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we kind of know that e.g. the founders of the U.S. had some sort of guilty conscience going on with regards to slaveryFrom where did that secret sense of guilt come?
Maybe there was no sense of guilt at all and it was all performative, cynically done for the sake of his reputation. But perhaps the arguments from contemporary abolitionists were secretly eating away at him from the inside, inspiring a real sense of guilt. I think that is congruent with the theory that some small kernel of universal morality exists in all humans capable of feeling empathy, derived from that innate empathy instinct. People can be conditioned to ignore it, and this explains much of the variability in human morality across cultures. A slave owner can ignore that part of his mind that empathizes with the slaves because ignoring it is socially and financially convenient. Some can be needled and prodded into acknowledging it, while many go their entire lives with this empathy buried and ignored. But however ignored, that kernel of universal morality still exists in them. It's universal to humans because we're all the same species of social ape evolved to have instincts which facilitate social cooperation, particularly empathy. This morality is universal in the sense that all humans have the theoretical ability to access it within themselves, but actually doing so obviously isn't universal.
(Actual psychopaths, if such people even exist, may be the exception. If they truly lack the ability to empathize they would be unable to tap into this otherwise universal morality. Furthermore, the empathy instinct is not as perfect as might be wished; in almost all people empathy is felt more strongly for people who are close (socially and geographically.) Less empathy is felt for people who are distant or "other". Innate empathy has limits, and therefore universal morality is woefully flawed and incomplete.)