The marginal cases argument (MCA) has been used very effectively in defense of the direct moral standing of sentient non-humans. Any being that has direct moral standing is such that our treatment of them matters directly, independently of how it affects others. Direct moral standing is worth wanting: it ensures protection against, among other things, cruelty, mistreatment, destruction and exploitation, etc. Probably works of art and other possessions have no more than indirect moral standing; certainly normal adult humans have direct moral standing.
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