William MacAskill, Krister Bykvist and Toby Ord, <i>Moral Uncertainty</i> (Oxford: Oxford University Press, 2020), pp. viii + 226.

Some time ago, I lived near a family of brown rats (Rattus norvegicus) who spent their days foraging in the streets.The rats were friendly and harmless creatures, and the young rats were nearly tame.But for reasons that I have never fully understood, my human neighbours thought the rodents should be killed.In fact, they were so certain that the rats lacked moral standing, that killing them did not strike them as in any way morally problematic.But just how certain were they?Suppose that my neighbours were as much as 99.9% certain that the rats lacked moral standing.Even so, killing the rats might still turn out to be both unreasonable and irrational, even from their own point of view.To see this, note that these people, who were otherwise kind and considerate, would clearly judge it morally wrong to place a poisoned bait that had a small risk to mistakenly kill a human child.But at the same time, these situations seem comparable, such that there is a sense in which: