What would it mean to treat other living creatures fairly?

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An elephant roams the streets of Bangkok, Thailand, in 2008

The numbers say it all: Nearly two-thirds of global mammalian biomass is currently made up of livestock, the majority raised and killed in intolerably cruel factory farms. The domesticated chicken is now the world’s most populous bird, whose discarded bones will define the fossil record of our human-dominated age. Driven by habitat loss, climate change, and other human causes, the ongoingSixth Mass Extinctionrepresents not just a crisis of biodiversity but a source of immense suffering for millions of individual creatures.

“Animals are in trouble all over the world,” University of Chicago professor Martha Nussbaum writes inJustice for Animals: Our Collective Responsibility,her new book out this month. In her half-century as a moral philosopher, Nussbaum has tackled an enormous range of topics, including…