Therefore in divine law there are some enjoined [praecepta] because good and prohibited because bad, and also some good because enjoined and bad because prohibited.
