A decade ago, I traveled to Princeton to spend some time with a young philosopher who had decided to start scanning people’s brains. I was working on a book about the history of neurology, called Soul Made Flesh, and I was fascinated by how the study of the brain had emerged from a scientific attempt to save souls. I wanted to end the book with a look at how scientists study the brain 350 years later, and during my research I discovered the work of Joshua Greene. He was taking the arguments that moral philosophers had developed over many years and testing them out on flesh-and-blood brains, monitoring neural activity as people worked through moral problems.

In addition to putting Greene