ABSTRACT
This chapter argues that sentience, the capacity to have experiences that can be positive or negative, is what matters for respecting someone. What most of us value in our own lives is not the mere fact of being alive, but the things that we experience while we are living. The chapter then addresses the evidence and arguments available to examine what animals are sentient. It considers behavior and evolutionary logic, but focuses especially on physiology. The chapter argues that sentience requires some processing of information such as the one performed in centralized nervous systems. This makes animals with complex nervous systems, such as vertebrates and some invertebrates like cephalopods, quite obvious candidates for being sentient. However, other invertebrates with simpler nervous systems with brains are sound candidates for that as well, such as arthropods. Moreover, animals lacking brains but nevertheless having other types of centralized nervous structures, such as cerebral ganglia, bivalves, and gastropods, cannot be ruled out as sentient either. The question remains open as to whether they can have experiences, and will not be solved any time soon, but acting as if they were not could have disastrous effects.
