Here's how we proceed.
- In section 1, we lay out the basic thought experiment, showing how Moral Twin Earth is supposed to be a variation on Putnam's original thought experiment.
- In section 2, we take up H&T's version of the argument from queerness. In its revised form, the argument is supposed to show that, under naturalistic assumptions, ethical properties are unacceptable because their supervenience1 on physical properties cannot be explained.
- In section 3, we turn to H&T's version of the open question argument. In its revised form, the argument is supposed to show that there is an important asymmetry between paradigmatic a posteriori identity claims such as water = H2O and claims that offer corresponding identities between moral properties and natural ones. The former, but not the latter, can be established by reflecting in a prescribed way on our semantic competence with the terms involved. Moral Twin Earth figures in these first two arguments by supporting crucial premises.
- But it isn't until we get to the direct argument, in section 4, that we can see how powerful the thought experiment is supposed to be. At this point H&T argue that it simply follows from the thought experiment that moral terms can't be rigid designators and that ethical naturalism is flawed for this reason alone. Again, we'll argue that, despite H&T's persistent efforts to undermine ethical naturalism, not one of their arguments is successful. Ethical naturalism may have its problems, but Moral Twin Earth is not among them.
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