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Consider how different defending an ethical theory will have to be from defending a scientific one. In choosing between scientific theories, we can appeal to empirical evidence. We can do experiments, testing predictions against observation. But none of this seems available in ethics. Imagine trying to do an experiment to test some fundamental moral claim, for exam-ple, that killing is wrong. Suppose I grab someone off the street, stab him in the chest, and watch him die. What am I looking for? What kind of observation will support (or undermine) the claim that killing is wrong? (Would it help if I gathered other observers as well?)
—Kagan, Normative Ethics