Consider the sequence:
Body → growing body → animal → human
'Animal' is the proximate genus of human. 'Rational' marks the human species off from other animals, making it proximate. 'Growing body' is a more remote level; being sentient marks a distinction at that earlier stage.
'Sentient' cannot be the proximate differentia of human because other animals are sentient too. It does not distinguish the human species within its nearest genus.
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The Five Predicables: Differentia, Property, Common Accident, Essential and Accidental Attributes
Consider the sequence:
Body → growing body → animal → human
'Animal' is the proximate genus of human. 'Rational' marks the human species off from other animals, making it proximate. 'Growing body' is a more remote level; being sentient marks a distinction at that earlier stage.
'Sentient' cannot be the proximate differentia of human because other animals are sentient too. It does not distinguish the human species within its nearest genus.