Slavery is the ownership of one person by another. Since a person no more owns another than a thief owns the purloined goods, there has never been any slavery. But of course there have been institutions thought to be slavery: institutions in which a person was thought to be the property of another. These were not institutions of slavery in the above strict sense but forms of unjust imprisonment, kidnapping, etc.
This seems to be merely a point about words, and a mistaken one at that. “Of course, Alexander II ended serfdom in Russia while Lincoln ended slavery in the US. Words mean what they are used to mean, and to dispute historical claims like these is to be like the fusty grammarian who claims that ‘It’s me’ is bad English.”
I agree that the question of words is unimportant. But here is what is important. Institutions are defined in large part by their norms. It is a defining feature of the norms of slavery (and, with some differences, of serfdom) that one person has property-style rights over another who has onerous obligations corresponding to these rights. But in fact, nobody has such rights over another, and the supposed obligations do not obtain. The institution that the “masters” saw themselves as a part of did not in fact exist, because the rights and obligations that they took to be integral to the institution did not, and could not, in fact exist.
We can use the term “slavery” (and cognates in other languages) for that non-existent institution, just as we use the term “phlogiston” for the substance that chemists mistakenly believed in before oxygen was discovered.
But we could also use distinguish and use two terms. Maybe slaveryh is the historical form of social organization that actually (and deplorably) existed and slaveryn is the normative institution that the mastersh (and maybe some of the slavesh, as well) incorrectly thought to exist and thought to be coextensive with slaveryh.
Again, the words don’t matter, but it matters that there was a morally condemnable attempt to create a certain social institution which attempt failed because the norms that were attempted to be instituted were incapable of institution.
This is a pattern we find in many other cases. There is no such thing as a forced marriage, since the norms of love and sexuality that define marriage do not come into existence apart from the free consent of the parties. But of course over the course of history there have been morally condemnable attempts to force people—especially women—into the institution of marriage. These attempts always failed, and what the victims were forced into was a different institution, one subjecting them to such injustices as kidnapping, unjust imprisonment, rape, etc.
Thomas Aquinas, similarly, holds that there are no unjust laws. Of course, legislators may attempt to enact laws that would be unjust (or they may simply be exercising power and not even trying to legislate). But when they do so, they fail to enact laws. What they enact are mere demands masquarading as laws (philosophical anarchists think all “laws” are like that). Again, the question of words is unimportant, but what is important is the pattern: the legislator is deplorably attempting to create a social institution—a law—and failing to do so, but instead creating another institution.
The particular cases of this pattern are interesting, and so is the pattern itself. A central part of the pattern is an attempt to create an institution (or an instance of an institution) that misfires, and instead another institution is created that is widely but mistakenly thought to be the one that was the target of the attempt. But the cases of slavery, forced marriage and unjust laws also share another feature that not all the cases of misfiring do. For instance, suppose due to an honest mistake in the counting of ballots, there is a mistake as to who the mayor of a town is. The false mayor then attempts to legislate something quite just. The attempt fails, because the false mayor lacks the standing to legislate. But there need be nothing morally deplorable here, as there is in the slavery, forced marriage and unjust law cases.
Moreover, the three cases I started with are not just morally deplorable, but there seems to be an important connection between moral evil and performative misfire. Slaveryh is morally horrific, but slaveryn would be even worse, as the slavesn would be under genuine obligations to do the enforced labor required of them and not to escape. This would, as it were, make morality itself complicit with the master, and the properly formed conscience of the slave into a whip in the master’s hand. And the same holds in the other two cases: morality itself would be a tool of oppression.
There are, alas, times when morality is a tool of oppression. The duties that exist between relatives are frequently exploited by repressive regimes as a means of social control: If you are an Uyghur or Tibetan defecting to a free country and speaking out against the Chinese regime, your relatives back home will suffer, and this restricts your activity because of the duties you have to your relatives. But the kinds of cases where the wicked use morality as a lever against the righteous seem different and less direct from what would be the case if slavery, forced marriage and unjust laws had the normative force that they pretend to. It is a mere coincidental effect of duties to family when these duties make it morally impossible or difficult to stand up to a wicked regime. But it would be of the very nature of the norms induced by slavery, forced marriage and unjust laws—if these norms really came into existence—that they would oppress.
This still leaves an interesting puzzle, which different moral theories will answer differently: Why is it the case that morality does not innately oppress?
Objection: Maybe slaveryh does create norms, but not moral norms.
Response: I myself don’t think there are any non-moral norms. But in any case slaveryh does not create any kind of obligation on the slave to obey the master, whether moral or not, except in some, but not all, cases a prudential one.