Would You Kill the Fat Man? The Trolley Problem and What Your Answer Tells Us About Right and Wrong by David Edmonds Princeton University Press, 220 pp., $19.95

Are certain actions intrinsically wrong, or are they wrong only because of their consequences? Suppose that by torturing someone, you could save a human life, or ten human lives, or a hundred. If so, would torture be morally permissible or perhaps even obligatory? Or imagine that capital punishment actually deters murder, so that with every execution, we can save two innocent lives, or three, or a dozen. If so, would capital punishment be morally permissible or perhaps even mandatory? And how, exactly, should we go about answering such questions?

On one view, the best method, and perhaps the only possible one, begins by examining our intuitions. Some people have a firm conviction that it is wrong for the government to torture or execute people, even if doing so would deter murder. Some people think that it is plain that a nation should not bomb a foreign city, and thus kill thousands of civilians, even if the bombing would ultimately save more lives than it would cost. If we are inclined to agree with these conclusions, we might test them by consulting a wide range of actual and hypothetical cases. That process could help us to refine our intuitions, eventually bringing them into accord with one another, and also with general principles that seem to explain them, and that they in turn help to justify.

Many philosophers are inclined to this view. To test our moral intuitions and to see what morality requires, they have been especially taken with a series of moral conundrums that sometimes go under the name of “trolleyology.” Here are two of the most important of these conundrums.

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. You are standing by the side of a railway track, and you see a runaway train coming toward you. It turns out that the brakes have failed. Five people are tied to the track. They will be killed unless you do something. As it happens, you are standing next to a switch. If you pull it, the train will be diverted onto a side track. The problem is that there is a person tied to the side track, and if you pull the switch, that person will…