Agreement in essence does not mean that all members of a species are identical in every attribute. Were that so, Zayd and Amr could not be distinguished as persons.
Species-level agreement concerns basic nature. Individuals can vary in their individuating and accidental attributes.
Two humans: same species despite different heights.
Human and horse: different species despite both being animals.
Keeping these levels distinct prevents genus and species from being confused.
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The Five Predicables: Genus, Species and Hierarchy of Genera
Agreement in essence does not mean that all members of a species are identical in every attribute. Were that so, Zayd and Amr could not be distinguished as persons.
Species-level agreement concerns basic nature. Individuals can vary in their individuating and accidental attributes.
Two humans: same species despite different heights.
Human and horse: different species despite both being animals.
Keeping these levels distinct prevents genus and species from being confused.