Morality gives a certain structure to society. It puts everyone, or at least the majority of people, on the same page concerning how to treat others. Those that do not fit within this code of how to treat others are restricted by the rest of the population in some way. Without this structure in place how would the world be different? If our current moral structure were completely reversed and bad became the new good and good became the new bad I think this would be detrimental to society. People would not have empathy or compassion for anyone else and only care about themselves. How would society learn to work together? Furthermore, what exactly would it mean to reverse our current moral orientation? Would that mean killing, stealing, hating and lying would become things to strive for? Would that mean honesty, love, kindness, and service would become things to avoid? If this would be the case, then I think we can all agree that a world with these preferences would crash and burn fairly quickly. A world like this would not promote the growth or well-being of our species. Right?
A free translation of a passage from Kant's 'Perpetual Peace' suggests that even a group of devils could form a perfectly tolerable society so long as they were rational. They could have the worst of moral orientations, as you might describe it, but provided they were not completely self-dependent (in which case they they would not need to form a society) they would have to adjust their behaviour to suit each others' needs. If they lie, they will not be believed, therefore their lies will not work, and so they will be constrained to tell the truth. They might not care in the least about others' interests but unless they helped others in need, when they themselves were in need others would not help them : and so they would have to practise a form of mutual aid.
I could extend the story but you can see how as the details build up, and the consequences of rationality unfold, a society of devils would form a society in which the requirements of present morality would still be followed. There would be no altruism, no benevolence, since the 'diabolic' moral motivation would be different from and exactly inimical to ours, but truth-telling, mutual aid and the rest would, I venture to suggest, be steered swiftly into place by rational self-interest. Hate one another they might, but help one another they will.
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