No, no, it’s not an oxymoron.
The problem with most philosophical ethics is that it takes a single rule and wants to rule the entire world with it.
This is noticeable in utilitarianism. “The most good for the most people.” This rule, applied, can lead to ethical abominations, to treatment of minorities and so on which is beyond the pale.
The story “The Ones Who Leave Omelas,” by Ursula LeGuin, is an examination of this problem (and highly recommended). In the real world, this leads to things like torture (what is a few people’s pain compared to the benefit for all?). It leads to accepting dire poverty as the result of our economic system due to an assumption that our system is the best and because, after all, the system benefits those in charge and in core nations the most. Call it hypocritical utilitarianism, but it’s real enough as a justification.
The discipline of economics, as it stands, is an exercise in hypocritical utilitarianism. Capitalism produces the best outcomes, therefore its shortcomings should be overlooked. It’s not clear it produces the best outcomes (certainly not if you are an Ethiopian or Congolese), but the point is larger than that. Even if it did, can a system which appears wedded to so much poverty, inequality, violence, and so on really be acceptable? By utilitarian standards it might well be.
And yet utilitarianism has a core of hard truth in it. It is easy to see the cases where utilitarian reasoning leads to horrible outcomes; it is intuitive to think that society should be run for the most benefit for the most people.
Again, the requirement to have one rule is what kills philosophical ethics. You must have bright lines, duties, and principles and you must know what the good is.
The first step is to have those bright lines. Do not rape. Do not torture. Stuff you do not do, no matter what. You unilaterally take some behavior off the table. You establish a minimum. If you do not, utilitarianism will always descend into barbarity, usually through specious argumentation, though not always.
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The second thing you do is establish positive duties. These are meant more for the social level than the individual, but cannot exclude the individual if they are to have efficacy. You might say here that everyone should eat if there is enough food in society and agree on how to triage in the very rare case that there is not. Women and children first, perhaps. Or workers in manual trades first. Or whatever. It doesn’t really matter, so long as it is generally regarded as fair.
You can extend this. If society has the means, everyone should have housing and clothes. Everyone who wants to work, and is capable of working, should have a job, perhaps. In a society which requires more work (not ours, we need less, too much of our work is harmful), you can say that everyone who is able should work. Add to this much of civil liberties: the right to jury trial and face your accusers, maybe.
Most of this boils down to an ur-rule. People all deserve to be treated with dignity, and everyone deserves the basics of life when they can be provided, which, in a society with vast over-surplus like our global society today, is the case.
A duty is something that applies to everyone. Everyone gets to face their accusers. Everyone gets to eat.
Having established your bright lines and your duties, you add your principles.
One of them might be: Civilians should not be targeted in war and every reasonable care should be taken not to harm them.
Another one might be: War is acceptable only in self-defense might be another one.
Yet another one: Everyone should have equal access to the determinants of success like education.
And so on.
Principles are about things you can’t achieve. You will never have a totally equal society, nor would you want one. We might want no war, but giving up the right of self-defense puts us at the mercy of the worst people in the world. If war is necessary, some civilians will be killed, but we should strive in every way to keep that to a minimum.
After having completed these steps, you are ready to add utilitarianism. Given meeting all the above, “the greatest good for the most people” is now the goal. You can use utilitarianism at this point because its worst associated problems have been taken off the table. You do the above things even if you think they don’t produce “the most good for the most people.” They are non-negotiable.
If you wish, you can order the above principles for triage, from most important to least important. But save in grave periods of crisis, it is almost never true that you can’t do all of them. You can both feed people and give them fair trials. If you can’t, you’ve gone wrong somewhere, as in the US, where the drug war has led to locking up millions of people who should never have been charged with a crime. You can’t give everyone in the US a fair trial, because there aren’t enough judges, juries, lawyers, prosecutors, and so on. That is an indication you need to be arresting less people, not that you need to compromise their right to a fair trial.
When making utilitarian calculations, you must always resist the urge to turn common sense and basic decency on their heads. If you find yourself excusing cruelty, in any way, you have gone wrong. If you find yourself excusing fraud, you have gone wrong.
There is no one ethical rule to rule them all. But at the same time, this need not be particularly complicated. Be kind. Treat everyone with dignity. See to everyone’s needs. Fight only in defense.
The further you extend this, the better your society will be. A decent concern for the fate of both other humans and other living beings would have served us well, and likely led to us avoiding much of the worst of the environmental disasters which have already come and which are yet to come, while avoiding almost the entire mess in the Middle East.
Do not descend to sophistry. Do not defend the indefensible. And if you want the most good, extend your circle of belonging out as far as it can go.