Consider the case that Parfit refers to as “Bridge,” a variant on a muchdiscussed scenario. In the canonical version, five people are bound to a track and threatened by the approach of a train. On the rail of the bridge over the track sits a fat man, whose heft would be sufficient to stop the train. Would it be right to push him from his perch onto the track below, thus using him as a buffer to protect the five? Of course, if you imagine yourself on the bridge faced with this choice, all sorts of awkward and practical questions arise. Would you be able to dislodge the fat man? (For the puzzle case to work, you have to be of lesser girth—otherwise you would have the option of sacrificing yourself.) If you pushed him, would he fall in a way that would halt the train? Is there some other way to prevent the deaths—a signal that can be given or a switch that can be thrown? Could you persuade the fat man to jump? Could you say, “Fat man, let us leap together”?

To avoid some of these questions, Parfit’s variant of the story stipulates a remotecontrol device that you can use to launch someone from the bridge on to the track. In this way he seeks to dodge or escape certain questions—but his modification introduces many others. How can you tell what will happen if you use whatever device you have? Could you stop the train simply by opening the trap, without anything falling through? Can you signal to the potential victim and arrange for some appropriate substitute object to fall through the trap? Are there other devices you should seek that would allow you to communicate directly with the driver, or to stop the train in less messy ways? Your response to any actual situation would depend on how you would answer or address questions such as these—on how you would cast around in attempting to avoid any death or injury (just as, in the original story, you would seek alternatives to the stark choice assigned to you). Parfit’s emendation of the canonical scenario is guided by no standard of objectivity for evoking reliable responses, and thus it generates further versions of the disease it is intended to cure.

You cannot respond to the imagined predicament without thinking hard, but hard thinking leads through a cloud of questions to a state of confusion. A few conditions are simply declared: the outcomes are known and the options limited. But since that sort of certainty and limitation is exceedingly remote from the circumstances in which we make our practical decisions, our judgmental capacities cannot be put to work in their normal ways. Readers are pitched into a fantasy world, remote from reality, in which our natural reactions are sharply curtailed by authorial fiat. When we are called on to render a verdict, the dominant feeling is a disruption of whatever skills we possess, and a corresponding distrust of anything we might say-often publicly visible when lecturers ask their audiences to respond to some puzzle case: only partisans of some particular theory answer confidently, while the rest sit in uncomfortable silence. The reader may even be left with a deep sense of unease that matters of life and death are to be judged on the basis of such cursory and rigged information. (Allen Wood makes similar points trenchantly in his contribution to Parfit’s book. This part of Wood’s critique goes unaddressed by Parfit.)

Parfit’s only defense of his use of puzzle cases occurs in passing, in a much earlier passage in which he proposes that thought-experiments are as valuable in ethics as they are in the sciences. The comparison prompts an obvious response—that many scientists think of thought-experiments as motivational rather than probative, as preludes to real experiments that will elicit genuine evidence. (It should also be noted that the great thoughtexperiments in the history of science occur in domains in which the questions can be precisely defined.) And matters are made even worse when the puzzle cases are used to interrogate the reasons that an imagined agent within the story might have for acting in a particular way. The vagueness of Parfit’s concept of a reason—a concept he takes to be indefinable—vitiates any serious attempt to survey the range of reasons someone has at his disposal. Even after the many claims about reasons Parfit makes, a sensitive reader should still wonder if those claims are justified. Moreover, his extensive discussion of issues about ethical truth and ethical knowledge renders our capacities for arriving at judgments, whether about individual situations or about general principles, so mysterious that the reader’s sense of hopeless floundering is further compounded.