Where is the margin between homeostasis and suffering in humans, mammals, fish, invertebrates, and even AI? Demarcating where feelings of pain or pleasure begin and end is not simple. As an example, sentience could appear in complex machines and software, like AI. For Jonathan Birch, a philosopher of science and animal ethics, sentience includes a “capacity to have valenced experiences” (p. 1) ranging from distress to satisfaction. This definition helps identify welfare risks and assess ways to avoid inflicting gratuitous harm. That’s The Edge of Sentience in a nutshell; Birch takes readers on a compelling, engaging, and at times controversial journey through the sentient life of humans, a variety of nonhuman animals, and artificial intelligence. The word “edge” in the title is important in several respects. Birch is cautious not to make grand pronouncements; rather, in situations where evidence about sentience is not yet clear, there should be debate to...

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