I would argue that the inability to have a science of morals has been recognized since Aristotle (and would disagree with Hume in any case). The realm of human affairs is simply too complex to easily derive the relevant oughts, apart from some very broad precepts (it's usually bad to kill, it's usually bad to lie, etc.) Combined with the difficulty in separating out self-interest in mode so tied up with our way of life, any attempt to develop a science of ethics is bound to fail.
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"Die Deutschen meinen, daß die Kraft sich in Härte und Grausamkeit offenbaren müsse, sie unterwerfen sich dann gerne und mit Bewunderung:[...]. Daß es Kraft giebt in der Milde und Stille, das glauben sie nicht leicht."
"The Germans believe that power must reveal itself in hardness and cruelty and then submit themselves gladly and with admiration[...]. They do not believe readily that there is power in meekness and calm."
-- Friedrich Nietzsche
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