Objection 1. It would seem that justice, as a general virtue, is essentially the same as all virtue. For the Philosopher says (Ethic. v, 1) that "virtue and legal justice are the same as all virtue, but differ in their mode of being." Now things that differ merely in their mode of being or logically do not differ essentially. Therefore justice is essentially the same as every virtue.
Objection 2. Further, every virtue that is not essentially the same as all virtue is a part of virtue. Now the aforesaid justice, according to the Philosopher (Ethic. v. 1) "is not a part but the whole of virtue." Therefore the aforesaid justice is essentially the same as all virtue.
Objection 3. Further, the essence of a virtue does not change through that virtue directing its act to some higher end even as the habit of temperance remains essentially the same even though its act be directed to a Divine good. Now it belongs to legal justice that the acts of all the virtues are directed to a higher end, namely the common good of the multitude, which transcends the good of one single individual. Therefore it seems that legal justice is essentially all virtue.
Objection 4. Further, every good of a part can be directed to the good of the whole, so that if it be not thus directed it would seem without use or purpose. But that which is in accordance with virtue cannot be so. Therefore it seems that there can be no act of any virtue, that does not belong to general justice, which directs to the common good; and so it seems that general justice is essentially the same as all virtue.
On the contrary, The Philosopher says (Ethic. v, 1) that "many are able to be virtuous in matters affecting themselves, but are unable to be virtuous in matters relating to others," and (Polit. iii, 2) that "the virtue of the good man is not strictly the same as the virtue of the good citizen." Now the virtue of a good citizen is general justice, whereby a man Is directed to the common good. Therefore general justice is not the same as virtue in general, and it is possible to have one without the other.
I answer that, A thing is said to be "general" in two ways. First, by "predication": thus "animal" is general in relation to man and horse and the like: and in this sense that which is general must needs be essentially the same as the things in relation to which it is general, for the reason that the genus belongs to the essence of the species, and forms part of its definition. Secondly a thing is said to be general "virtually"; thus a universal cause is general in relation to all its effects, the sun, for instance, in relation to all bodies that are illumined, or transmuted by its power; and in this sense there is no need for that which is "general" to be essentially the same as those things in relation to which it is general, since cause and effect are not essentially the same. Now it is in the latter sense that, according to what has been said (5), legal justice is said to be a general virtue, in as much, to wit, as it directs the acts of the other virtues to its own end, and this is to move all the other virtues by its command; for just as charity may be called a general virtue in so far as it directs the acts of all the virtues to the Divine good, so too is legal justice, in so far as it directs the acts of all the virtues to the common good. Accordingly, just as charity which regards the Divine good as its proper object, is a special virtue in respect of its essence, so too legal justice is a special virtue in respect of its essence, in so far as it regards the common good as its proper object. And thus it is in the sovereign principally and by way of a mastercraft, while it is secondarily and administratively in his subjects.
However the name of legal justice can be given to every virtue, in so far as every virtue is directed to the common good by the aforesaid legal justice, which though special essentially is nevertheless virtually general. Speaking in this way, legal justice is essentially the same as all virtue, but differs therefrom logically: and it is in this sense that the Philosopher speaks.
Wherefore the Replies to the First and Second Objections are manifest.
Reply to Objection 3. This argument again takes legal justice for the virtue commanded by legal justice.
Reply to Objection 4. Every virtue strictly speaking directs its act to that virtue's proper end: that it should happen to be directed to a further end either always or sometimes, does not belong to that virtue considered strictly, for it needs some higher virtue to direct it to that end. Consequently there must be one supreme virtue essentially distinct from every other virtue, which directs all the virtues to the common good; and this virtue is legal justice.
Source: Summa Theologica (New Advent)