This chapter argues that Darwin's thought plays a central role in the history of the conscience and that the history of the conscience plays a central role in Darwin's thought. A core project of his later works is to show how the human moral faculties could have evolved, since such a faculty seemed to pose a decisive objection to the theory of natural selection. But the theory of group selection Darwin developed to explain the origins of morality had the inadvertent effect of inducing skepticism about instinctive moral feeling. Such skepticism transformed Western moral thought: although appeals to moral “intuitions” and naturalistic theories of ethics would return, after Darwin's analysis of the conscience never again could the bare fact of moral feeling offer evidence of the divine design of humanity. In ways thinkers are still considering, Darwin forced moral philosophy to confront its fundamental earthliness.

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.

The common-sense tradition holds that among the things we know are various facts about the external world and some epistemic facts – for example, that we know there are other people, that people know their names, and that we know that they know their names. This chapter makes two claims. First, that the common-sense tradition should include among the things known various common-sense moral claims as well as various particular moral claims that are no less evident. Second, that these moral claims are more reasonable to believe than any philosophical view that implies either that they are false or that we do not know them. In short, it suggests that the common-sense philosopher should treat some moral claims as having the same weight as some epistemic claims and claims about the external world. The last three sections consider some philosophical objections to this view. These include the objections that no evaluative claims are true or false, that we cannot know particular moral claims without knowing some general moral criterion, and that the appeal to our moral intuitions is illegitimate in philosophical inquiry.