Summa summae S. Thomae, Tomus III

The name of right is taken in various ways: first, for that which is just. But the just is taken in two ways: generally, for everything that is consonant with law; strictly, for the equality owed to another; for those things that are equalized are vulgarly said to be justified. And right so taken is the object of justice, as we shall say below.

Secondly, right is taken for the legitimate faculty of doing something, acquiring, possessing, alienating, etc.: in which sense one is said to use his right. But right taken in this second way is the foundation of right taken in the first way. For nothing is owed to another by way of equality without a right in the other, because what is owed is owed to a right; right and debt are correlatives.

Thirdly, right is taken for law, which is the rule of the just in general. Hence right taken in the second way depends on this right; for that anyone have a right to something is attributed to him either by the law of nature, or by positive law, or at least by free concession of God or men. Wherefore

I say secondly: Right taken in these three ways can be divided, by the same division, into natural and positive right.

And as to right taken for law, it is evident; because law is one thing natural, another positive. Eternal law is not included, because this division is made into members entirely diverse; but natural law is not a thing entirely diverse from eternal law, rather it is its participation in a rational creature, as is taught in Ia IIae, q. 91, a. 2, ad 1.

Positive right is divided into divine and human. Divine is one thing of the Old Testament, another of the New. Human positive right is divided into the law of nations, canon law, and civil law. Canon law, which especially concerns us, is compounded from the canons of councils, the decrees of Pontiffs, and the judgments of the Fathers.

Right taken for the just is likewise adequately divided into natural right and positive right¹ ; because right so taken is the just, or what is adequate to another. Now something can be adequate to another in two ways. In one way, from the nature of the thing, as when someone gives as much that he receive the same amount, and this is called natural right. In another way something is adequate or commensurate to another from the good pleasure of those who establish it, as when two private persons so covenant between themselves, or the community consents that such a thing be held as just, or the prince ordains by law that it be so; and this is called positive right. The other subdivisions are as in the division of law.

Right taken for "the legitimate faculty, etc.," is also divided, on the part of the principle, into natural, which belongs by law or by the consent of nature, and positive, which belongs by positive law or by the free concession of God or men. The other subdivisions are as in the division of law. Likewise this right, on the part of the term it regards, is divided into right in a thing and right to a thing.

Right in a thing is what one has in a thing already his and obtained. Hence it gives a real action upon the very thing. Three conditions are required for it: the existence of the thing, a legitimate title, such as purchase, donation, etc., and delivery of the thing. So has right disposed, except in certain cases.

Right to a thing is what one has in order that some thing become his. For this no other condition is required than a legitimate title. It does not give an action upon the very thing, but upon the person only. Such right² is acquired by donation, purchase, etc., before delivery. Also by election, presentation to an office, by collation of a benefice before acceptance, etc.