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.

The first Chapter is the Introduction. I start with a puzzle stemming from the fact that even though the Textbook View of moral theories conceives of the main traditions as both explanatorily and extensionally incompatible, many philosophers have recently argued that much more agreement is possible. I argue that this puzzle might be best explained by the fact that just as scientific theories can be underdetermined by the evidence, so can moral theories be underdetermined by their extension. This analogy to the philosophy of science has received little attention so far, and the book provides the first comprehensive analysis of it.I characterize the main idea in some more detail by distinguishing it from several others. The idea is neither that the moral is underdetermined by the non-moral, nor that moral theories leave our particular choices underdetermined, nor that theories are underdetermined because the evidence is somehow lacking. Finally, I give an outline of the structure of the book.