Greene’s “dual-process theory” of moral decision-making posits that rationality and emotion are recruited according to the circumstances, with each offering its own advantages and disadvantages. He likens the moral brain to a camera that comes with manufactured presets, such as “portrait” or “landscape,” along with a manual mode that requires photographers to make adjustments on their own. Emotional responses, which are influenced by humans’ biological makeup and social experiences, are like the presets: fast and efficient, but also mindless and inflexible. Rationality is like manual mode: adaptable to all kinds of unique scenarios, but time-consuming and cumbersome.

“The nice thing about the overall design of the camera is that it gives you the best of both worlds: efficiency in point-and-shoot mechanisms and flexibility in manual mode,” Greene explains. “The trick is to know when to point and shoot and when to use manual mode. I think that this basic design is really the design of the human brain.”