It really isnt so ethical as it is a warning of revenge and saying that those who like others shall be liked. It is not morality. Imagine we have a masochist. Hilarity ensues.
I already said it doesn't cover all cases, Kant tied himself in knots trying to get it to cover all eventualities. Maybe his "universal law" formulation has it covered, I couldn't say. And anyway I don't believe it is about being liked or wreaking revenge on those who harm you.
It's very simply an explanation of why it's wrong to do so-and-so.
Let's put it like this:
CASE A: Don't steal from me or I'll steal back from you, or worse.
That's a threat of revenge and it will probably work if you believe I can really exact my revenge on you. If not, it will probably cause you to "get your revenge first" and eliminate me as a problem.
CASE B: Don't steal from me because it's wrong. It's wrong because if everyone in the world was stealing, property would cease to have any meaning and we'd be living in chaos.
There's no threat here, nor is it important to be popular or well liked for the argument to apply. All that's necessary for the argument to be effective is that you recognise that the world would be a different place if the "wrong" action was universally accepted. This argument will only work if it is universally accepted, which in fact it is.
(By universally I don't mean everyone, I mean nearly everyone everywhere).
Ethics doesn't have to be holy or sacred to work. It only has to be recognized as a universal "same rule for everyone" thing.
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