The answer is: Accidental murder

Josh 20:3

The cities of refuge established in the Book of Joshua were essentially ancient sanctuaries that provided protection for individuals who committed accidental murder. This wasn't just a random legislative idea; it mirrored the deep-rooted principle of justice that existed in ancient Israel. You would think if someone accidentally caused death, they’d automatically be doomed to face the vengeance of the deceased’s family. Instead, these cities ensured that a person wouldn’t be punished for something that wasn’t intentional.

Almost like the ancient version of our modern legal system, this concept underscores two crucial aspects of justice: the distinction between intention and impact, and the provision of mercy. The six cities of refuge—notably Shechem and Hebron—were strategically distributed in Israel, making them accessible. Isn’t that interesting? It shows that accessibility was a priority; they wanted people to reach help when they needed it.

Also, recall how ancient laws evolve over time—this notion isn't isolated to Joshua. The parallel with later religious teachings, such as the New Testament emphasis on mercy and forgiveness, illustrates humanity’s continuous quest for understanding and compassion. Perhaps this idea sparks curiosity about how human societies value intention versus action throughout history and today, driving home the timeless principle that justice must also have a compassionate side. The cities of refuge stand not only as places of safety but also as an emblem of the balance humanity strives for in seeking justice and mercy.