If a creature can suffer and has a subjective experience of the world, the welfare argument says "minimize the suffering." The rights argument says "they are someone, not something."

If you are looking for the origins of the modern movement, these seminal works are essential: Animal Liberation " (Peter Singer, 1975) : A foundational text that introduced the concept of speciesism

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The modern animal rights movement exploded into the public consciousness with the publication of Peter Singer’s Animal Liberation (1975). Although Singer is a utilitarian (focused on suffering, not theoretical "rights"), his work galvanized a generation. This was followed by Tom Regan’s deontological rights theory in the 1980s. Activists began abandoning the term "welfare" as a compromise, arguing that "kinder" slaughter was still slaughter.