Chapter 3 develops a theoretical grounding for the PDE, the solidarity rationale. Taking a cue from Philippa Foot, I situate the PDE within a larger region of morality, one structured around the concept of solidarity. I propose that our conduct is measured by a standard of human solidarity toward each other person, that is, a standard of concern and goodwill that is based on our common humanity, and that a person’s conduct is wrong when it deviates from this standard. I argue that the solidarity standard grounds more specific moral norms, including the precept of beneficence, the strict constraint against intentional harm, and the Principles of Proportionality and Due Care. Finally, I argue that the normative foundation for the solidarity standard is the dignity of human beings.

According to the PDE, there is a strict moral constraint against bringing about serious evil (harm) to an innocent person intentionally, but it is permissible in a wider range of circumstances to act in a way that brings about serious evil incidentally, as a foreseen but nonintended side effect. In Chapter 1, I discuss the key terms in this definition and I distinguish between an absolutist version of the PDE and a nonabsolutist one. I then introduce two principles that help guide agents when they foresee their conduct will cause incidental harm: the Principle of Proportionality and the Principle of Due Care. Finally, I describe the role of the PDE in just war theory and international humanitarian law, and I set out the plan for the remainder of the book.