The Pyramid and the Horizon illustrate the persistence of social injustice, but the argument takes for granted an obligation toward justice. A society should be just for the same reason that a person should be moral. The terms identify the ideals toward which a society or an individual should strive. However, such a simple account risks question-begging, so one also needs an account of justice that demonstrates its desirability.

Morality and justice originate in the need to live in harmony with others. Like all primates, humans live in groups. Our survival is tied to both our ability to fend for ourselves and our ability to help one another. Fundamental moral norms such as honesty, generosity, and harmlessness create a safe environment for individuals to cooperate. A norm of reciprocity ensures that individuals cultivate beneficial connections and sever harmful ones. While social units will define more specific norms, the root of morality is behavior that facilitates living in harmony with one another.

When social units mature into political entities, laws supplement morality by defining norms that political power will enforce. Since cooperation remains important to survival, laws should provide a basis for cooperation among citizens to distribute resources, solve problems, and resolve conflicts. A just society has laws that facilitate beneficial cooperation among citizens, just as a moral person behaves in ways that facilitate living in harmony with others.

If political power concentrates in a small group, the law will reflect the interests of that group primarily. If control over resource distribution enables a group to force bondage on the rest of the citizens, that group can cooperate within itself to exploit all other citizens. Such laws would be unjust insofar as systematic disadvantage poisons cooperation. If an individual is not protected by law, they must use extra-legal means to protect themselves. They must be suspicious of any cooperation because they would be unlikely to prevail in a legal dispute.

Injustice threatens the motive to cooperate, a motive inseparable from the urge to survive. Citizens should push their society toward justice because a just society benefits all citizens. We strive to be moral so that we can live in harmony with others, and we strive for a just society so that we can share the advantages of that harmony.