How do we know that some beings have a status as 'persons' and some beings do

How do we know that some beings have a status as 'persons' and some beings do

How do we know that some beings have a status as 'persons' and some beings do not? If we attempt to delineate certain characteristics of personhood, we run into the quandary of, say, labelling the mentally ill as non-persons or labelling cancer cells as persons. Is this a problem? Is there a way to avoid this? Must we have the rights which personhood entails in the first place?

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