Very often, we are unsure about what we ought to do. Under what conditions should we help to improve the lives of distant strangers rather than those of our family members? At what point does an embryo or foetus become a person, with all the rights that that entails? Is it ever permissible to raise and kill non-human animals in order to use their meat for food?

Sometimes, this uncertainty arises out of empirical uncertainty: we might not know to what extent non-human animal...