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How do we know what is right? Do we follow a set of rules or just go with our gut? That might seem a funny question, coming from a lawyer. Lawyers are trained to rely on an intricate system of rules that are meant to be universally applied. This isn’t so different from the work of ethicists, theologians, and philosophers; in their own ways, they try to set forth definitive precepts to guide behavior in every situation. Law and ethics are both based on an assumption that certain basic, logical principles underlie all permissible behavior. And perhaps it is possible to create a complete system based on such principles. But could it ever conform exactly to our humanity? Or will there always be situations in which we have to abandon the rules in favor of what we sense is right?

In my first year in law school, we discussed the “Trolley Problem”, a philosophical puzzle created by Philippa Foot and enhanced by Judith Jarvis Thomson. Imagine you could flip a switch to divert a runaway trolley from one track, on which five unsuspecting track workers lie, to a side track, on which a lone worker sits. Would you do it? Most people say yes; it is better to trade one life for five. The professor who posed the Trolley Problem to us asked whether, by hitting the switch and actively choosing to kill the lone worker, you would be committing murder. A few students said yes, and that therefore you should not flip the switch; but most students (like most people asked in other contexts) still said it was justifiable, or even that it was unjustifiable not to do it.

What if, instead of flipping a switch to divert the train, the only way to save the five workers was to push an extremely fat man standing next to you into the path of the trolley, so that the fat man would be killed but his bulky body would halt the trolley’s progress? Would that be okay? This time, most of the class said no – even though the calculus was exactly the same: take no action, and five people die, or act, and through your action kill one person who was not otherwise in danger. When challenged by the professor to explain the difference between the two scenarios, one student suggested the fat man was an innocent bystander, while the lone worker had accepted a job that he knew might lead to death-by-trolley. Another said that killing the lone worker was an unintended consequence of diverting the trolley, but killing the fat man was deliberate.

These attempts at explanations led to discussions of concepts like intent and assumption of risk. The implication of the discussion was that there must be some rational distinction between diverting the trolley and stopping it with a fat man’s corpse – some underlying reasoning that most people are able to access subconsciously – and that, if we could identify the universal principles underlying that reasoning, we could then use those principles to create theories of morality and codes of law that almost everyone would agree upon. If only we could find the common thread, we could sew everything up nicely, and devise a coherent set of rules to govern all of our behavior, in every situation.

But the work of Joshua Greene, a psychology professor at Harvard, suggests that there is not one coherent set of rules, and that rationality may not always satisfy our desire for the good. Greene posed the Trolley Problem to people while their brains were being scanned by an MRI machine, which showed what parts of the brain were active. He discovered that there is one part of the brain that responds consistently and rationally to both scenarios in the Trolley Problem. It appears to perform the calculus mentioned above: in both cases, it reasons that it is better to sacrifice one life in order to save five.

However, there is another part of the brain that responds negatively to the push-the-fat-man scenario, but is quiet when faced with the flip-the-switch scenario. Apparently, this latter part of the brain is sympathetic to those in close contact; it produces pleasurable feelings when helping those near us, but uncomfortable feelings when doing them harm. In the flip-the-switch scenario, this part of the brain remains silent, since imagining the switch scenario does not generate an image of interacting with the lone worker on the side track. That person is not directly connected with us. However, when the push-the-fat-man scenario is addressed, our brains imagine physically making contact with a person in close proximity, and then doing him harm. Even though this is only imagination, it generates discomfort, so that most people say not to push the fat man.

Man is a social animal, so this kind of brain activity is not really a surprise. We are born to help those near to us and to avoid hurting them. But notice what this means: we cannot hope to create a set of rational rules that would guide all “good” behavior, because a big part of what we sense is good behavior is actually triggered and rewarded by an impenetrable, irrational portion of our brains. That portion may sometimes be responsible for our “gut feelings”, and the sense that we must sometimes fly in the face of logic to select an outcome that just “feels right”. Of course, the different parts of our brains do usually work together, so if you are inclined to go with your gut, it would be wise to also examine the situation rationally to make sure you aren’t overlooking something. But don’t automatically discount what your gut is telling you!