One way of motivating the PDE is by appealing to our intuitions about cases. We might wonder, however, whether there is any deeper rationale for the principle. In Chapter 2, I argue that proponents of double effect face the grounding challenge: The challenge of providing a theoretical rationale that explains why the distinction between intentional harm and incidental harm is relevant for evaluating the moral permissibility of human conduct. I first show how the PDE accounts for common intuitions about a number of cases that might otherwise seem puzzling. Next, I provide four reasons why the PDE cannot be sufficiently justified solely on the basis of its agreement with case-based intuitions. I then examine six proposed rationales, and I argue that each is vulnerable to serious objections. The thrust of the discussion is therefore critical, but at the end of the chapter, I draw some lessons from the failures of these proposals. This sets the stage for Chapter 3, where I turn to the constructive project of developing a novel rationale.