Summa summae S. Thomae, Tomus III

Justice, as such, is divided into general or legal, by which it orders citizens to the general or common good, as parts to the whole, and particular, which orders citizen to citizen¹.

I say 2°: Particular justice is adequately subdivided into commutative and distributive, as into essentially distinct species²

Proof. first part³. Particular justice is the virtue by which one is ordered to some particular person. Now the order of one to another particular is twofold, namely either of part to part, or of whole to part. The first commutative justice directs, the second distributive. Therefore.

Proof. second part⁴. 1° Since it pertains to the reason of justice that one render what is owed to another, where there is found a special reason of debt to some person, there is a special virtue. Now in commutative and distributive justice a special reason of debt is found. Therefore. — Proof. of the minor. In commutative justice there is rendered to each what is his and simply owed, namely what is proper and appropriated to him and within his dominion, although held by another. In distributive justice truly there is rendered to each what is his and owed, not simply, but secundum quid, by the right namely of condignity and of the community of which he is a part ; for the things of the whole are, in a certain way, the things of the parts. Now this reason of debt is diverse, as is plain. For in one way is owed the thing which is the creditor's own and already committed to his dominion ; in another because it belongs to the whole community of which he is a member, and for that reason, his condignity being considered, he has the right that it become his own. Therefore.

Nota. And this is the reason why violation of commutative justice induces the obligation of restitution, not violation of distributive justice precisely qua such.

Proof. 2°. The general form of justice is equality⁵. Now a diverse equality is found in commutative and distributive justice. Therefore. — Proof. of the minor. In commutative justice there is an absolute equality of thing to thing, which is called arithmetical proportion, as if you owe a hundred, you render a hundred ; in distributive there is an equality not absolute, but of proportion, which is called geometrical, as if, v. g., in the distribution of the common good ten coins be given to him who has six degrees of merit, five be given to him who has three degrees. Therefore.

You will ask whether between father and son not emancipated, between master and slave, between husband and wife there is justice properly so called.

I answer. There is justice properly so called, yet not altogether perfect⁶ ; because, although the son, as son, is something of the father, as a part of him, the slave something of the master, as his instrument, the wife something of the husband, who is her head and to whom she is subject, from which aspect there is not between them perfect justice toward another : nevertheless, insofar as these men are distinct supposita, the son from the father, the slave from the master, the wife from the husband ; justice under that aspect is toward another, and thus the reason of justice properly so called is saved, though not altogether perfect.