Genus is an essential universal common to different specific natures.
Species is an essential universal common to individuals of one specific nature.
'Animal' is a genus when humans, cattle and goats are compared.
'Human' is a species when Zayd, Amr and Bakr are compared.
The decisive question is whether the specific natures involved are different or one.
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The central genus–species distinction
The Five Universals: Genus, Species, Differentia, Property, Common Accident
Genus is an essential universal common to different specific natures.
Species is an essential universal common to individuals of one specific nature.
'Animal' is a genus when humans, cattle and goats are compared.
'Human' is a species when Zayd, Amr and Bakr are compared.
The decisive question is whether the specific natures involved are different or one.