«وإن كان موجودا في الذهن»
And if the subject exists in the mind.
A proposition treating a concept precisely as a concept in the intellect is called mental (dhihniyyah).
«الإنسان كلي»
Human is universal.
Universality here does not belong to a particular human as an external characteristic. Zayd and Amr are individuals, not universals in their own persons. Being universal is a logical feature of the mentally considered human concept.