"All men are brothers." "All men are created equal." "One man, one vote." "When Adam delved and Eve span, who then was the gentleman?"
It is possible for an idea to be both very old and revolutionary. I offer as an example the notion that men ought to be politically equal. These days the idea is widespread enough and popular enough that even tyrants tend to pay it lip service. With a few exceptions, such as Kim Sung Il and almost anybody who runs a sharia shop--and even they pretend that everybody is equally under God's law. I'll assume we're all familiar with the history here.
The huge question is: "Is political equality the only way this fundamental equality between people is to be displayed?" Is it enough that everyone (even the lawmakers themselves) is equal under the law? Or ought there be a tendency to treat people similarly (or even the same) socially and economically (and spiritually?) as well?
I will use the word "equity" to describe the principle that one ought to treat people similarly. It tries to govern our actions towards our neighbors based on what they are: human beings like us. I am aware that this is not the usual definition from legal philosophy.
Of course, though all men may be brothers, some are certainly bothers as well. You can easily find radically different approaches to life, some of which are wonderful and some downright evil. And whether you like it or not, we have to make judgements about behavior. And, whether you like it or not, rewarding those who benefit the rest of us and punishing those who hurt us turns out to be essential to running a society. If you don't reward the benefactors, they quit working; and if you don't punish the malefactors, they keep up the bad work.
Justice demands that you recognize and reward people based on what they do.
The greatest horrors of the twentieth century were perpetrated in the name of equity. ("From each according to his ability . ..") That doesn't invalidate the principle, of course, but does warn us that equity as a principle cannot stand alone.
Justice as a guiding principle doesn't stand alone either. Justice doesn't take opportunity into account, so the aristocratic society with a few rich rulers can be just. A single peasant isn't a great benefit to the rest of society, and so isn't rewarded with much. A duke organizing the defense of the region is an irreplaceable asset, and is rewarded with a great deal; which of course means he can build stronger fortresses and buy better horses and be even more irreplaceable. When you say that this society isn't just, you really mean it isn't equitable. The peasant's opportunities are nowhere like as great as the duke's, and he can't possibly be as great a benefit (or as great a disaster) to the rest of the society, even if his God-given talents and his personal dedication may be far greater.
As formulated here, justice and equity are in tension. I think my approach is justifiable. You could try to use a different definition of justice which takes opportunity and intention into account, but that demands knowledge of someone's thoughts, which only God has.
Though sometimes parents can come close to knowing the thoughts. Within a family I think we begin to see some reconciliation between the two principles. Each child is equally valued and loved (at least we hope so), but since each one is different the disciplines and duties required are also different, and tailored to that child's abilities and age and attitude: equitable, in a word. In effect, we wind up with slightly different standards of justice for each child. What from the 15-year-old is insultingly sloppy shows praiseworthy concentration from the 5-year-old. And we share in the common meal because of what we are: members of the same family. Within the family we are closer to that "from each according to his ability, to each according to his need" that wreaked so much havoc in the world.
But when we move from the family to the neighborhood the model breaks down. I don't know the neighbor's son as well as my own, and I can't tailor my expectations to match his ability. We have to have clearer rules; more abstract rules. And my disapproval isn't enough to discourage bad behavior among the neighbors as it can with my own children.
Expand the scope beyond a block in the city and the model of adjustable rules of justice goes completely to hell. I cannot know that many people well enough to guess their motives, and I'm guaranteed that some of them are going to be bad actors. And if I have this kind of flexible judging authority to deal with them, I might succumb to temptation and wind up being the bad actor myself.
And this is the only model that successfully reconciles justice and equity. And only God knows enough to make it work.
I reject the Unabomber's approach (killing off enough people to reduce society to individual families which then presumably live in wonderful harmony). So I have to accept that society will always be, to varying degrees, both unjust and inequitable.