Summa Theologiae 1.57.1, my very rough translation. The Domincan Fathers translation is here, the Latin is here. This passage is quite tricky to render into English! English, unlike some European languages, has long since dropped the distinction between 'right' and 'law' -- what we call 'philosophy of law', for instance, is called 'philosophy of right' in some other languages. We usually use 'right' terms for what is sometimes called 'subjective right', not, as they would originally have been used, for 'objective right', as here. And we have no word at all that corresponds to fas; perhaps in some very limited contexts, a word like 'religion' is used in something like this sense, and in others 'divine law' or 'eternal law', but there is no consistency at all on the point. In addition, in this article Aquinas is sorting through words that are closely related, and sometimes nearly but not exactly synonymous, and the shades of similarity and difference are difficult to preserve in translation. There are also a few words that have both common meanings and technical meanings, and sometimes they are both in view.
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It seems that the right [ius] is not the object of justice. For the jurist Celsus says that the right is the art of the good and equitable. But an art is not the object of justice, but is by itself an intellectual virtue. Therefore the right is not the object of justice.
Further, law, as Isidore says (Etym.), is a kind of right. But law is not the object of justice, but rather of prudence; thus the Philosopher posits 'legislative' as a part of prudence. Therefore the right is not the object of justice.
Further, justice principally subordinates man to God; and Augustine says (de mor. Eccles.) that 'justice is love serving God alone, and from this ruling well all other things that are subordinate to man'. But the right does not pertain to the divine, but only to the human; thus Isidore (Etym.) says that 'the divinely ordered [fas] is divine law, but the right [ius] is human law'. Therefore the right is not the object of justice.
But contrariwise is what Isidore says, in the same, that the right [ius] is so called because it is the just [iustum]. But the just is the object of justice; thus the Philosophers says (Ethic. V) that 'all call justice the disposition by which just things are done'. Therefore the right is the object of justice.
I reply that it must be said that it is proper to justice among other virtues to order man in those things in which he is 'to the other'. Thus it implies a sort of equality, as its name shows, so that it is commonly said that what is made adequate is just, for equality is 'to the other'. But other virtues complete man only in the things which befit him in himself. Therefore what is right [rectum] in the works of other virtues, to which the disposition of virtue tends as to its proper object, is not attributed except by comparison to the agent. But what is right [rectum] in the work of justice, besides being by comparison to the agent, is constituted by comparison to the other, so that what is said to be just in our works is that which is according to some equality to another, like the payment of a wage owed for a performed service. Therefore something is said to be just [iustum], as having the rightness [rectitudinem] of justice, which is that in which the act of justice terminates, even without considering the way it is done by the agent, but in other virtues nothing is determined to be right [rectum] save in that way that it is done by the agent. And because of this justice is specifically determined beyond other virtues according to its object, which is called 'the just' [iustum]. And this is the same as the right [ius]. Thus it is obvious that the right is the object of justice.
Therefore to the first it must be said that it is customary for names to be turned from their first imposition to another meaning, just as the name 'medicine' is in the first place imposed to mean that which is given to the sick to make them healthy, but is then applied to meaning the the art by which it is done. So also this name, 'the right', is first imposed to signify the just thing itself [ipsam rem iustam], and afterward is applied to the art which knows what is just; and further to note the place in which the right is administered, as when someone is said to be in jure; and even further to saying that the right is administered by one whose office it is to do justice, even if his judgment is wicked [iniquum].
To the second it must be said that just as there preexists some proportion [ratio] in the mind of the artisan for what is done externally by an art, which is said to be the rule of the art, so also for the just work itself there preexists in the mind some proportion [ratio] by which its proportion [ratio] is determined, as it were some rule of prudence. And if this is collected in writing, it is called law, so that law, according to Isidore, is written system [constitutio scripta]. And thus law is not the same as the right, properly speaking, but a sort of proportion of the right [ratio iuris].
To the third it must be said that because justice implies equality, but we are not able to give God equivalent recompense, therefore we cannot give what is just, according to complete proportion [secundum perfectam rationem], to God. And because of this, 'the right' [ius] is not properly said of divine law, but 'the divinely ordered' [fas], because God is satisfied if we do what we can. But justice tends toward man, insofar as he can, giving recompense to God, by totally subordinating his soul to him.
