أن المراد بالصدق في تعريف الكلي هو الصدق على وجه الاجتماع
The predicability intended in the definition of a universal is applicability through a shared meaning, not mere substitution of one candidate for another.
The concept “human” applies to Zayd and ʿAmr together in the same respect: both are human. One need not withdraw it from Zayd to apply it to ʿAmr.
A vague personal image differs: the observer may take it as Zayd or instead as ʿAmr, but the image's intended identity is not thereby a nature commonly predicable of both. The commentary marks this contrast as collective common applicability versus alternation.