Moral progress, as it is discussed in the Stoic sections of Cicero's On Goals and in Lawrence Becker's reconstruction of Stoic ethics, is a process by which the moral agent develops gradually from egocentric and pursuit of private endeavors to one that takes into account the larger communities to which one belongs, e.g., the nation-state of which one is a citizen, and finally reaches a perspective above particular historical communities that makes it possible to criticize the limitations not only of individual ethical outlooks but also of the implied values of particular national communities.
The values we prefer insofar as they aid us in pursuit of the ends we happen to have are instrumental-relative, that is, they gain their meaning for us from the ends we have as a matter of fact. But when we identify with, say, the national community, they are no longer merely a means to our ends but also a means to peace (even if an imperfect peace) in the national community. At this point our ideas tend to be shaped by the ideas dominant in our nations (hence relative to current public opinion there).
To take a position above, and in potential contradiction to, the sentiment of the national community is entirely possible. Socrates did it when he opposed actions of the Athenian council when he once happened, by lot, to be chairing the council. To do this is to risk short-term disruption; yet it is sometimes justified in service of justice. The service of justice is also normally in service of preferred values, since injustice begets even more social disorder and ill-will in the long run than honest criticism.
The Stoics, who in many respects were *not* Platonists, nevertheless had their own standard beyond public opinion, that is, the understanding of virtue approximated by the Stoics who made the most moral progress. Of course, this standard is related to the ideal of the Stoic sage, whose will is the same as the will of Zeus. But I formulated the standard without direct reference to the Stoic sage or to Zeus since none of us have ever met a Stoic sage (correct me if I am wrong) and none of us can be 100% sure of the will of Zeus (say, with reference to the current NATO bombing of Belgrade, which just today hit a civilian train, probably killing quite a few civilians).
I will venture to guess that were we to discuss current international events or say, the human rights situation in various countries even among the folks represented on the list (I think we have No. and So. America, Europe, Australia, and Asia represented), we would have differences of opinion reflecting differences in "information" that is available to us.
Suppose we could allow for that, say, by dialogue that could correct each other's misinformation or spur us each to doublecheck what we regard as facts, about which we could easily be wrong if we got them through government- and business-approved channels. Suppose that through careful inquiry we could position ourselves where we were all operating from the same factual starting points. I suspect that our differences would be diminished considerably. Would there still be differences based not on a different assessment of the facts but on values?
Probably some, but the more I give myself to the dialogue that listens to other persons and makes the other person's biography part of my own social memory, the more I will include among my values the values that I had left out before now. I am speaking here of preferred values.
But the recognition of the diversity of preferred values, followed by the intellectual effort to devise ways to avoid conflict between them, leads to an increasingly perfected notion of justice, generosity, moderation, and so forth; which is to say, of virtue. At the ideal limit, this is no Platonic form seated in an intelligible heaven beyond space and time, but it is not arbitrary either; it is not limited by having been formulated in an English- or Greek- or Latin- or Turkish-speaking culture.
Therefore, from where I stand, Stoics are not Platonists, but they are not, in the last analysis, relativists either.