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.