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How does Peter Singer use the idea of speciesism to criticize unequal treatment of animals?

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Three PETA protesters in animal masks and green clothing hold signs reading No Fur at a demonstration against fur in clothing at a shopping district in Seoul, South Korea. — Animal rights A PETA (People for the Ethical Treatment of Animals) demonstration in Seoul, South Korea, against the use of animals for fur in clothing.

Peter Singer uses the concept of speciesism to argue that discriminating against beings based solely on their species membership is morally indefensible, drawing a direct parallel to racism and sexism.21

His critique is grounded in the principle of equal consideration of interests (PEC), which asserts that one must give equal weight to the like interests of all affected beings. Singer argues that because both humans and animals are capable of feeling pain, they share a fundamental interest in avoiding it. He maintains that the capacity to experience suffering is the essential condition for having interests, and therefore, species membership is a morally irrelevant characteristic.21

By favoring human interests over the like interests of animals, speciesists commit an irrational prejudice. Singer challenges the notion that human-specific traits like rationality or autonomy justify this unequal treatment, noting that such arguments often fail when confronted with "marginal cases"—such as human infants or individuals with severe cognitive disabilities—who may lack these same capacities. Ultimately, Singer contends that all beings capable of enjoyment or suffering deserve equal moral consideration.21