A thing is just simply when it is between equals, since justice is a kind of equality, and he calls this the politic or civil just, because all citizens are equal, in the point of being immediately under the ruler, retaining their freedom.
 Thomas Aquinas, Summa Theologica (1274). copy citation

add
Author Thomas Aquinas
Source Summa Theologica
Topic equality politics
Date 1274
Language English
Reference
Note Translated by Fathers of the English Dominican Province
Weblink http://www.gutenberg.org/cache/epub/19950/pg19950-images.html

Context

“Wherefore it is evident that penance, as a virtue, is a part of justice. It must be observed, however, that according to the Philosopher (Ethic. v, 6) a thing is said to be just in two ways, simply and relatively. A thing is just simply when it is between equals, since justice is a kind of equality, and he calls this the politic or civil just, because all citizens are equal, in the point of being immediately under the ruler, retaining their freedom. But a thing is just relatively when it is between parties of whom one is subject to the other, as a servant under his master, a son under his father, a wife under her husband. It is this kind of just that we consider in penance.” source