لِأَنَّهُ يَقْتَضِي التَّشَخُّصَ وَمَنْعَ الِاشْتِرَاكِ
A proper name carries individual determination rather than ordinary distribution across members.
“Human” is a general noun whose instances are many; “Ḥātim” identifies a particular person. Calling someone else a Ḥātim cannot simply mean that this different human is literally the same historical individual. Its figurative interpretation must rest on the relevant resemblance, such as generosity.
Distinguishing the logical status of a proper name from that of a common noun safeguards rhetorical meaning. Treating every proper name as freely generic would dissolve the special effect of invoking an exemplary person.