Objection 1. It would seem that judgment is not an act of justice. The Philosopher says (Ethic. i, 3) that "everyone judges well of what he knows," so that judgment would seem to belong to the cognitive faculty. Now the cognitive faculty is perfected by prudence. Therefore judgment belongs to prudence rather than to justice, which is in the will, as stated above (Question 58, Article 4).
Objection 2. Further, the Apostle says (1 Corinthians 2:15): "The spiritual man judgeth all things." Now man is made spiritual chiefly by the virtue of charity, which "is poured forth in our hearts by the Holy Ghost Who is given to us" (Romans 5:5). Therefore judgment belongs to charity rather than to justice.
Objection 3. Further, it belongs to every virtue to judge aright of its proper matter, because "the virtuous man is the rule and measure in everything," according to the Philosopher (Ethic. iii, 4). Therefore judgment does not belong to justice any more than to the other moral virtues.
Objection 4. Further, judgment would seem to belong only to judges. But the act of justice is to be found in every just man. Since then judges are not the only just men, it seems that judgment is not the proper act of justice.
On the contrary, It is written (Psalm 93:15): "Until justice be turned into judgment."
I answer that, Judgment properly denotes the act of a judge as such. Now a judge [judex] is so called because he asserts the right [jus dicens] and right is the object of justice, as stated above (Question 57, Article 1). Consequently the original meaning of the word "judgment" is a statement or decision of the just or right. Now to decide rightly about virtuous deeds proceeds, properly speaking, from the virtuous habit; thus a chaste person decides rightly about matters relating to chastity. Therefore judgment, which denotes a right decision about what is just, belongs properly to justice. For this reason the Philosopher says (Ethic. v, 4) that "men have recourse to a judge as to one who is the personification of justice."
Reply to Objection 1. The word "judgment," from its original meaning of a right decision about what is just, has been extended to signify a right decision in any matter whether speculative or practical. Now a right judgment in any matter requires two things. The first is the virtue itself that pronounces judgment: and in this way, judgment is an act of reason, because it belongs to the reason to pronounce or define. The other is the disposition of the one who judges, on which depends his aptness for judging aright. On this way, in matters of justice, judgment proceeds from justice, even as in matters of fortitude, it proceeds from fortitude. Accordingly judgment is an act of justice in so far as justice inclines one to judge aright, and of prudence in so far as prudence pronounces judgment: wherefore synesis (judging well according to common law) which belongs to prudence is said to "judge rightly," as stated above (Question 51, Article 3).
Reply to Objection 2. The spiritual man, by reason of the habit of charity, has an inclination to judge aright of all things according to the Divine rules; and it is in conformity with these that he pronounces judgment through the gift of wisdom: even as the just man pronounces judgment through the virtue of prudence conformably with the ruling of the law.
Reply to Objection 3. The other virtues regulate man in himself, whereas justice regulates man in his dealings with others, as shown above (Question 58, Article 2). Now man is master in things concerning himself, but not in matters relating to others. Consequently where the other virtues are in question, there is no need for judgment other than that of a virtuous man, taking judgment in its broader sense, as explained above (ad 1). But in matters of justice, there is further need for the judgment of a superior, who is "able to reprove both, and to put his hand between both" [Job 9:33]. Hence judgment belongs more specifically to justice than to any other virtue.
Reply to Objection 4. Justice is in the sovereign as a master-virtue [Cf. 58, 6], commanding and prescribing what is just; while it is in the subjects as an executive and administrative virtue. Hence judgment, which denotes a decision of what is just, belongs to justice, considered as existing chiefly in one who has authority.
Source: Summa Theologica (New Advent)