Good and Evil

[Ben Davis]

Oddly that ethics is largely subjective, yet it does admit analysis using measurable and meaningful criteria, giving it an element of objectivity you wouldn't see in art history, for instance.

Potentially but it depends on the framework. The ethics of Feudalism, for example, are almost immeasurable in every respect, dependent as they are on the whims of the feudal Lord. Humanist ethics, in contrast, lends themselves well to objective measurement because they deliberately try to identify 'maximally positive' positions on matters; even then there's so much potential variation that there will always be some subjectivity (e.g. promotion of physical health vs the right to smoke - a microcosm of State vs Individual). 

But their very conception of "harm" may well have differed from ours.

Their conception of harm would likely have been similar. Remember that humans have undergone very little physical/neurological change over the past couple of millennia; what hurts us now would have hurt us then. The difference was in the perception and acceptance of that harm (an example of what I referred to earlier as 'the trade-off') as you intimate here:

For ancient peoples, the safety of family or tribe was more important than the safety of the individual. The latter could be sacrificed if needed. And there were good reasons for it to be that way: The world didn't have strong central governments or police in that era, leaving people to band together for their own protection usually under a system of kinship ties. Most of these child marriages were arranged to cement alliances between one group and another. High-status people of any age weren't allowed to marry for love, and the marriages had to take effect while relations were good; waiting risked the possibility the groups would become enemies in meantime. (Though curiously, low laborers may have had more freedom of mate choice since no one would be concerned with diplomatic results.) Basically the individual wasn't safe unless her kin group was.

True. People were driven by social mores to make different sacrifices but that hits the point right on the head: people knew that they were sacrificing. They had an appreciation of harm similar to ours today (think of the modern resonance of ancient literature as an analogy) but their ethical frameworks discounted individualistic concerns in favour of tribal ones. Imagine if the weighting been in the other direction.

Which makes me careful regarding anachronistic projection of contemporary moral standards onto earlier eras.

Whereas my understanding of our similarities lends me to greater confidence.

I won't style myself a child psychology expert, and freely concede the known hazards of child sex you have listed. Yet I doubt these hazards were well appreciated in medieval times

Not necessarily. If we look at medieval literature, we sometimes see themes of childhood trauma being stated as the cause of both adult derangement in villains and drivers for heroism. That suggests to me a clear appreciation of the harm that children could suffer and the consequences of success/failure in dealing with the emotional fallout of that harm. Here, the social framework is the issue, as you outlined above.

It took more self-sacrifice and pain to live in that era than it does in ours.

Not necessarily. There are still many impoverished parts of the world, where pain & self-sacrifice are standard currencies. We're not without insight to medieval standards of tribal totalitarianism.

I can see an argument where sexual use of others by powerful people became excessive even by whatever standards they did have, so that Muhammad may not be completely off the hook.

Well, there were standards of debauchery back then, too! His marriage to Aisha wasn't officially considered abuse but I'm sure that there were people back then who thought that child marriage was 'sad but necessary'.