أَعْجَبَنِي زَيْدٌ عِلْمُهُ
Zayd's knowledge impressed me.
This is badal ishtimāl, substitution of an associated quality. «His knowledge» is not a physical portion of Zayd; it is something meaningfully connected to him and is the actual object of admiration.
Compare it with «most of them»: the latter names members of an initial group, whereas «his knowledge» names an associated attribute. Treating them both as merely a second name would obscure the textbook's distinction.
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ʿAṭf al-Bayān, Badal and Coordination: Clarifying, Reinforcing and Specifying the Subject
أَعْجَبَنِي زَيْدٌ عِلْمُهُ
Zayd's knowledge impressed me.
This is badal ishtimāl, substitution of an associated quality. «His knowledge» is not a physical portion of Zayd; it is something meaningfully connected to him and is the actual object of admiration.
Compare it with «most of them»: the latter names members of an initial group, whereas «his knowledge» names an associated attribute. Treating them both as merely a second name would obscure the textbook's distinction.