We have come to the end of six chapters that have documented the historical decline of violence. In them we have seen graph after graph that locates the first decade of the new millennium at the bottom of a slope representing the use of force over time. For all the violence that remains in the world, we are living in an extraordinary age. Perhaps it is a snapshot in a progression to an even greater peace. Perhaps it is a bottoming out to a new normal, with the easy reductions all plucked and additional ones harder and harder to reach. Perhaps it is a lucky confluence of good fortune that will soon unravel. But regardless of how the trends extrapolate into the future, something remarkable has brought us to the present.

One of Martin Luther King's most famous quotations was adapted from an 1852 essay by the abolitionist Unitarian minister Theodore Parker:

I do not pretend to understand the moral universe; the arc is a long one, my eye reaches but little ways; I cannot calculate the curve and complete the figure by the experience of sight; I can divine it by conscience. And from what I see I am sure it bends towards justice.

A century and a half later our eyes can see that the arc has bent toward justice in ways that Parker could not have imagined. I do not pretend to understand the moral universe either; nor can I divine it by conscience. But in the next two chapters, let's see how much of it we can understand with science.