<< > Since capitalism was absent in ancient Athens, there was no material
condition that would make for the hegemony of the idea of equality; but in
America, slavery was embedded in capitalism, so slaves, free blacks, and
abolitionist whites never thought that slavery could be morally justified,
in the face of the idea of human equality.
It occurs to me I may have misread you. You seem here to be equivocating between the two views:
(a) the (im)morality of slavery depends upon, and is relative to, the views thatw ere held in these different epochs, and what views were held in those epochs depended on the mode of production--in other words, classical Marxian relativsim of the sort defended by Allen Wood, and
(b) an argument that slavery was inevitable in the ancient world, perhaps required to bring up the accumulation of surplus to a higher ;evel, whatever, but was not inevitable in early modern America, and since there was an alternative toi it, it was unjustified.
I reject (a) wholeheartedly, but (b) is a different story. it is not a relativist view at all. It does not depend on what people thing, but on what the possibilities are. I agree that there is no point advocating an unrealizable ideal, and so is there si no alternative to slavery--not, if no one thinks that there is an alternative, but if there is in some suitably strong sense, no real alternative--then there is no point criticizing the institution.
In that sense, the morality of slavery may be historically relative in a realist sense: if there is no feasible alternative to slavery at time t, then slavery is OK, or anyway immune to criticism, but if there is an alternative (a better one, it says without going)., at time t+1, then slavery is immoral and can be criticised. Note here tjhe moralirt or imnmorality of slavery, although not immuatble and the same in all circumstances, is objective. It does not depend on what people think or want.
Now, whether slavery was ever necessary, whether there was an alternative, is a complex empirical questioon on which I have no string views. But it doesn't matter for my purposes here.
Perhaps if what you mean is (b), what you really object to in what you mistake to be my viuew is the idea that exactly the same things are moral and not in all circumstances regardless of whether they are feasible. But of course I do not think that. No one does. Kant and the utilitarians will agree that what is right oin their lines will vary with the circumstances, jsu not with people's beliefs.
However, if you acknowledge that, then do you need any more historicism than that? In particular, do you need relativization to views rather than just to circumstances?