There's no actual evidence for this, and using the same logic everything we currently consider immoral was always known to be immoral "because there were marks on people's consciences FROM THE GET-GO".
Wrong again - as stated, the evidence is in the fact that things changed. They changed. They did. You can't deny that. It is history, and modernity in action. Done deal. The very fact that it changed means that people were swayed. Meaning that there obviously had to be a first, and more likely many minds already against the ideas. Had a mind never had those thoughts, then things wouldn't have changed. There would have been no need. So, at some point historically, there were people who were, personally, against slavery - even if they didn't act out against it directly. Hell - even The Bible (that much morally depraved pile of wasted pages) contains some understanding that it is not great to be enslaved. Hence the Exodus. Hence "Let my people go." The evidence abounds, honestly. The Golden Rule, when that was coined, intrinsically contains the idea that slavery isn't something that should be done. If you wouldn't want it for yourself...
Perhaps everyone also always knew it was wrong to eat animals as 'meat is murder' but they were just used to letting aholes be aholes.
Did I say "EVERYONE?" Nope. No I didn't. Which makes you hyperbolic - a form of strawman. And even then, therewere people even in antiquity who
did find there to be an issue with killing animals to eat them. Hence the reason you had cultures develop who thanked the animal for their sacrifice. The understanding is there that there is an unsavory type of deed being done in the killing. That idea is ancient.
Of course you can. You can assume that almost everybody would fall within the boundaries of the typical range of morality in the society.
But not everybody. And that was my point anyway. You even used the qualifier "almost" yourself. That is tacit agreement with my point. You can't assume how a single individual will react. To lump them altogether with their respective "group", or "tribe" may be stereotypically "accurate" - that is, accurate only in approximation. But in the end, you can't be entirely sure. And even those people who were the standard would take offense at being called the standard in the way that you are so flippant about it. "Of course you can." We have a genius on our hands everybody!
Assuming you would be some remarkable historical outlier is pure conceit, and why such thought exercises are important.
Oh please. I'm not claiming to be the outlier - I didn't exist in history. All I am saying is that there were such outliers. Hence the reason we find ourselves with slavery being abolished from law. When the hell did I insert myself into the historical scenario at all? I have been talking about how I would handle and react to these ideas in the here and now - given this idea of teaching someone about the past. You really want to teach them AND walk the line of our current, modern moral standards (this is what I am advocating, by the way)? Then you have them think about slavery from the point of view OF THE SLAVE. Not the slaver. Not the master. Screw that. There is absolutely no reason for it. Have them view it from the point of view of the slave.
One of the smartest things I have ever heard is an idea about who and how one might go about formulating a fair/equitable society. And the idea is that you have the lawmakers, those in charge, those writing the charter think and assume that they are among those who would be the most underprivileged within the societal scenario being examined. Write your rules and laws with that perspective held firmly in your mind. YOU, however, are advocating the opposite. That it might be really awesome to formulate the standards for society from the point of view of, instead, the money lenders, corporate executives, stock brokers, etc. Again - I don't think it's a great idea, and I am telling you so.
You can assume that given not a single person actually wrote about it prior to the 4th C and that basically every society practiced it that it wasn't very common for people to be morally opposed to slavery.
So what? Appeal to numbers much?
No, I believe it is pompous and self-righteous to assume you would be some kind of historical outlier rather than being typical of the society you were born into.
Again genius, when did I insert myself into the role of a person from these past societies? YOU are the one who keeps insisting that I did this, or that I am doing this. I am stating that in the here and now, I don't want to see slavery given morally ambiguous treatment of any sort.
Do you believe it remotely probable you would have been anti-slavery had you been born in ancient Babylon? Or would you most likely have been typical of the time and place?
Uh... I have no idea. Never said I did. I live in this time. I live now. In modernity, and I don't want to see people being told to think like a master. Think like a slave. That's what I want. I (the modern person, who has the benefit of thousands of smarter minds having worked this all out before me historically) want you to think like a slave when you contemplate slavery. Done.
Teaching people that they are morally special for holding the typical morality of the day but that everyone in the past was a 'sociopath' for holding the typical morality of the day is not the smartest ploy to avoid moral complacency if you ask me.
I am for everyone using their own morality to judge "the way forward" for themselves, and the rest of us reviewing and peer-testing each others solutions and coming up with what we think the best for the masses. And this includes using your past mistakes and the things you found to be bad or even horrible to gauge what you TEACH to your progeny as part of that way forward. Kind of like EXACTLY WHAT HAPPENS. What I am against is treating lightly those things that we have found to be "bad" or "wrong" or detrimental to our masses. Prescribing a view of things from the point of view of the slaver or master, instead of from the point of view of the slave is just not cool in my opinion, and I am telling you so.