The Meaning of the Poet's Saying: "Like abstractions in the imagination, and a people have built upon them the weakest of structures, thinking that they have an external existence, and if their existence were true, it would be in..."

The Shaykh : He says: "Like abstractions in the imagination, and a people have built upon them the weakest of structures"
He is referring to the philosophers who denied that Allah is described by affirmation or described by negation.
"Thinking" means that these people "believed that they have an external existence", meaning they believed that there exists an essence that is not described by affirmation or negation. Ibn al-Qayyim says: "And if their existence were true", in what? "In the minds"
He means that the mind is what posits an essence that has no attributes, whereas in external reality, it is not possible for an essence to exist without attributes. If its only attribute were existence, that would be sufficient. For example: there is a stick standing in front of us. Can we say that this stick is neither existent nor non-existent? It is not possible. But the mind may posit impossible things. Therefore, he says: "If their existence were true, it would be in the minds".