Chapter 3 is concerned with what is arguablythe most in-depth case study of moral underdetermination to date: Derek Parfit’s argument in On What Matters to the effect that the best versions of three of the most famous moral traditions arrive at the same verdicts about what matters. I start with some historical background, sketching how both J. S. Mill and Richard Hare challenged the assumption that Kantianism and consequentialism are necessarily incompatible. I then turn to Parfit.Since Parfit himself does not think of his project in terms of underdetermination, my argument here requires a two-step approach. First, I outline how Parfit challenges the Textbook View, focusing on his Convergence Argument. Second, I turn to the interpretation of these results. I discuss two interpretations that are suggested by Parfit’s writing, arguing that neither of them is convincing. I then argue that contra Parfit’s own understanding, the best way to think of the results of the Convergence Argument is in terms of moral underdetermination.If my argument is successful, the chapter thus introduces one very detailed and very prominent case study of moral underdetermination.

Chapter 8 concludes the book. I first consider two hypotheses about how we ended up with underdetermination in ethics. One hypothesis refers to advanced dialectics, that is, the way in which defenders of the main traditions have modified their theories in order to avoid counterexamples. The other hypothesis points to the method of reflective equilibrium and how methodological changes might have led to a convergence between the traditions. If correct, these hypotheses help alleviate some doubts about the fact that the analogy to science has only become relevant quite recently. Finally, I return to the puzzle and explain why I take the underdetermination view to offer the best answer to it. Two features make the underdetermination interpretation preferable to its alternatives. First, it can uphold a more plausible semantics, allowing us to take moral explanatory claims at face value. Second, it can uphold the adversarial spirit of the Textbook View, making it less of a miracle why this view has been accepted by a majority of ethicists.