According to the source, 'human' is an essential universal with respect to Zayd, Amr, and other humans because it signifies their complete specific essence.
Calling Zayd human identifies his kind rather than adding an optional attribute.
The same universal applies to different individuals despite their varying ages, knowledge, appearance, and habits.
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An essential universal as the whole essence
Essence, Quiddity, and Essential vs Accidental Universals
According to the source, 'human' is an essential universal with respect to Zayd, Amr, and other humans because it signifies their complete specific essence.
Calling Zayd human identifies his kind rather than adding an optional attribute.
The same universal applies to different individuals despite their varying ages, knowledge, appearance, and habits.