It is morally acceptable to redirect a speeding trolley from a track on which there are five people onto a track with only one person. On the other hand, it is not right to shoot one innocent person to save five. What is the morally relevant difference between the two cases? If we denied the PSR, then we could simply say: "Who cares? Both of these moral facts are just brute facts, with no explanation." Why, indeed, suppose that there should be some explanation of the difference in moral evaluation if we accept the denial of the PSR, and hence accept that there can be facts with no explanation at all?
Almost all moral theorists accept the supervenience of the moral on the nonmoral. But without the PSR, would we really have reason to accept that? We could simply suppose brute contingent facts. In this world, torture is wrong. In that world, exactly alike in every other respect, torture is a duty. Why? No reason, just contingent brute fact.
The denial of the PSR, thus, would bring much philosophical argumentation to a standstill.
An interesting thing about this argument is that it yields a PSR not just for contingent truths but also for necessary ones.
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